THE UNIVERSITY OF MANCHESTER
Investigating Effect of Clay Composition on Safety Function Performance in a Geological Disposal Facility (GDF) A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering 25th of September 2018
Adam Peter Sims School of Chemistry
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Table of Contents
Table of Contents...... 2 Table of Figures...... 8 Table of Tables...... 15 Commonly used Abbreviations...... 17 Abstract...... …...... …...... 19 Declaration of Work...... …....20 Copyright Statement...... …21 Acknowledgements...... …...... …22 About the Author...... 23
1. Introduction and Thesis content ...... 24
1.1. Project Significance ...... 24
1.2. Aims and Objectives ...... 27
1.3. Thesis Structure ...... 29
1.4. Papers Submitted and Collaborators ...... 32
1.5. Conferences, external work, and user facilities visited ...... 33
1.5.1. Conferences (presentations and posters) ...... 33
1.5.2. External work ...... 34
1.5.3. User Facilities ...... 34
1.6. References ...... 35
2. Effects of Composition, Radiation Environment, and Corrosion Products on Performance of an Engineered Clay Barrier ...... 38
2.1. The Nuclear Waste Story ...... 38
2.1.1. The Nuclear Fuel Cycle ...... 39
2.1.2. UK Nuclear Legacy ...... 42
2.1.3. Current Radioactive waste Storage ...... 44
2.1.4. Geological Disposal ...... 45
2.2. Engineered Clay Barrier ...... 50
2.2.1. Clay Minerals ...... 50
2.2.2. Smectites ...... 52
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2.2.3. Bentonite ...... 53
2.2.4. Montmorillonite, Nontronite and Beidellite ...... 54
2.2.5. Colloids ...... 55
2.3. Challenges from heat generating HLW in a geological disposal facility ...... 55
2.3.1. Heating ...... 56
2.3.2. Irradiation ...... 57
2.3.3. Groundwater Infiltration ...... 59
2.3.4. Corrosion products ...... 66
2.4. References ...... 66
3. Materials and Methods ...... 84
3.1. Materials ...... 84
3.1.1. Clay Source Repository Clays ...... 84
3.1.2. Chemicals ...... 88
3.2. Methods ...... 88
3.2.1. Size Fractionation ...... 88
3.2.2. Homoionisation ...... 88
3.2.3. Clay Pellets ...... 89
3.2.4. Heating ...... 89
3.2.5. γ-irradiation ...... 89
3.2.6. α-irradiation ...... 90
3.2.7. Colloidal Suspensions ...... 92
3.2.8. Sorption Experiments ...... 92
3.3. Clay Characterisation ...... 94
3.3.1. Major Element Analysis...... 94
3.3.2. X-ray Diffraction (XRD) ...... 94
3.3.3. Infra-red Spectroscopy (IR) ...... 96
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3.3.4 Cation Exchange Capacity (CEC), Sorption Properties, and Extractable Fe2+ Measurements ...... 97
3.3.5. Electron Paramagnetic Resonance spectroscopy (EPR) ...... 99
3.3.6. X-ray Computed Tomography (XCT) ...... 100
3.3.7. Colloidal Measurements ...... 102
3.3.8. Synchrotron Measurements ...... 104
3.4. Safety ...... 121
3.4.1. Laboratory Safety ...... 121
3.4.2. Radiological Hazards ...... 121
3.4.3. Safety Courses ...... 121
3.5. References ...... 122
4. Molecular- to meso-scale effects of heat and subsquent gamma radiation on engineered clay barrier performance for radioactive waste disposal ...... 129
4.1 Abstract ...... 130
4.2. Introduction ...... 131
4.2.1. Effect of heating on clay performance ...... 133
4.2.2. Effect of γ-irradiation on clay performance ...... 137
4.3. Methods and Materials ...... 142
4.3.1. Clay preparation ...... 142
4.3.2. Heat treatment and irradiation experiments ...... 142
4.3.3. Clay characterisation before and after heat treatment and irradiation .... 143
4.4. Results ...... 146
4.4.1. Clay chemical composition and exchangeable cations ...... 146
4.4.2. Combined effects of heat and γ-radiation on clay structure ...... 147
4.4.3. Effect of γ-irradiation on CEC and extractable Fe2+ ...... 150
4.4.4. EXAFS analysis of STx-1b and NAu-1 before and after γ-irradiation ...... 151
4.4.5. EPR analysis of combined heating and γ-irradiation effects on STx-1b...... 155
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4.4.6. Effect of γ-irradiation on STx-1b colloid formation ...... 157
4.5. Discussion ...... 159
4.5.1. Effect of chemical composition on clay barrier performance ...... 159
4.5.2. Effect of heat and γ-irradiation on clay barrier performance ...... 160
4.6. Conclusions ...... 164
4.7. Acknowledgements ...... 164
4.8. References ...... 165
4.9. Paper Figures ...... 177
4.10. Supplementary Information:...... 189
5. The coupled physicochemical effects of alpha irradiation and heating on clay barriers for radioactive waste disposal ...... 203
5.1. Abstract ...... 204
5.2. Introduction ...... 205
5.2.1. Effects of heat on ECB performance ...... 206
5.2.2. Effects of Radiation on ECB performance ...... 206
5.2.3 Coupled effects of heat and radiation on ECB performance ...... 210
5.3. Methods and Materials ...... 210
5.3.1. Clays ...... 210
5.3.2. Alpha irradiation ...... 211
5.3.3. Clay characterisation before and after heat treatment and irradiation .... 212
5.4. Results ...... 213
5.4.1. Clay composition ...... 213
5.4.2. Effect of interlayer cation on resistance to heat and α-irradiation damage ...... 214
5.4.3. Effect of chemical composition on alpha irradiation damage in clays ...... 222
5.5. Discussion ...... 229
5.5.1. Effect of composition on clay interactions with α-irradiation ...... 229
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5.5.2. Effect of heat on a model montmorillonite under α-irradiation ...... 231
5.6. Conclusions ...... 231
5.7. Acknowledgements ...... 232
5.8. References ...... 233
5.9 Supplementary information ...... 241
6. The effect of γ-irradiation on the sorption capacity of a model montmorillonite and nontronite under simulated repository conditions with respect to U(VI) and Cr(VI) in solution (research article) ...... 260
6.1 Abstract ...... 261
6.2. Introduction ...... 262
6.2.1. γ-irradiation ...... 264
2- 6.2.2. Chromium (CrO4 ) ...... 264
2+ 6.2.3. Uranyl Studies (UO2 ) ...... 266
6.2.4. Current Cr(VI) and U(VI) sorption study ...... 272
6.3. Methods and Materials ...... 272
6.3.1. Chemicals ...... 272
6.3.2. Clay minerals ...... 272
6.3.3. γ-irradiation ...... 273
6.3.4. Sorption experiments ...... 273
6.3.5. Solid and solution phase characterisation ...... 275
6.4. Results ...... 276
6.4.1. Characterisation ...... 276
6.4.2. Chromium Sorption ...... 277
6.4.3. Uranium Sorption Study...... 278
6.5. Discussion ...... 287
6.6. Conclusions ...... 291
6.7. Acknowledgements ...... 292
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6.8. References ...... 293
6.9. Supplementary Information ...... 305
7. Summary, conclusions and future work ...... 313
7.1. Summary ...... 313
7.2. Conclusions ...... 318
7.3. Future Directions ...... 322
7.4. References ...... 327
8. Appendices ...... 329
8.1 Unpublished work in progress ...... 329
8.1.1. Notes on: Impact of canister corrosion products on the structure and chemical composition of the engineered clay barrier in a geological disposal facility ...... 329
8.1.2. Notes on: The production of H2 gas as a factor of γ-radiation dose and Fe content on a series of montmorillonites and nontronites...... 336
8.2. Paper contributions ...... 339
8.3. Other relevant courses and volunteering ...... 352
8.3.1. Roles of responsibility ...... 352
8.3.2. Additional technical training ...... 352
Word Count: 62,741
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Table of Figures
1.1. The challenges faced after closure by an ECB in a GDF from stored HLW and the surrounding environment...... ……………...... 25
2.1. The Nuclear Fuel Cycle...... 41
2.2. Sites of existing and proposed nuclear power stations...... ………….....42
2.3. Contribution to radioactive waste stock pile from current endeavours...... …...43
2.4. Information from the 2016 waste Inventory on percentage volume (m3) and radioactivity of different types of radioactive waste...... 45
2.5. Reprint of the possible design for a GDF in the UK...... ……..…….…....46
2.6. Reprint of a basic schematic of the multiple barrier system...... 47
2.7. Hydrated smectite clay layer structure representation ……………………...….….....52
2.8. 238U radioactive decay series………………………………………...... ……..…………....62
2.9. 235U radioactive decay series……………………………………………...... …………………..63
2.10. Speciation of U(VI) at 1 ppm and dissolved carbonate concentration as a function of pH...... ……...... …...... …...... 64
3.1. Pellet press used in experiments and a 13 mm pellet of STx-1b and NAu-1...... 89
3.2. Example of pellet sample sets (natural clays and treated sets) prior to γ-irradiation and a sediment set after γ-irradiation...... …………..….…...90
3.3. Foss Therapy Services Model 812 60Co γ-irradiator at DCF……..…...... 90
3.4. α-irradiation samples on a glass slide, mounted on a pin, and a SCa-3 pellet sample before and after α-irradiation...... 91
3.5. NEC 15SDH-4 tandem Pelletron ion accelerator at DCF; accelerator hall and target station……………………………………………….………………………………………………...92
3.6. Bruker D8 advanced diffractometer...... …….....…….…...95
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3.7. Bragg’s law interactions within a crystal lattice……………………………….……...…….96
3.8. Bruker EMX Micro X-band EPR Spectrometer...... …………………..….99
3.9. Zeiss Xradia Versa 520 XCT system...... ………………………...... 101
3.10. Generalised overview of XCT………………...... …....102
3.11. Beamline I18 XRD set up...... …………………………………..106
3.12. Example of a pre-edge feature in NAu-1 clay from a 1s-3d transition……….....109
3.13. A Ca-K edge XAS spectrum, highlighting the different sections of interest…...111
3.14. Uranium L3-edge XANES of schoepite and uraninite...... ………....112
3.15. 1st derivative of U L3-edge data…………………………………………………………………..115
3.16. Showing a Fe K-edge XAS spectrum of montmorillonite...... ………………………115
3.17. Showing Ca k-edge data before and after γ-irradiation...... 116
4.1. Basic schematic of 2:1 (tetrahedral: octahedral) swelling montmorillonite…..177
4.2. d001 X-ray diffraction data for cation exchanged smectites...... 178
4.3. X-ray diffraction measurements for Ca-montmorillonite at 25 °C…...... 179
4.4. XAS spectrum of STx-1b and NAu-1 Ca K-edge before and after γ-irradiation, and STx-1b fits in Artemis...... …...... 180
4.5. XAS spectrum of STx-1b and NAu-1 K K-edge before and after γ-irradiation and STx-1b fits in Artemis...... …...... 181
4.10. EPR spectrum of STx-1b at 25 °C……………………………………………………………...... 182
4.11. EPR spectra of STx-1b compared to STx-1b heated to 500 °C...... 183
4.12. EPR spectra showing the predominant defect centres of untreated STx-1b γ-irradiated to 5 MGy at 25 °C., including structural representation...... 184
4.13. EPR spectra of STx-1b and γ-irradiated STx-1b to doses of 0.2 MGy, 2.7 MGy and 5 MGy all at 25 °C…………………...... …...………….185
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4.14. EPR spectra of γ-irradiated STx-1b (5 MGy) at 25, 90, 160 and 500 °C...... 186
4.15. AFM morphology pictures of dried colloid suspensions on the starting STx-1b and the γ-irradiated (5 MGy) STx-1b…………...... …...... 187
4.16. Force vs. separation graph between polystyrene particles and clay particles showing the effect of γ-irradiation at pH 9...... …………………………188
4.1SI. Comparison of irradiated vs natural X-ray diffraction patterns of Mg-montmorillonite (STx-1b) heated to 90 °C………………………………………………191
4.2SI. Assigned infra-red spectrum of a nontronite and montmorillonite...... …..….192
4.3SI. Effects of γ-irradiation on montmorillonite (STx-1b) shown for unirradiated, STx-1b 0.2 MGy, 2.7 MGy, and 5 MGy...... …………….192
4.4SI. XAS spectrum of SCa-3 Ca K-edge before and after γ-irradiation…...... 193
4.5SI. XAS spectrum of NAu-2 Ca K-edge before and after γ-irradiation...... …..193
4.6SI. XAS spectrum of STx-b potassium K-edge before and after γ-irradiation...... 194
4.7SI. XAS spectrum of SCa-3 potassium K-edge before and after γ-irradiation...... 194
4.8SI. XAS spectrum of SCa-3 Fe K-edge before and after γ-irradiation...... 194
4.9SI. XAS spectrum of NAu-2 Fe K-edge before and after γ-irradiation...... …195
4.10SI. Calcium K-edge EXAFS fit for NAu-1 before and after γ-irradiation...... …....195
4.11SI. Potassium K-edge EXAFS fit for STx-1b before and after γ-irradiation.....….....196
4.12SI. Iron K-edge EXAFS fit for NAu-1 before and after γ-irradiation...... …...... 197
4.13SI. Force vs. separation graphs showing the effect of pH and γ-irradiation between polystyrene particles and clay particles…………………………………..………………...…200
4.14SI. AFM images of colloidal STx-1b detachment at pH 5 and pH 9……………..….....201
4.15SI. SEM images of colloidal STx-1b before and after γ-irradiation…...……...... 201
5.1. Showing the set up for tomography samples before and after α-irradiation…211
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5.2. Showing the effect of α-irradiation on the low and high energy XRD measurements on unheated Na+-substituted and Mg2+-substituted STx-1b...216
5.3. Showing the Ca K-edge XAS data for the STx-1b before and after α-irradiation; and effect of prior heat treatment after α-irradiation...... 217
5.4. Showing the Fe K-edge XAS data for the STx-1b before and after α-irradiation; and effect of prior heat treatment after α-irradiation...... 218
5.5. Tomography images for STx- before and after α-irradiation and heating with corresponding pore density plots from small sections within the clays...... 220
5.6. Showing a selection of images of Mg treated STx-1b from before and after α-irradiation...... …………………………………………….………………………………………….221
5.7. Showing 2D slices of SAz-2 and STx-1b before and after α-irradiation...... 222
5.8. Showing the low and high energy XRD d-spacing scans of STx-1b, mapping from an un-irradiated area to an area of high α-damage.……………………...... …..…223
5.9. Showing the XRD map of NAu-1 from an area of high damage to an area of lower damage...... ………………….….225
5.10. Showing d001 spacing in the natural and treated clays (STx-1b) as a function of displacements per atom caused through differing α-irradiation doses...... 228
5.1SI. Showing the effect of α-irradiation on the high and low energy XRD measurements on Na+ substituted and Mg2+ substituted STx-1b both heated prior to α-irradiation...... ……….241
5.2SI. Showing untreated STx1b, both unheated and heated, high and low energy XRD scans after α-irradiation...... …...... ……………………...……..242
5.3SI. Showing the Ca K-edge XAS data for Mg STx-1b befor and after α-irradiation; and the effect of prior heating...... 243
5.4SI. Showing the Fe K-edge XAS data Na STx-1b before and after α-irradiation; and the effect of prior heating...... 243
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5.5SI. Tomography cross sections for Mg STx-1b unheated and heated before and after α-irradiation with corresponding pore density plots...... ………...244
5.6SI. Tomography cross sections for Na STx-1b unheated and heated before and after α-irradiation with corresponding pore density plots...... 244
5.7SI. Tomograpghy images of Mg STx-1b heated from before and after α-irradiation...... 245
5.8SI. Tomograpghy images of STx-1b from before and after α-irradiation...... …..245
5.9SI. Tomograpghy images of STx-1b heated from before and after α-irradiation...... 246
5.10SI. Tomograpghy images of Na STx-1b from before and after α-irradiation…...... 246
5.11SI. Tomograpghy of Na STx-1b heated from before and after α-irradiation...... 247
5.12SI. Showing the low energy XRD d-spacing scans of SCa-3; mapping from an area of low α-irradiation damage into an area to an area of high α-damage...... 248
5.13SI. Showing the low energy XRD d-spacing scans of SBld-1, before and after α-irradiation...... ……………………...... ….....249
5.14SI. Showing the low energy XRD d-spacing scans of NAu-2, before and after α-irradiation...... ……………………...…….....249
5.15SI. Showing the low energy XRD d-spacing scans of SWy-1, before and after α-irradiation...... ………………………………...….250
5.16SI. Showing the Ca K-edge XAS data for STx-1b along a damage track from an area of no visible α-damage to an area of high α-damage...... 250
5.17SI. Showing the Ca K-edge XAS data for STx-1b along a damage track from an area of no visible α-damage to an area of high α-damage...... 250
5.18SI. Showing the Fe K-edge XAS data for NAu-1 before and after α-irradiation.....251
5.19SI. Showing the Fourier transform of the Ca K-edge EXAFS for SWy-1, NAu-2, and SBld-1; before and after α-irradiation...... ……………...…....251
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5.20SI. Showing the Fourier transform of the Fe K-edge EXAFS for SWy-1, NAu-2, and SBld-1; before and after α-irradiation...... ………………...... 251
5.21SI. Beam profile image used to derive a Gaussian fit for α-damage.……………...... 254
5.22SI. SRIM pathway generated in montmorillonite type material…………………...…...256
6.1. A comparison of chromium sorption onto a montmorillonite and nontronite with and without prior γ-irradiation over 12 months in AGW...... 277
6.2. Showing the uptake of uranium onto a montmorillonite and nontronite with and without prior γ-irradiation and in low and normal carbonate solutions of AGW over 12 months...... 279
6.3. Normalised uranium L3-edge XANES analysis of STx-1b at 5 weeks, NAu-1 and STx-1b at 9 months and STx-1b at 12 months for both natural and γ-irradiated samples...... 280
6.4. U L3-edge EXAFS data and fits from Artemis for STx 1b in contact with U(VI) solution for 9 months; samples without prior γ-irradiation and after γ-irradiation...... 284
6.5. U L3-edge EXAFS data and fits from Artemis for NAu-1 in contact with U(VI) solution for 9 months; samples without prior and after γ-irradiation...... 285
6.1SI. Control results for Cr(VI) sorption experiment in AGW over 12 months…...... 307
6.2SI. Control results for U(VI) sorption experiment, low and high carbonate systems in AGW over 12 months……………………………………………………………………………....308
6.3SI. U L3-edge EXAFS data and fit from Artemis for STx-1b in contact with U(VI) solution for 9 months without prior γ-irradiation...... …………………...308
6.4SI. U L3-edge EXAFS data and fit from Artemis for STx-1b in contact with U(VI) solution for 9 months with prior γ-irradiation...... …………………...... 309
6.5SI. U L3-edge EXAFS data and fit from Artemis for STx-1b in contact with U(VI) solution for 12 months with prior γ-irradiation...... …………...... 309
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6.6SI. U L3-edge EXAFS data and fit from Artemis for NAu-1 in contact with U(VI) solution for 9 months without prior γ-irradiation...... …………...309
6.7SI. U L3-edge EXAFS data and fit from Artemis for NAu-1 in contact with U(VI) solution for 9 months with prior γ-irradiation...... ………………………...310
6.8SI. U L3-edge XANES for the STx-1b 9M low carbonate system; average, 1st and last scan…………………………………………………...... ……………………...... 312
7.1. Key research links and further research questions to be answered...... 322
8.1A. Set up for controlled atmosphere corrosion experiments...... 330
8.2A. Showing Fe foil within the heat treated STx-1b pellet and Fe foil damage...... 331
8.3A. Showing 56Fe Mössbauer of Cl-treated STx-1b after heating at 90 °C for 21 months...... …………………...…..…332
8.4A. Showing the un-normalised XANES plots of the Un1, MgCl1, Cl1, ANO1 STx-1b samples and an aerobic blank (cross section mapped)……………….…………….....333
8.5A. Showing the normalised XANES and EXAFS plots for the aerobic STx-1b sample run and Un1 STx-1b sample……………………………………………………………………...….334
8.6A. Showing diffraction pattern of STx-1b MgCl1 at 12 KeV...... 335
8.7A. The production of H2 in clays as a function of dose and Fe content (μM)...... 338
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Table of Tables
2.1. Showing a summary of planned GDF designs...... 49
2.2. Key modes of decay showing changes in charge, neutron number and atomic number...... ……...... ……………..……………………………...... 62
3.1. Summary of data on relevant clays used throughout thesis experiments...... 86
3.2. Showing the interpretation of R-factor values, quantifying fit mismatch...... 120
4.1. Effect of γ-irradiation on clay: review of studies reported in the literature.....138
4.2. Smectite chemical composition measured using XRF…...... 146
4.3. d001 spacing for heat-treated and γ-irradiated montmorillonite (STx-1b) and nontronite (NAu-1)……………………………………………………………………………………....148
4.4. CEC and ferrozine assay results for natural and cation-exchanged STx-1b and SWy-1 before and after irradiation…...... …………………...... 150
4.5. Ca and Fe K-edge EXAFS fit results for STx-1b and NAu-1 before and after irradiation…………………………………………………………………………………………………….153
4.6. Zeta potential measurements on STx-1b before and after irradiation...... 158
4.1SI. Weight loss of ignition data for natural smectites……………………………………...…191
4.2SI. CEC and ferrozine measurements on natural clays……………………………...……….192
4.3SI. EXAFS fits for STx-1b, NAu-2 and SCa-3 from Artemus………………………...….……197
4.4SI. Colloidal zeta potential and size measurements……………………………………...…..199
5.1. Smectite chemical composition measured using XRF…...... 214
5.2. Showing the effect of interlayer cation on the d-spacing of the clays...………..214
5.3. Ca K-edge EXAFS fit results for STx-1b before and after α-irradiation.....…...…217
5.4. Fe K-edge EXAFS fit results for treated series of STx-1b before and after α-irradiation………………………………………………………………………………………………...219
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5.5. Ca and Fe K-edge EXAFS fits for STx-1b along a α-irradiation track from an area of low damage to an area of higher damage...... ……………………………...224
5.6. Fe K-edge EXAFS fit results for NAu-1 for a point off the α-damaged area on the pellet and a point at in the highest area of damage...... …………………...... ….225
5.7 Showing the effect of dpa that was caused by α-irradiation on d-spacing of the clays...... ……………………………………………………………………………………………...227
5.1SI. Ca K-edge EXAFS fit results for Mg treated STx-1b after α-irradiation…….…....243
5.2SI. Ca and Fe K-edge EXAFS fits for NAu-1, NAu-2, SBld-1, SCa-3, and SWy-1 before and after α-irradiation……………………………………………………………………………...….252
5.3SI. Distribution of 8.36x1015 He2+ ions over a beam profile (Figure 21SI)...... ……..255
5.4SI. Showing the values used to calculate dpa...... …………………………………………...257
5.5SI. Showing the calculated area of α-damage across the Gaussian distribution of the beam profile………………………………………………………………………………………..…258
6.1. Showing the linear combination fitting from Athena, comparing the U(VI)/U(IV) ratios in each of the measured systems, with and without γ-irradiation.....….282
6.2. U L3-edge EXAFS fit results for STx-1b with and without γ-irradiation and NAu-1 with and without γ-irradiation...... ……....………...... …285
6.1SI. U L3-edge EXAFS fit results for STx-1b with and without γ-irradiation...…...... 310
6.2SI. Showing the linear combination fitting of the STx-1b 9M low carbonate system and evidencing beam reduction…………………………………………………………………...312
8.1A. Showing each of the clays used and calculation of the Fe concentration in each sample…...... 337
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Commonly used Abbreviations
Abbreviation Term
AFM Atomic Force Microscopy
ALI Annual Limit of Intake
CEC Cation Exchange Capacity
CRR Centre for Radiochemistry Research
DU Depleted Uranium
EBS Engineered Barrier System
ECB Engineered Clay Barrier
EPR Electronic Paramagnetic Resonance
EPSRC Engineering and Physical Sciences Research Council
EXAFS Extended X-ray Absorption Fine Structure
GC Gas Chromatography
GDF Geological Disposal Facility
HGW Heat Generating Waste
IC Ion Chromatography
ICP-AES Inductively Coupled Plasma – Atomic Emission Spectroscopy
ICP-MS Inductively Coupled Plasma – Mass Spectroscopy
ILW Intermediate Level Waste
IR Infrared Spectroscopy
KIT-INE Karlsruhe Institute of Technology – Institute for Nuclear Waste Disposal
LLW Low Level Waste
MBS Multi-Barrier System
NERC Natural Environment Research Council
RID Radiation Induced Defect
SEES School of Earth and Environmental Science
SEM Scanning Electron Microscopy
17
SF Spent Fuel
TEM Transmission Electron Microscopy
TGA Thermogravimetric Analysis
XANES X-ray Absorption Near Edge Structure
XAS X-ray Absorption Spectroscopy
XCT X-ray Computed Tomography
XPS X-ray Photoelectron Spectroscopy
XRD X-ray Diffraction
XRF X-ray Fluorescence
18
Abstract
Adam Peter Sims Investigating Effect of Clay Composition on Safety Function Performance in a Geological Disposal Facility (GDF) Doctor of Philosophy (PhD) The University Of Manchester, 2018
A legacy of radioactive waste has accumulated since the late 1940s and safe containment of long lived, highly radioactive waste is crucial for the future of nuclear power. A geological disposal facility (GDF) is the preferred method for the safe disposal of radioactive wastes; a multifaceted approach, using both engineered and natural barriers, to maximise the time between the breakdown of barriers and the final interaction with the environment and subsequently people.
Clay is likely form an integral part of the engineered barrier system (EBS) surrounding the waste canisters in many proposed GDFs for heat generating radioactive wastes. The clay selected for this purpose would need to have the necessary physical and chemical properties to protect the waste container against corrosion and also to limit the release of radionuclides from the waste after container failure.
Clays have a number of advantageous properties, such as high sorption capacity for radionuclides, small pore structure restricting microbial activity, and stability over geological time scales. Substitution of cations (Fe2+/3+, Mg2+, Al3+) into octahedral and tetrahedral (Al3+ and Si4+) sheets give a net negative charge on the clay layers giving interlayer spaces in-between; hydrated cations balance the negative charge within the interlayer space and cause the clay to swell filling surrounding gaps/cracks, avoiding advective flow, stabilizing the canister, and making diffusion the predominant transport mechanism within the barrier.
A number of challenges such as heat (from the high level wastes <130 °C), radiation (α/γ), waste package corrosion products (Fe/ Cu release), and ground water infiltration (ion exchange, swelling) would be present in the GDF environment and need to be examined with respect to the clay barrier material. Clay characterisation is fundamental to understanding the limitations of the barrier as each challenge is confronted. The coupled effects of heating (25-500 °C) and γ-irradiation on a number of montmorillonites and pseudo-alteration products (nontronites), heating (120 °C) and α-irradiation on a model montmorillonite, and the effect of γ-irradiation on U(VI) and Cr(VI) sorption on a model montmorillonite and a nontronite, were studied using a combination of methods including XRD, IR, XRF, XCT, XAS, zeta potential and EPR.
The results provided a positive outlook for the use of montmorillonite (bentonite) as an engineered clay barrier material. Montmorillonite was shown to mitigate potential detrimental effects whilst maintaining its bulk advantageous properties. Heating studies showed limited effects to the clay (>160 °C), with small changes being resisted further by divalent interlayer cations. γ-irradiation was shown to generate charge defects within the clay, increasing surface potential and activating redox properties (Fe); α-irradiation showed localised amorphisation of the clay structure with long range order maintained. Maximising the ability of the clay barrier to withstand the challenges expected in the GDF environment would allow for the strengthening of public opinion and a faster, smaller (footprint), cheaper and safer GDF for high level, heat generating, radioactive wastes to be produced.
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Declaration
That no portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning.
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Copyright Statement i. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii. The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and tables (“Reproductions”), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=24420), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.library.manchester.ac.uk/about/regulations/) and in The University’s policy on Presentation of Theses.
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Acknowledgements
First and foremost I would like to thank my academic supervisors Carolyn Pearce, Francis Livens, and Kath Morris. All of whom have offered key guidance and advice throughout the duration of my PhD, keeping me on the straight and narrow through some hard times, and showing continued patience with drafts. I would also like to thank Richard Pattrick and Simon Pimblott who gave advice at key stages too. I have really enjoyed the studies I have undertaken in my PhD and would like to acknowledge the Dalton Nuclear Institute for funding these and the University of Manchester (UoM) for the use of facilities. Further to this, I appreciate the chances I was given to travel and attend stimulating conferences at which I was able to present my research to like- minded peers and world leading experts.
I would like to thank Carolyn and Adrian Parker for helping organise research trips to Karlsruhe, Washington, and Indiana; allowing expansion of my research abroad and the opportunity to meet a number of extremely clever and kind people. A special thanks to Sarah, Cheryl, George and Gemma over these trips. The chance to work at a number of facilities in the UK such as DCF, Diamond and Newcastle University was appreciated and thank you to all that helped, especially Fred Mosselmans for dealing with ‘angry Adam’.
A number of other people have contributed to my PhD journey at the UoM. William Bower helped numerous times and should be thanked greatly. Technical staff should be thanked for the help given with the large number of analysis techniques used throughout my thesis. Nick Bryan and Nick Sheriff for pointing me down the academic path and all the members of Law, Natrajan and geomicrobiology group. Special thanks to Connaugh, Adam F, Rosie, Mark, Sophie, Vanessa and Nick for help with grammar and common sense over the years.
I would not have made it to the end of this journey without the support of family and friends. Therein, I would like to thank Jess, Dutton, Jamie, Gainey, Hannah, Elisa, Beth, and Rusty as well as Sierra Lima Crossfit and the UoM basketball team for keeping me in shape and giving an outlet for stress. My Family were continually encouraging so thank you Robert, Ian, Hannah, Mum and Dad, I hope this does you proud.
Lastly, Meg you have kept me healthy and been my rock through the ups and downs of the past few years, so for that you have my endless appreciation and love.
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The Author
The author graduated from the University of Manchester in July 2014 with a 1st class honours degree in Chemistry (MChem), which included a year of study in North America at the University of Illinois Urbana-Champaign. The author then joined the Centre for Radiochemistry Research (CRR) in the School of Chemistry at the University of Manchester and started the PhD presented here in September 2014.
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1. Introduction and Thesis content
1.1. Project Significance
An engineered geological disposal facility (GDF), using a multi-barrier system to encapsulate wastes safely and securely, is the UK’s chosen option for safe isolation and containment of heat generating radioactive wastes, such as high level waste (HLW) or spent nuclear fuel (SNF)(DECC, 2014). The engineered clay barrier (ECB) acts as a buffer and is a critical element of this system. Clay proposed GDFs incorporate the use of a multi-barrier system (Baldwin et al., 2008; Beattie and Williams, 2012) and are designed to isolate the waste from the surrounding environment and to contain the slowly decaying radioactivity for 105-106 years (Sellin and Leupin 2013). Clay is an integral component of many HLW GDFs worldwide, both as the engineered barrier and as a potential host rock for an engineered facility (Holton et al., 2012). The safety functions of the ECB are to: (i) fill the gaps in-between the host geology and canister, mitigating detrimental effects from the wastes, and protecting the waste container against corrosion by limiting groundwater flow and microbial activity; and (ii) limit the release of radionuclides after container failure. The clay selected for this purpose needs to have the necessary physical and chemical properties, which must be maintained in spite of challenges from heat, irradiation, corrosion products, and infiltration of groundwater, continuing to retard the release of radionuclides after canister decay (Figure 1.1). The long-term prediction of engineered barrier performance in response to the coupled effects of heat and radiation from radioactive waste is essential to a GDF safety case (Bernier et al., 2007, Holton et al., 2012).
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Waste Heat Geosphere package Groundwater Radiation
Waste package components Engineered Clay
Safety Functions: Protect the waste container (100s of years) Limit radionuclide transport (1000s of years)
Figure 1.1. The challenges faced after closure by an ECB in a GDF from stored HLW and the surrounding environment (Reprint, Pearce, 2012).
The clay currently selected for use as an ECB in the KBS3 concept is an aluminium phyllosilicate, bentonite (Sellin and Leupin, 2013). Bentonite has advantageous characteristics in relation to storage; low hydraulic conductivity, high swelling properties, high ion exchange, and small pore structure make diffusion the main form of transport from the waste canister (Hicks et al., 2009; Missana et al., 2008). Bentonites show a varied chemical composition depending on source and on the accessory minerals present, and these can impact greatly on the characteristics observed (Lee and Shackelford, 2005; Karnland, 2010). Bentonites used as geological disposal materials (Posiva, 2009) all predominantly contain montmorillonite (>75%), a dioctahedral smectite, with a weak net negative charge on the layers, giving rise to the interlayer spaces within the mineral that hold hydrated cations to balance charge (Brigatti et al., 2011); montmorillonite is the source of the majority of the advantageous properties observed and as such needs to be resistant to threats from heat and irradiation, with a common and duplicable composition. The experiments conducted in this project use a number of montmorillonites with varying compositions, as well as a couple of nontronites (higher Fe) as pseudo-alteration products. These materials are used to investigate possible changes in the ECB performance at a fundamental level.
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The initial challenges faced by the clay barrier in the GDF are those of heat and γ-irradiation generated from the encapsulated radioactive wastes (Chapter 4). Heat generating wastes have been postulated to cause maximum temperatures of ≈90 °C in the buffer material after 10-15 years (ANDRA, 2005), with heating studies using temperatures up to 150 °C (Honty et al., 2012, Wersin et al., 2007). If canisters were to be stored earlier and closer together, increasing the temperature the buffer material experiences but reducing the storage time of wastes at the surface as well as the footprint of the GDF, then this would reduce lifetime costs and allow for a smaller and cheaper GDF to be developed. The ionizing γ-irradiation given off from the wastes over the first 1000 years give estimated minimum doses of 5 MGy (SKB, 2006a). Alterations would manifest predominantly as changes to physicochemical properties through charge defects, partial reduction of Fe3+ in minerals through radiolysis product interaction, and increased colloid stability (Allard et al., 2012; Gournis et al., 2000; Holmboe et al., 2009; Pattrick et al., 2013). The coupled effects of both heating and γ-irradiation have been studied through the γ-irradiation of samples followed by a heating step causing the annealing of these charge defects, visualised through EPR (Sorieul et al., 2005). In this project the heating was conducted prior to γ-irradiation to examine the differences in the behaviour within the clay samples as a combined effect, giving a realistic view of the impacts that would be seen within the GDF environment during the initial duration of storage. Comparing this with previously reported data leads to more accurate predictions on the characteristics of the buffer material during the initial periods of storage in which small but measureable changes on a meso-scale are observed.
After an initial period of storage (1000 years for Fe-based canisters), the canister can no longer be assumed to be intact due to inevitable chemical and physical degradation processes. The secondary ECB function would evolve; challenges from elevated heat of ≈40-70 °C at canister surface (Yang and Yeh, 2009), but higher if waste was exposed, and radiation would be incurred in the form of α-particles from actinides (U, Np, Pu, Am) and fission products (I, Tc, Cs, Sr); and the release of radionuclides into the surrounding environment would need to be inhibited (Chapter 5 and Chapter 6). The radiation damage from α-irradiation come from α-particles (4–5.5 MeV) and the recoil of heavy actinide nuclei (70–100 keV) created as by-products (Farnan et al., 2007). This
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caused local amorphisation in smectites at low dose (dpa, displacements per atom) (Sorieul et al., 2008; Kitamura and Takase, 2016). A study on the coupled effects of heating (120 °C), increased above that of the storage temperature from the reduced shielding of the decaying waste (canister breach event), and α-irradiation would allow for the characterisation of changes within the clay buffer that would occur after canister decay. At the time of canister breach any contaminants released from the waste, both anionic and cationic, would need to be sequestered effectively (Greathouse and Cygan, 2006; Brookshaw et al., 2015). Clay would have already undergone γ-irradiation leading to a change in surface properties (Holmboe et al., 2009: Allard et al., 2012) whose effects on contaminants can usefully be explored under conditions relevant to a GDF. A broader understanding of alteration processes, and of the environment and surface chemistry within the clay, would allow for more realistic estimation of damage and behaviour, confirming the diminution, maintenance or enhancement of characteristics relevant to geological disposal.
A number of fundamental studies are strategically vital and timely, meeting the
Government demand for lower CO2 emissions by 2050 (DECC, 2011) and the growing problem of radioactive legacy waste (Wilson et al., 2011). The crucial knowledge gaps within the GDF safety case would be addressed and ultimately allow the accurate prediction of ECB evolution. The effects of heating and radiation, in the form of γ-irradiation (Chapter 4) and α-irradiation (Chapter 5), on clay performance are examined to simulate initial disposal conditions and those following a canister breach, as well as the concurrent release of radionuclides at this point (Chapter 6).
The aim of this research programme was to demonstrate that the ECB would continue to perform its safety functions regardless of challenges that are apparent in a GDF environment. Through a mechanistic understanding of these processes, a detailed and thorough safety case can be built that would allow the safe disposal of radioactive wastes in the UK and at sites worldwide.
1.2. Aims and Objectives
This project was focussed on understanding the impacts of heat and radiation (α,γ) on montmorillonite, the main component of bentonite, for future use as a buffer material in a GDF for heat generating radioactive wastes. It will provide information for the 27
safety case required for the Engineered Clay Barrier (ECB), looking to minimise any changes seen in the buffer material with respect to challenges from heat and radiation (α,γ), making sure that no adverse effects to the advantageous properties demonstrated under ambient conditions occur. The physical and chemical properties are examined through a number of mineralogical, geochemical, analytical, and spectroscopic techniques to allow a mechanistic understanding of these processes under simulated GDF conditions.
This will be achieved by:
(i) Developing a matrix of clay samples with different chemical compositions that have been subjected to elevated temperatures relevant to a GDF.
(ii) Finding a model clay sample, with minimal accessory minerals, to study the effects from α/γ-irradiation on the main component of the buffer material.
(iii) Finding a pseudo-alteration product to examine the effects of canister corrosion (Fe incorporation) on the buffer material with respect to challenges from α/γ-irradiation.
(iv) Interrogating the chemical, mechanical and physical characteristics on both a macro- and meso-scale before and after γ-irradiation.
(v) Analysing the bonding environment of structural (Fe) and interlayer (Ca/K) cations using synchrotron techniques (XAS), and probing surface chemistry changes after γ-irradiation (EPR, zeta potential, AFM)
(vi) Interrogating the chemical, mechanical and physical characteristics on both a macro- and meso-scale before and after α-irradiation.
(vii) Probing the effects of γ-irradiation on the sorption of cationic (U(VI)) and anionic (Cr(VI)) contaminants.
(viii) Analysing the bonding environment and oxidation state of uranium, in high and low carbonate loaded systems, through synchrotron techniques (XAS).
(ix) The assessment of the ability of the buffer material to protect the waste container and limit radionuclide migration under conditions relevant to a GDF.
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1.3. Thesis Structure
This thesis has been submitted in alternative format and comprises a general background and a section on the methodologies. Three research style papers (author contributions 1.4) are included, each of which have been prepared for publication, presented in report format with figures included in the text and supplementary sections following each paper for ease. The papers address the aims and objectives of the project, which are summarised in a final section that includes a direction for future work.
Chapter 2 gives a review of the effects of composition, radiation environment, and corrosion products on the performance of an Engineered Clay Barrier (ECB) with regards to a Geological Disposal Facility (GDF) for High level Waste (HLW). It introduces the concepts of the nuclear fuel cycle and describes waste classification and waste management. A review of clay mineralogy in relation to geological disposal was also included. Chapter 3 presents information on the source of the materials and methods used, together with a description of the key theoretical aspects of the measurement techniques.
The first research paper, ‘Molecular- to meso-scale effects of heat and gamma radiation on engineered clay barrier performance for radioactive waste disposal’, (in review in Applied Geochemistry) was presented in Chapter 4. This paper examined the initial stages of disposal and the first challenges that would be faced by the containment facilities (heat, γ-irradiation). The principal hypothesis tested was that the clay will withstand the twin challenges of heating (160 °C) and γ-irradiation (5 MGy) under GDF conditions, continuing to perform its safety function. To test this hypothesis a two part approach was taken. Initially the clays were subjected to a range of heat treatments; 90 °C (expected GDF temperature), 160 °C (upper reported temperature), 500 °C (de-hydroxylation of clays), and 1000 °C (destruction of clay structure) with samples measured after 2 days through XRD, IR, WLOI, EPR, and XRF. Secondly, a model montmorillonite and a pseudo-alteration product (nontronite) were homo- ionised (Un/ Na/ Mg/ K/ Ca) and heated to a range of temperatures (90, 160, 500 °C) with the effect of subsequent γ-irradiation (5 MGy) and interlayer cations studied using XRD, XAS, EPR, zeta potential, and AFM. Some other clays were also used in a
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comparison of irradiated (2.7 MGy) and unirradiated materials. It was found that there were minimal changes to the clay structure on a macro-scale, as a function of the coupled effects of temperature (up to 160 °C) and γ-irradiation (up to 5 MGy). However, γ-irradiation led to small, but measureable, changes in surface properties.
In Chapter 5, a research paper entitled ‘The coupled physicochemical effects of alpha irradiation and heating on clay barriers for radioactive waste disposal’ for publication in Journal of ACS Earth and Space Science was presented. This chapter focuses on the effects of canister breach in a heat generating waste GDF and the influence that heat and α-emitters, produced by actinide decay products in the wastes, will have on the engineered clay barrier. The hypothesis proposed was that α-irradiation would cause crack formation within the clays and alter the local chemical composition. The chemical and physical characterisation of the clay was examined initially using a model montmorillonite, STx-1b, through an X-ray tomography study of clay structure and pore distribution; measurements were taken prior to and after α-irradiation (5 MGy) on heat treated (120 °C) and unheated (25 °C) sample sets (natural/Mg/Na), using XRD and XAS to characterise chemical changes. Further changes were mapped within a set of natural clays after α-irradiation through XRD and XAS analysis, studying effects as a function of interlayer cation identity and average displacements per atom (dpa) delivered. Heating alone showed minimal changes to the clay structure but α-irradiation caused amorphisation in some areas of moderate dose (>0.15 dpa) with a collapse or loss of the interlayer (d001 basal spacing), a small increase in the pore density, and visible damage to the clay surface; this was not seen in all of the clays and the effects of α-irradiation were dependent on chemical composition as well as interlayer cation of the clays. Modelling (SRIM) showed the damage to be limited to the topmost 26 μm of the clay surface, but, if sorption-desorption cycles of α-emitting actinides occurs within a GDF, this localised damage could be propagated away from the containment area allowing potential migration paths for radionuclides.
Chapter 6, ‘The effect of γ-irradiation on the sorption capacity of a model montmorillonite and nontronite under simulated repository conditions with respect to U(VI) and Cr(VI) in solution’, for submission to the Journal of Environmental Radioactivity, extends the studies in Chapter 4 and Chapter 5. The paper examines the effect of prior γ-irradiation (5 MGy) on subsequent sorption experiments on smectite 30
clays, using examples of both a cationic species, U(VI), in solution relevant to radioactive waste disposal and a anionic species, Cr(VI), in solution found at contaminated land and some nuclear sites. The hypothesis tested was that the sorption of contaminants (anionic and cationic) will increase after γ-irradiation, under conditions relevant to geological disposal, due to the increased surface potential and Fe reduction within clay structures seen previously with γ-irradiation (Chapter 4). This was studied through a set of sorption experiments, using a model montmorillonite and 2+ 2- pseudo alteration product (nontronite), with UO2 and CrO4 aqueous species on both γ-irradiated (5 MGy) and natural clay samples. Sorption was measured through liquid phase analysis (ICP-MS) and changes in bonding environment and oxidation state changes on the solid in the U(IV) experiment were followed through XAS. Small increases in sorption were observed overall but these varied with the experimental conditions. The largest effects were seen in the anionic sorption experiment (Cr), whereas the sorption of uranyl showed limited changes and was heavily dependent on the carbonate content of the system. The interactions of both a positively charged 2+ 2- species (UO2 ) and negatively charge species (CrO4 ), after canister failure, in a GDF environment are relevant to safety performance since sorption-desorption cycles could have an large influence on the transport of radionuclides (Chapter 5).
Finally, in Chapter 7, a short summary of the research conducted, the conclusions drawn, and the implications for the safe disposal of radioactive wastes was presented. The future directions for further studies are examined and two on-going research projects are summarised.
Chapter 8 includes some additional information and unpublished notes as well as some background information on the author.
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1.4. Papers Submitted and Collaborators
Chapter 4:
Molecular- to meso-scale effects of heat and gamma radiation on engineered clay barrier performance for radioactive waste disposal
A. P. Sims, J. M. Devine, W. R. Bower, K. Morris, R. A. D. Pattrick, R. Edge, S. M. Pimblott, A. J. Fielding, T. Schäfer, G. K. Darbha, J. F. W. Mosselmans, F. R. Livens, C. I. Pearce – Submitted to Applied Geochemistry 29.03.2018 – In Review.
A. P. Sims – Principal author, concept development, experimental work, all experimental measurements and data analysis. J. M. Devine – helped with sample preparation, XRD and IR measurements. W. R. Bower – XAS collection. R. Edge & S. M. Pimblott – γ-irradiation and use of DCF facilities. A. J. Fielding – helped with EPR analysis and review. T. Schäfer – zeta potential measurements and review G. K. Darbha - AFM measurements, analysis, and review. J. F. W. Mosselmans – helped with XAS collection and interpretation. K. Morris, R.A.D. Pattrick, and F.R. Livens – extensive manuscript review and development of concept. C. I. Pearce – initial concept, extensive manuscript review.
Chapter 5:
The coupled physicochemical effects of alpha irradiation and heating on clay barriers for radioactive waste disposal
A. P. Sims, W. R. Bower, K. Morris, R. A. D. Pattrick, A.D. Smith, J. Behnsen, M. Miller, J. F. W. Mosselmans, F. R. Livens, C. I. Pearce - In preparation for submission to ACS Earth and Space Science
A. P. Sims – Principal Author, initial concept and development, experimental work and measurements, and data analysis. W. R. Bower – XAS collection, assistance with α-irradiation and dpa calculation review. A.D. Smith – assistance with α-irradiation and dpa calculation review. J. Behnsen and M. Miller – help with XCT collection and analysis. J. F. W. Mosselmans –XAS collection and analysis. K. Morris, R. A. D. Pattrick, 32
and F. R. Livens - manuscript review and concept development. C. I. Pearce – assistance with some experimental, concept development, and extensive manuscript review.
Chapter 6:
The effect of γ-irradiation on the sorption capacity of a model montmorillonite and nontronite under simulated repository conditions with respect to U(VI) and Cr(VI) in solution (research article)
A. P. Sims, K. Morris, J. F. W. Mosselmans, S. Hayama, F. R. Livens, C. I. Pearce – In preparation for submission to the Journal of Environmental Radioactivity.
A. P. Sims – Principal author, initial concept, experimental work and data analysis. J. F. W. Mosselmans and S. Hayama – assistance with XAS collection and analysis. F. R. Livens, K. Morris, and C. I. Pearce – conceptual guidance and extensive manuscript review.
Microscopic X-ray imaging techniques applied to mineral systems and catalyst particles
J. F. W. Mosselmans, C. I. Pearce, W. R. Bower, R. A. D. Pattrick, S. W. T. Price, A. M. Beale, A. P. Sims, L. Barrio. The Clay Mineral Society Workshop Lectures Series (2016) 21, 6, 65-77.
Co-Author: assisted with XRD analysis and presentation in relation to α-irradiation on a smectite clay (Appendix 8.2).
1.5. Conferences, external work, and user facilities visited
1.5.1. Conferences (presentations and posters) Actinide XAS, 2017. COGER, 2016. Frontiers in Environmental Radioactivity, 2016. RSC Radiochemistry 50th Anniversary meeting, 2016 RSC Radiochemistry group meeting, 2015
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BELBaR Winter School, 2015 Euroclay, 2015. Environmental mineralogy group & Geo-microbiology Network, 2015 RSC Radiochemistry Group Young Researcher Meeting, 2015 Postgraduate research showcase UoM, 2015
1.5.2. External work
1.5.2.1. Research Fellow at Pacific Northwest National Lab (PNNL), 2016 Conducted a three month placement at PNNL as an alternate sponsored fellow in the geosciences group looking at sorption and desorption kinetics of iodine (ICP-MS) in relation to various ‘getters’ and assisting with the analysis for μXRD samples run in lab on site. A basic experiment for U(VI) and Cr(VI) sorption (ICP-MS) was completed as proof of concept for a larger scale investigation at the University of Manchester. A scientific collaboration was initiated with novel ideas on the processing of tomography data presented in Chapter 5.
1.5.2.2. Visiting Researcher at University of Notre Dame, 2017 A two month research project under Jay LaVerne in the Radiation Laboratory at the University of Notre Dame looking at the effect of γ-irradiation on hydrogen production in clays in relation to the chemical composition and surface chemistry using GC, BET and XPS, starting a collaboration on surface chemistry effects of γ-irradiation with respect to geological disposal of radioactive wastes.
1.5.3. User Facilities
1.5.3.1. Diamond Light Source
I18 January, 2015: Sequential radiation effects on phyllosilicate minerals in a GDF environment. The effects of γ-irradiation were examined in clays using μXRD and μXAS.
July, 2015: Influence of radiation damage on the sorption capacity and redox reactivity of phyllosilicate minerals in a GDF environment. The effects of α-irradiation were examined in clays using μXRD and μXAS.
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October, 2017: Impact of canister corrosion products on the structure and chemical composition if the engineered clay barrier. Investigated the impact of accelerated corrosion conditions, on a model montmorillonite, through μXRD and μXAS.
I20 December, 2017: XAS Analysis of U, Np and Pu Samples in Engineered and Natural Environments: Exploitation of I20 Capabilities. The oxidation state and bonding environment of uranium in un- and γ-irradiated clays was examined through μXAS.
1.5.3.2. Manchester X-ray Imaging Facility (MXIF) April-June, 2015: XCT on samples before and after α-irradiation
October, 2016-2017: XCT on long-term Fe corrosion samples.
1.5.3.3. EPSRC National Service for Electron Paramagnetic Resonance Spectroscopy August, 2015: EPR on γ-irradiated samples to examine the effect and extent of radiation induced charge defects in clays.
1.5.3.4. Dalton Cumbria Facility (DCF) June, 2015: α-irradiation of clays at DCF using the 5 MV tandem pelletron ion accelerator. Various uses of 60Co γ-irradiator over the duration of the PhD.
1.5.3.5. Advanced Photon Source, 20-IDBC July, 2017: Source Term Chemistry and Biological Impacts on Technetium Speciation and Transport in the Subsurface. Mapped for active Tc(VII) particles in Hanford site sediments.
1.6. References
Allard, T., Balan, E., Calas, G., Fourdrin, C., Morichon, E. and Sorieul, S. (2012) Radiation-induced defects in clay minerals: A review. Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms 277, 112- 120.
ANDRA (2005) Andra Research on the Geological Disposal of High-Level Long-Lived Radioactive Waste: Results and Perspectives. Andra, France, 1-40.
Baldwin, T. Chapman, N. Neall, F. (2008) Geological Disposal Options for High-Level Waste and Spent Fuel: Report for the UK Nuclear Decommissioning Authority. NDA.
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Beattie, T.M. and Williams, S.J. (2012) An overview of near-field evolution research in support of the UK geological disposal programme. Mineralogical Magazine 76, 2995- 3001.
Bernier, F., Li, X.L., Bastiaens, W., Ortiz, L., Van Geet, M., Wouters, L., Frieg, B., Blümling, P., Desrues, J., Viaggiani, G., Coll, C., Chanchole, S., De Greef, V., Hamza, R., Malinsky, L., Vervoort, A., Vanbrabant, Y., Debecker, B., Verstraelen, J., Govaerts, A., Wevers, M., Labiouse, V., Escoffoer, S., Mathier, J. F., Gastaldo, L., Bühler, C. (2007) Fractures and Self-healing within the Excavation Disturbed Zone in Clays (SELFRAC), Nuclear Science and Technology. European Commission.
Brigatti, M.F., Malferrari, D., Laurora, A. and Elmi, C. (2011) Structure and mineralogy of layer silicates: recent perspectives and new trends. Layered Mineral Structures and Their Application in Advanced Technologies 11, 1-71.
Brookshaw, D.R., Pattrick, R.A.D., Bots, P., Law, G.T.W., Lloyd, J.R., Mosselmans, J.F.W., Vaughan, D.J., Dardenne, K. and Morris, K. (2015) Redox Interactions of Tc(VII), U(VI), and Np(V) with Microbially Reduced Biotite and Chlorite. Environmental Science & Technology 49, 13139-13148.
Department of Energy & Climate Change (DECC). (2011) The Carbon Plan: Delivering our low carbon future. UK Government, London, 1-220.
Department of Energy & Climate Change (DECC). (2014) Implementing Geological Disposal, A Framework for the long-term management of higher activity radioactive waste. UK Government, London.
Farnan, I., Cho, H., and Weber, W.J. (2007) Quantification of actinide α-radiation damage in minerals and ceramics. Nature 445, 190.
Gournis, D., Mantaka-Marketou, A.E., Karakassides, M.A. and Petridis, D. (2000) Effect of gamma-irradiation on clays and organoclays: a Mössbauer and XRD study. Physics and Chemistry of Minerals 27, 514-521.
Greathouse, J.A. and Cygan, R.T. (2006) Water Structure and Aqueous Uranyl(VI) Adsorption Equilibria onto External Surfaces of Beidellite, Montmorillonite, and Pyrophyllite: Results from Molecular Simulations. Environmental Science & Technology 40, 3865-3871.
Hicks, T.W., White, M.J., Hooker, P.J. (2009) Role of Bentonite in Determination of Thermal Limits on Geological Disposal Facility Design. NDA.
Holmboe, M., Wold, S., Jonsson, M. and Garcia-Garcia, S. (2009) Effects of gamma- irradiation on the stability of colloidal Na+-Montmorillonite dispersions. Applied Clay Science 43, 86-90.
Holton, D., Dickinson, M., Hoch, A., Cowper, M., Thetford, R., Allinson, H., Crockett, G., Cairns, M., Roberts, D., Padovani, C., Johnson, M., Carr, N., Jowett, J., Finch, C., Walsh, C., Southgate, B., Wood, P., Young, A. (2012) Project Ankhiale: Disposability and full life cycle implications of high-heat generating UK wastes. NDA, 1-291.
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Honty, M., Wang, L., Osacky, M., Uhlik, P., Czimerova, A. and Madejová, J. (2012) Experimental interactions of the Opalinus Clay and Boom Clay with various repository relevant solutions at 90 degrees C under closed conditions. Applied Clay Science 59-60, 50-63.
Karnland, O. (2010) Chemical and mineralogy characterization of bentonite procedure in KBS-3 repository. SKB.
Kitamura, A. and Takase, H. (2016) Effects of α-radiation on a direct disposal system for spent nuclear fuel – (1) review of research into the effects of α-radiation on the spent nuclear fuel, canisters and outside canisters. Journal of Nuclear Science and Technology 53, 1-18.
Lee, J.-M. and Shackelford, C. (2005) Impact of Bentonite Quality on Hydraulic Conductivity of Geosynthetic Clay Liners.
Missana, T., Alonso, U., Garcia-Gutierrez, M. and Mingarro, M. (2008) Role of bentonite colloids on europium and plutonium migration in a granite fracture. Applied Geochemistry 23, 1484-1497.
Pattrick, R.A.D., Charnock, J.M., Geraki, T., Mosselmans, J.F.W., Pearce, C.I., Pimblott, S. and Droop, G.T.R. (2013) Alpha particle damage in biotite characterized by microfocus X-ray diffraction and Fe K-edge X-ray absorption spectroscopy. Mineralogical Magazine 77, 2867-2882.
Posiva Oy. (2009) Nuclear Waste Management at Olkiluoto and Loviisa Power Plants Review of Current Status and Future Plans. Posiva Oy.
Sellin, P. and Leupin, O.X. (2013) The use of clays as an engineered barrier in radioactive-waste management – a review. Clays and Clay Minerals 61, 477-498.
SKB (2006) Fuel and canister process report for the safety assessment SR-Can. 2006a. SKB, Sweden.
Sorieul, S., Allard, T., Morin, G., Boizot, B. and Calas, G. (2005) Native and artificial radiation-induced defects in montmorillonite. An EPR study. Physics and Chemistry of Minerals 32, 1-7.
Sorieul, S., Allard, T., Wang, L.M., Grambin-Lapeyre, C., Lian, J., Calas, G. and Ewing, R.C. (2008) Radiation-Stability of Smectite. Environmental Science & Technology 42, 8407-8411.
Wersin, P., Johnson, L.H. and McKinley, I.G. (2007) Performance of the bentonite barrier at temperatures beyond 100 degrees C: A critical review. Physics and Chemistry of the Earth 32, 780-788.
Wilson, J., Bond, A.E., Savage, S., Pusch. R., Bennett. D.G. (2011) Bentonite: A Review of key properties, processes and issues for consideration in the UK context. NDA.
Yang, S.Y. and Yeh, H.D. (2009) Modeling transient heat transfer in nuclear waste repositories. Journal of Hazardous Materials 169, 108-112.
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2. Effects of Composition, Radiation Environment, and Corrosion Products on Performance of an Engineered Clay Barrier This section contains an introduction to the nuclear industry, clay mineralogy and to the concepts of geological disposal and the engineered clay barrier. More detailed introductions are included in each research chapter (4, 5, and 6).
2.1. The Nuclear Waste Story
The nuclear industry is expanding in the UK with outlooks internationally depending on regional trends and political policy (IAEA, 2017). Already used in 30 countries, it accounts for approximately 10-15% of electricity generation worldwide (DECC, 2016; DECC, 2014; Curtis and Morris, 2012), and is viewed as a ‘newer’ and sustainable form of cleaner energy low greenhouse gas emissions. However, public perception of the use of nuclear, in any form, has connotations from past endeavours that saw the production of nuclear weapons and fuel with scant regard towards the environment and safe storage of the waste produced (Grimston, 2008).
Nuclear power plants across the UK (and worldwide) vary with the fuel used and coolant system in place. Magnox reactors were used predominantly in early stages of development in the UK using un-enriched uranium. Further development and research lead to the implementation of advanced gas cooled reactors (AGRs) and light water reactors (LWR) including pressurized water reactors (PWRs) that are used principally worldwide. The use of enriched uranium oxide activates the fuel, producing more fission reactions, creating a higher working temperature and hence outputting far greater power per unit volume than previously. A number of reactor designs and concepts are used, with differences in fuels and coolant systems. Due to the high variability in design the repair and treatment of fuels has been a large problem, and many of the older reactors have already been decommissioned or in the process of being decommissioned (DTI, 2007). However, a new nuclear build has been planned in the UK (DECC, 2016), giving modularised, standardised, and reproducible reactors from privately funded companies at old nuclear sites across the country; Hinkley Point (Somerset) and Sizewell (Suffolk) are two examples of current projects underway, run by EDF, to build a number of European pressurised water reactors (EDF, 2013). Other privately owned companies have planned builds such as Horizon at Wylfa Newydd and
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Oldbury and NuGeneration at Moorside, implementing a number of advanced boiling water reactors or APR1000/APR1400 respectively (WNA, 2018).
The UK government believes nuclear power stations are a key low carbon power source; with target of a 60% cut of CO2 emissions by 2050 (DECC, 2011). There are many advantages to the generation of electricity from nuclear sources, namely the huge energy density, for example one uranium pellet (≈20 g) contains as much energy as ≈800 kg of coal or ≈680 L of oil (NEI, 2018). The foremost drawback for the nuclear sector are the high level (heat generating) and intermediate level radioactive wastes, produced by reactors, in research and used medically, which need to be disposed safely, isolating harmful by-products of nuclear endeavours from the environment and preventing potential damage to humans (Wilson et al., 2011; DECC, 2014; BERR, 2008; Ewing et al., 1995; Svenke and Nilsson, 1983).
The necessity for power generation from the nuclear sector will become apparent as the growing need for energy is predicted to rise, with energy production frocasted to increase by 56% by 2040 (EIA, 2013). As fossil fuel stocks dwindle a new, secure, high output and low carbon alternative energy supply will need to supplement the increasing use of renewable energy sources and reduced use of coal, oil and gas that are economically favoured at present (DECC, 2014; BERR, 2008).
2.1.1. The Nuclear Fuel Cycle The discovery of nuclear fission in 1938, followed by atomic weapon production (1940s) and then energy generation (from heat production), was coordinated by the Atomic Energy Research Establishment in the UK from 1946 (Bayliss and Langley, 2003). The United Kingdom Atomic Energy Authority (UKAEA) was formed in 1954 to run the nuclear energy program, later splitting into British Nuclear fuels Ltd and the UKAEA through the demerger of its production division (Bayliss and Langley, 2003). The nuclear fuel cycle (Figure 2.1) has been overseen by the International Atomic Energy Agency (IAEA) ever since the United Nations formed it in 1957 to ensure peaceful and safe use of nuclear processes internationally. The nuclear fuel cycle comprises a number of interrelated activities that together make up a complete route. These activities are variable depending country of origin. Some countries such as USA, Canada, and Sweden use the uranium once-through option whereby spent fuel is
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stored after use prior to disposal; others such as China, France, Russia and the UK (stopping in 2020) use reprocessing as part of their strategy for spent fuel management trying to maximise energy from fissile and fertile atoms before the need for disposal (Choppin et al., 2013).
The nuclear fuel cycle is made up of a number of steps. Mining and milling of uranium ore, most commonly uraninite (UO2) with variable portions of triuranium octoxide
(U3O8) interspersed. Refining and concentrating ore through conversion to uranium 19 hexafluoride (UF6), a toxic volatile form, used due to the mono-isotopic nature of F (only dependent of the U isotope). Uranium enrichment where 235U concentration is enhanced, typically to 3–5 atom %. Fuel fabrication where natural and enriched UF6 is converted to UO2, uranium metal alloys, or mixed oxide fuel (U + Pu oxides) for use in reactors. Use in reactor operation to generate heat and therefore electricity, or for use in research. Spent fuel storage where wastes are stored (semi-permanently) in reinforced containers to allow heat dissipation at facilities above ground (i.e. Sellafield, UK) or reprocessing where uranium and plutonium are extracted and re-enter the cycle (depending on cycle type). Final decommissioning of nuclear facilities and safe long term disposal of radioactive wastes at geological disposal facilities (NEA, 2005; Choppin et al., 2013; Sharrad et al., 2011; Morris et al., 2011; Degueldre et al., 2010).
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Figure 2.1. The Nuclear Fuel cycle (Reprint USNRC, 2017).
The Department for Business, Energy and Industrial Strategy (BEIS) oversees nuclear growth and decommissioning. Privately and government funded companies such as EDF, Sellafield, or NNL run the current nuclear sites and are involved in the implementation of new nuclear sites (Sharrad et al., 2011; EDF, 2013). The Nuclear Decommissioning Authority (NDA) is responsible for the safe dismantling and remediation of old nuclear sites contracting site licence companies (SLCs) to carry out the work (Kimber et al., 2011). All these compartments of the nuclear sector are regulated by the Office for Nuclear Regulation (ONR, 2016) as well as the Environment Agency (EA) in England, Scottish Environment Protection Agency (SEPA) in Scotland, and Natural Resources Wales (NRW) in Wales, summarised in Figure 2.2. Nuclear power generates 25% of the United Kingdom’s electricity (DECC, 2016) and as of 2018 there are 15 nuclear power reactors operational in the UK (7 sites, 14 AGRs and 1 PWR) and 17 undergoing decommissioning (BEIS, 2018), with current nuclear new build facilities being deployed at old nuclear sites as well as a number of ‘new build’ nuclear sites in the early planning stages (EDF, 2013).
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Figure 2.2. Sites of existing and proposed nuclear power stations, adapt from DECC (2012). Most operation sites (red) have 2 AGRs onsite; Sizewell (light red) has 1 PWR. *Reprocessing and HLW storage.
Although there are a number of sites in the UK where nuclear operations have occurred, the majority of the legacy waste and contamination is located at a few principal facilities, such as Sellafield, where the greatest contamination concerns arise and a plan for the long term storage of wastes needs to be implemented (Kimber et al., 2011; Banford and Jarvis, 2011; Morris et al., 2011)
2.1.2. UK Nuclear Legacy The clean-up of the UK’s nuclear legacy sites is managed by the Nuclear Decommissioning Authority (NDA), established through the Energy Act 2004, and beginning operations in 2005 (Kimber et al., 2011). The UK has accumulated a legacy of higher activity radioactive waste (including yet to be declared wastes at Pu and U stockpiles), estimated at 650,000 m3 (DECC, 2014), from weapons production, energy production, reprocessing of spent fuel, commercial uses in research activities, refining 42
of uranium ores, and defence wastes as by-products from the processing of Pu and tritium used in nuclear weapons, summarised in Figure 2.3 below (Donald et al., 1997; NDA, 2014; Sharrad et al., 2011).
Figure 2.3. Contribution to radioactive waste stock pile from current endeavours; reproduced from the 2013 radioactive waste inventory (NDA, 2014). Note: large amounts of legacy wastes included in spent fuel reprocessing section. Unclassified wastes not included (depleted uranium and Pu stores).
There have been a number of nuclear contamination events, some from inadequate planning of waste management at nuclear facilities like Hanford, Oak Ridge, and Windscale which is now Sellafield (Kimber et al., 2011), as well as some large scale accidents, such as Three Mile Island in 1979, Chernobyl in 1986, and Fukushima in 2011 (Donald et al., 1997; Sharrad et al., 2011; WNA, 2018). Treatments such as bioremediation, electrokinetic remediation, chemical redox reactions, or sediment washing have been implemented at contaminated sites to try to reduce, reverse, and inhibit the effect of nuclear contamination events but this also causes an increase in wastes for disposal (Kimber et al., 2011; Qafoku et al., 2015; Riley and Zachara, 1992;
Williamson et al., 2014).
At present 96% (4,700,000 m3) of all types of radioactive wastes (including VLLW and LLW) forecast already exist either in need of processing at decommissioned sites or at sites close to closure. The other planned 4% (170,000 m3) is still to be produced (BEIS, 2018). In addition, the UK has materials which may be declared wastes in the future such as separated plutonium, uranium (depleted, natural and reprocessed) and spent fuel prior to reprocessing (Morris et al., 2011; BEIS, 2016).
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2.1.3. Current Radioactive waste Storage Radioactive wastes are categorised into 4 main types of waste dependent on radioactivity content (Morris et al., 2011). Very low level Wastes (VLLW) are a sub- category of low level wastes (LLW) only very lightly contaminated with radioactivity, with single items containing less than 40 kBq of β/γ activity (or each 0.1 cubic metre of material containing less than 400 kBq), comprising mainly of small amounts of wastes from universities and hospitals as well as large amounts of structural building materials from nuclear sites. These can be disposed of at permitted landfill sites. Low Level Wastes (LLW) have a radioactive content below 4 GBq per tonne of α activity or 12 GBq per tonne of β/γ activity; these comprise of contaminated materials from the running of nuclear sites or from decommissioning activities at these sites. The majority of these waste materials are decontaminated or processed before disposal to reduce the capacity required at the current Low Level Waste Repository (LLWR) in Cumbria or at engineered sites near nuclear locations (Dounreay), but some are stored above ground for future disposal due to concentration of specific radionuclides present (237Np, 239Pu and 241Am). Intermediate Level Wastes (ILW) exceed the upper threshold for LLW, but do not require heat to be taken into account in their management; these arise from reprocessing of spent nuclear fuel, fuel cladding materials, or treatments of radioactive effluents. There is no existing disposal route for ILW but they are often treated in a solid form, conditioned in a cement based material, and stored in stainless steel containers to be disposed separately. High level wastes (HLW) are comprised of wastes that are sufficiently radioactive so the decay heat significantly increases waste temperature and the temperature of its surroundings (thermal power above about 2 kW m-3), requiring heat to be accounted for in disposal; these wastes arise from spent nuclear fuel and highly radioactive liquids from reprocessing. The aqueous HLWs are converted to stable solid forms through the vitrification of wastes into borosilicate glasses and then stored above ground to allow for heat dissipation prior to final disposal (BEIS, 2016; DECC, 2014; DEFRA, 2002; NDA, 2015; DEFRA, 2008; Morris et al., 2011; Ewing, 1999; Ewing, 2001).
In the UK, the radioactive waste is currently stored at the Earth’s surface (Figure 2.4). High level wastes (HLW) and spent nuclear fuel (SNF) currently comprise only 0.03% by volume of the wastes, but these hold 84% of the total radioactivity within them (BEIS,
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2016). The ILW comprises a higher percentage volume of the wastes (10.35%), but only a fraction of the radioactivity (16%). LLW and VLLW make up the bulk of the waste volume (89.62%), but with <1% of the total radioactivity (DECC, 2016; Morris et al., 2011; BEIS, 2016; Weber et al., 1997).
Total wastes: 4.77 million m3
Figure 2.4. Reproduction of information from the 2016 waste Inventory on percentage volume (m3) and radioactivity of different types of radioactive wastes that have been collected and are currently stored or that have been previously disposed (BEIS, 2016).
A number of disposal methods for indefinite disposal that have been considered: disposal at sea, disposal in outer space, deep borehole disposal and near surface storage (NDA, 2010; Baldwin et al., 2008). However, a geological disposal facility (GDF) has been chosen because after final shutdown and closure it would require no further human interaction minimising chances of unwanted releases (DECC, 2014), it is cost effective angaist some of the other opitions (space), and fits with current regualtionsn for disposal (London Protocol, 1972). The permanent and safe containment of long lived, highly radioactive waste is crucial for the future development of the nuclear industry (BERR, 2008; Bernier et al., 2007; Sellin and Leupin, 2013; BEIS, 2016).
2.1.4. Geological Disposal In many countries, geological disposal has been chosen as the preferred method for the long term management and disposal of higher activity, heat generating, radioactive waste (DECC, 2014). The overall aim of a GDF is to maximise the time between the breakdown of the barriers and the final interaction with people and the environment with a timescale of 1000s to millions of years (Sellin and Leupin, 2013; Holton et al.,
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2012). The current UK plan relies mainly on interim storage at Sellafield to allow wastes to cool over time and requires the implementation of a disposal facility before 2075 involving the re-packaging of legacy wastes (NDA, 2010; BERR, 2008; Holton et al., 2012; Bayliss and Langley, 2003).
Work on geological disposal facilities has been fully implemented in USA for HLW (Waste Isolation Pilot Plant), Finland (Onkalo) for spent nuclear fuel, Sweden (Forsmark) for ILW and Norway for NORM wastes; many other countries such as Canada, Sweden and France are close to site approval for geological disposal. Currently the UK is in the planning stages and in search of a suitable site (NDA, 2010; DECC, 2014; Sellin and Leupin, 2013). Construction of such a facility involves the emplacement of radioactive wastes (HLW and ILW separately) 200–1000 m underground in an isolated repository (Figure 2.5), where a stable geological setting provides long-term confinement of the wastes and comprehensively inhibits radionuclide migration (Morris et al. 2011; Ojovan and Lee, 2014).
ILW storage
HLW storage
Figure 2.5. Reprint of the possible design for a GDF in the UK (NDA, 2015).
Many proposed geological disposal facilities (GDFs) incorporate a multi-barrier system, using both engineered and natural barriers, to maximise the time between
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containment failure and the final interaction of released radionuclides with the biosphere (Baldwin et al., 2008; Svenke and Nilsson, 1983; NDA, 2010; Beattie and Williams, 2012). These barriers can be altered depending on host rock of the location, waste type (HLW or ILW), or design of the GDF, but all include (Figure 2.6) the waste form as a solid, the waste container, a metal over pack (HLW), and buffer and backfill materials which vary with design, waste type, and the host geology (Baldwin et al., 2008; Combarieu et al., 2011; ANDRA, 2005; Morris et al., 2011; Sellin and Leupin, 2013).
Figure 2.6. Reprint of a basic schematic of the multiple barrier system (Baldwin et al., 2008).
2.2.4.1. Waste and waste container The immobilisation of wastes for disposal through the conversion of liquids and sludge into solid waste forms varies with the waste type (Hicks et al., 2008; Holton et al., 2012); repository bound LLW and ILW are retained in cement, whereas HLW is vitrified into glasses (borosilicate glass but other materials being considered) or disposed directly as spent fuel confining the radionuclides to the structures (Bennett et al., 2001, DECC, 2014: Donald et al., 1997). These wastes are then placed in alloy containers (stainless steel - UK, copper - SKB, steel - Nagra) to form a durable waste package. Standardisation of design concpets, to simplify the disposal process internationally should be implamented, allowing consistent execution across multiple sites (Baldwin et al., 2008; NDA, 2010; Holton et al., 2012; Svenke and Nilsson, 1983; Nagra, 2002).
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2.2.4.2. Metal over-pack A metal-over pack is incorporated into the HLW design to reduce heat generation from the wastes through increased shielding (more distance for heat to dissipate through), mitigating the potential for enhanced dissolution under oxidising conditions, to minimising corrosion that may lead to alterations of the GDF environment, and to further the confidence in the safety case built from possible breaches of internal containment. The over-pack is made up from alloys of resistant metals such as copper, titanium, or stainless steel (Baldwin et al., 2008; NDA 2010).
2.2.4.3. Buffer, backfill and host geology The buffer and backfill materials vary depending on the waste type and environment of the GDF; both are linked and are ordinarily made up from the same material (Pusch, 2002). For LLW and ILW a cementitious buffer material is used to control the system chemistry through an increased pH (>11). Under these conditions the buffer decreases canister corrosion, allows gas escape, makes radionuclides less soluble after release, and provides a highly absorbing material for contaminants (NDA, 2010). In HLW disposal, clay (bentonite) based buffer materials are often used with a lower pH (8-9) and a high content (75%) of montmorillonite (Wilson et al., 2011; Posiva, 2009). Crushed rock (salt) has also been suggested in a dry repository concepts as well as cement in concepts that use larger containment vessels (Holton et al., 2012; ONDRAF/NIRAS et al., 2015). These barriers need to provide a stable zone of chemical and mechanical protection around the disposal canisters, limiting the inward transport of corrosive substances (groundwater and microbes) and eventual outward movement of radionuclides (SKB, 1983; Hansen et al., 2012; Savage and Authur, 2012).
A stable host geology would function as a natural barrier to decrease the effects of canister corrosion through low groundwater flow and reduced radionuclide migration after canister failure due to the occurrence of advantageous minerals or surfaces that sorb the contaminants (Morris et al., 2011; Holton et al., 2012). Some designs remove the buffer materials completely using advantageous host geology solely as a final barrier around the waste canisters (Coubarieu et al., 2011; ANDRA, 2005).
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2.2.4.5. Previous and Current Engineered barrier systems: Table 2.1: Showing a summary of planned GDF designs. Note: 100 °C used as baseline temperature expected (Holton et al., 2012; ONDRAF/NIRAS et al., 2015; IAEA, 2009; NEA, 2003).
Country Belgium Canada Germany France Finland Sweden Switzerland UK USA Body ONDRAF/NIRAS NWMO DBE ANDRA POSIVA SKB NAGRA NDA/RWMD DOE Waste HLW (SNF) HLW (SNF) HLW (SNF) HLW HLW (SNF) HLW (SNF) HLW (SNF) HLW (SNF) HLW (SNF) Canister Supercontainer - Carbon steel Standardised Carbon steel Standardised Standardised Standardised standardised stainless steel two waste inner carbon steel disposal canister, canister, carbon steel container canister with containers container canister canisters copper over copper over canister stainless an alloy 22 containing 4 spent with a pack, cast pack, cast steel inside a nickel-based fuel assemblies copper iron insert iron insert carbon steel corrosion inside a carbon outer shell. over-pack/ over-pack and steel over-pack, copper titanium drip surrounded by shields concrete inside a thin stainless steel envelope Buffer Cement Bentonite Crushed rock Clay Bentonite Bentonite Crushed rock Bentonite N/A (clay mix) (salt) (clay) Max temp 100 100 200 100 100 100 125 100 <96 in tunnel on buffer (>200 at (°C) surface) Host Lower strength Higher Evaporite Lower Higher Higher Lower Higher Volcanic rock geology sedimentary rock strength strength strength strength strength strength (Igneous) (Clay) sedimentary sedimentary sedimentary sedimentary sedimentary sedimentary rock rock (Clay) rock rock rock (Clay) rock (Granite) (Granite) (Granite) (Granite) N.B. Some concpets depend on the host geology of the siting (i.e. UK) and some countries have assessed multiple concpets (i.e. Germany – clay host rock too). 49
2.2. Engineered Clay Barrier
The use of compacted swelling clays, commonly bentonite, as an engineered buffer material is favoured in a number of proposed HLW disposal designs at GDFs, as shown above in Table 2.1 (Delage et al., 2010; Tournassat et al., 2015). The heat-generating radioactive waste canisters would be surrounded by a geo-engineered clay barrier; favourable stabilised conditions occur through the swelling of the clay matrix after GDF closure (groundwater infiltration) filling the gaps and cracks present at the buffer/host geology interface, avoiding advection, and making the favoured slow diffusion of contaminants after canister failure the predominant mechanism for transport (Wilson et al., 2011; Holton et al., 2012; Borisover and Davis, 2015: Savage and Authur, 2012).
The clay selected for this purpose would need to have the necessary physical and chemical properties to allow predictable and favourable conditions to be maintained around the wastes canister; protecting the waste canister against corrosion through a restriction of microbial activity due to a small pore structure, limiting the release of radionuclides after container failure from a low hydraulic conductivity, and the high swelling capacity of the clay (self-sealing). Challenges from heat, irradiation (α, β, γ), corrosion of the waste canister, and infiltration of groundwater would need to be faced at various stages of disposal both as individual threats and as coupled processes (Wilson et al., 2011). The Long-term prediction of ECBs ability to fulfil the safety functions in spite of the challenges presented by the evolving GDF environment are essential to a required safety case (Bernier et al., 2007; Holton et al., 2012; Morris et al., 2011; Tournassat et al., 2015).
2.2.1. Clay Minerals A number of descriptions for clays and clay minerals are widely used and accepted in the literature with no uniform standard description (Bergaya and Lagaly, 2013). Guggenheim and Martin (1995) described clays as ‘a naturally occurring material composed of primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden with dried or fired’ and Moore and Reynolds (1997) described clay minerals as hydrous aluminium silicates, classified as phyllosilicates or layer silicates, with a common morphology comprising of a perfect (001) cleavage plane and a small size fraction (<2 μm).
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Clays are characterised structurally by the ordering of the chemical units that make up the layers; tetrahedral layers (T) are ordinarily comprised of three corner sharing SiO4 tetrahedra linking in a hexagonal pattern in two dimensions, whereas octahedral layers
(O) consist of edge sharing AlO6 forming hexagonal or pseudo hexagonal symmetry in sheets variable with changeable cis/trans orientation of OH groups (Brigatti et al. 2013; Moore and Reynolds, 1997). The common tetrahedral and octahedral ions are Si4+, Al3+, or Fe3+ and Al3+, Fe3+, Mg2+, or Fe2+ respectively, with the substitution of cations into the structure causing the build-up of a negative charge on the clay layers balanced through interaction with cations (e.g. Ca2+, K+) in a interlayer space or at edge sharing sites (Brigatti et al., 2011; Koch, 2008). Clay structures contain vacancies and undergo structural substitutions within the sheets effecting source of the layer charge; for example in different smectites. Montmorillonite contains vacancies at cis-vacant octahedral sites and undergo substitutions at octahedral sites, inducing octahedral charge, whereas, nontronites containing trans-vacant octahedral sites and undergo substitutions at tetrahedral sites causing charge to be predominantly on the tetrahedral sheet (Brigatti et al. 2006). Octahedral sheets in layer silicates can have gibbsite-like (1:3 cation to anion) or brucite-like (1:2 cation to anion) arrangements leading to di-octahedral (C2 symmetry) or tri-octahedral clays (C2/m symmetry) where number of vacancies at each site differ (1 and 0 respectively). The separations between sheets, interlayer spaces, differ depending on clay structure with a number of different characteristics demonstrated depending on the ratio of sheets, cations present, and vacancies within the structures (Moore and Reynolds, 1997).
Clays with a 1:1 ratio (TO), like kaolinites or serpentines, are held together through hydrogen bonds and van der Waals forces (VDW). They contain a non-expandable interlayer allowing no swelling within the mineral, redcuing the interlayer distances seen, even in a hydrated halloysite showing a spacing of 7 Å (Brigatti et al. 2013). Clays show variable characteristics with 2:1 (TOT) layers depending on the interlayer species and hydration state of these species and charge on the structure, all exhibiting VDW forces to different extents: pyrophyllite and talc like structures are electrically neutral so require no charge balancing cation in the interlayer, held together by just VDW forces; mica and Illite like structures show anhydrous interlayer species balancing higher charges on the sheets (0.6-2 negative charge per layer); smectites
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(montmorillonite, nontronites, and beidellite) relevant to the studies undertaken within show a weaker negative charge on the layers (0.2-0.6) with hydrated cations balancing charges and promoting larger interlayer spaces (14-16 Å); and finally chlorite like structures that exhibit 2:1 (TOT) layers with positively charged octahedrally co- ordinated interlayer cations (sheet) linked between sheets through hydrogen bonding (Brigatti et al. 2013; Brigatti et al. 2006; Moore and Reynolds, 1997). Natural clays may form mixed-layer structures with interstratified layers of different composition either as an ordered regular pattern or an irregular random layering of the structures (Brigatti et al. 2013).
2.2.2. Smectites Smectites are 2:1 layered clay minerals, both dioctahedral and trioctahedral, that can expand and contract their structure whilst maintaining 2D-crystalographic integrity through hydration and substitution of interlayer cations, generically visualised in Figure 2.7 below (Moore et al., 1997). The expansive properties of smectites rely on the up take of water associated with cations into the interlayer, balancing the weakly negative charge within the structure of the smectites and causing expansion of the interlayer space to 14-16 Å, in comparison to higher charged clays that have interlayer associated anhydrous cations in a non-expansive interlayer (i.e. Illite). Freezing of smectites causes the interlayer to expand further (Svensson, 2015) and conversely heating causes the loss of associated interlayer water, causing a reduction in the interlayer spacing with progressive water loss giving distances comparable to those in higher charged clays such as illite or biotite, and promoting thermally induced mineral transformations of smectites (Section 2.3.1.)
Figure 2.7. Hydrated smectite clay layer structure representation (adapted from Bergaya et al., 2006)
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Smectites show a number of advantageous properties in relation to geological disposal: low hydraulic conductivity due to swelling from hydration of interlayer cations reduces the chance of crack formation and stabilises the waste canister, making diffusion the main means of transport (Nagra, 2002; Missana et al., 2008; Beattie and Williams, 2012; Wilson et al., 2011); high cation exchange capacity and consequently high sorption capacity for highly charged radionuclides and other contaminants (Grauer, 1994; Moore and Reynolds, 1997; Sylwester et al., 2000); small pore size distribution restricting microbial processes (Hicks et al., 2009); a wide availability naturally and stability over geological timescales (Sellin and Leupin, 2013; Wilson et al., 2011); and relative resistance to heating and radiation (Savage and Authur, 2012).
These favourable properties of smectites have led to the planned use of bentonite as a buffer material in a repository for heat generating wastes within a GDF (Posiva, 2009; Teich-McGoldrick et al., 2015; Baldwin et al., 2008).
2.2.3. Bentonite Bentonite is a impure dioctahedral smectite clay, that is internationally favoured as the buffer material for proposed GDFs (Dohrmann et al., 2013; Holton et al., 2012; ONDRAF/NIRAS et al., 2015; Guo, 2009) and is formed through the alteration of tuff and ash (glassy volcanic material) or silica bearing rocks such as granite or basalt (Stankovic et al., 2011). Bentonite is a general term used for hydrous aluminium silicates, varying from site to site, with a chemical composition dependent on the location, geochemical and hydrological processes that were involved in the formation of the deposit (Karnland, 2010; Savage and Authur, 2012). Fundamentally, bentonites used for GDFs consist of a high content of the diocthedral smectite montmorillonite (>75%), which is used an approximate formula for composition of bentonites, that alco contain small quantities of additional accessory minerals such as iron oxides, gypsum, illite, quartz, calcite, carbonates, feldspars, kaolinites, sulphides and organic matter (Karnland and Bigersson, 2006; Posiva, 2009). Sellin and Leupin (2013) postulated two mineralogical requirements to be met by the material intended for use as the buffer; a sufficiently high montmorillonite content to maintain advantageous properties manifested within the buffer and a limitation of detrimental accessory minerals avoiding undesired effects, such as oxidising conditions, with smectite content 53
dominating the bentonite behaviour observed (Ahn and Apted, 2010; Karnland, 2010; Stanković et al., 2011; Wold and Eriksen, 2007; Savage and Authur, 2012).
2.2.4. Montmorillonite, Nontronite and Beidellite A main objective of the studies within this thesis is to measure the changes to the ECB performance at a fundamental level allowing a clear and concise measurement of behaviours with respect to the challenges faced from the wastes. To do this a series of montmorillonites (STx-1b, SCa-3, SAz-2, SWy-1), a beidellite (SBld-1), and two nontronites (NAu-1, NAu-2) were used to compare how slight changes in chemistry effect the behaviour and hence the suitability of each of clay mineral intended for use as an ECB material, or as a possible pseudo-alteration product from Fe canister corrosion.
Montmorillonites make up the majority of the intended buffer material for geological disposal and beidellites are an end member of the same group. They are formed from alteration of volcanic glass under low pH or in tropical areas from surface weathering of rocks (acid tuffs) in the presence of groundwater containing Na, Ca, Mg and ferrous iron. The defining trait of montmorillonites is a variable Mg content in comparison to beidellites that have little or no Mg and both have variable Fe contents and interlayer cations depending on source. Nontronite is a higher Fe smectite, used as a pseudo- alteration product from the expected Fe corrosion of the canister, created by hydrothermal processes of weathering basalt and ultramafic rocks (Haldar and Tišljar, 2014; Brigatti et al., 2013).
Montmorillonites exhibit a layer charge predominantly in the octahedral sheet (-0.33) whilst nontronites and beidellites show the layer charge (-0.33) to be primarily in the tetrahedral sheet (Moore and Reynolds., 1997). Substitution of ions into the octahedral and or tetrahedral layers has a significant effect on clay layer charge and expansion properties exhibited key to retention of cations within the interlayer and at edge sites (Savage and Authur, 2012; Gorski et al., 2013). An increased Fe content, such as that of the nontronites, could lead to enhanced redox reactivity from electron transfer pathways in the octahedral sheet, but counter-productively may increase layer charge effecting the swelling expressed and therefore the cation exchange
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properties of the clay (Stucki et al., 1984; Gorski et al., 2013; Lantenois et al., 2005; Neumann et al., 2015; Rosso and Ilton, 2003).
2.2.5. Colloids The formation of colloids through interactions between the clay buffer and host rock and at cracks within the buffer transpire from the loss of density and swelling pressure at the water/clay interface that could occur in a GDF environment (Savage and Authur, 2012; Grindrod et al., 1999). Colloids are defined as particles indefinitely suspended in a fluid from Brownian motion, within the size range of 1–1000 nm (Wold, 2010; Hunter, 2001) and are of high importance in a safety case for disposal as they could lead to the migration of radionuclides away from areas of storage (Missana et al., 2008; Buddemeier and Hunt, 1988; Kunze et al., 2008). Although not a central theme of the research undertaken within this thesis, studies have shown the importance of the high charge density of colloids and associated affinity for radionuclides with respect to a comprehensive safety case for a GDF (Wold, 2010; Mori et al., 2003; Missana et al., 2003; Holmboe et al., 2009; Holmboe et al., 2011; Grambow et al., 2014).
2.3. Challenges from heat generating HLW in a geological disposal facility
The focus of this thesis is centred on the characteristics of the main fraction of the clay buffer, montmorillonite, at a fundamental level in relation to various challenges under GDF conditions. The challenges that face the ECB after closure include; heat generation, irradiation from the wastes (α/β/γ), re-saturation of clay (groundwater infiltration), gas generation, microbial processes, waste container corrosion, and radionuclide retardation (Sellin and Leupin, 2013; Brookshaw et al., 2015; Beattie and Williams, 2012; Wilson et al., 2011; Ewing, 1999).
Each of the research papers (Chapters 4, 5, 6), as mentioned previously, will look at different challenges that would be present within a GDF environment after disposal, relating effects initially as single symptoms and then as coupled processes in relation to buffer performance. Chapter 4 looks at the effect of heating with and without subsequent γ-irradiation on the clay buffer materials on a macro- and meso-scale.
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Chapter 5 studies the effect of heating and then the effect of heating with subsequent α-irradiation on a model montmorillonite at a physical and chemical level. Chapter 6 investigates the effect of prior γ-irradiation on cationic and anionic sorption on a model montmorillonite and pseudo-alteration product. An overview of each challenge is given within this chapter with more detailed and in-depth analysis of relevant challenges given within each research chapter.
2.3.1. Heating HLW self-generates heat from radioactive decay, in which α, β, and γ-irradiation are converted into thermal energy, resulting in an initial storage temperature outside the canister of ≈90–100°C reached over the first 10–15 years (Holton et al., 2012; ANDRA, 2005; Posiva, 2009; Ewing, 2001). The buffer must withstand heat from the HLW for over 1000 years, over which period temperatures should drop to ≈40°C, making sure preferable characteristics are maintained and preventing thermally induced mineral transformations of montmorillonite. Conversions to non-swelling illite, in the presence of K+, could lead to crack formation or possible containment failure. A large variety of studies have been undertaken on thermal-transformations showing great variability with an increased rate in lab studies verses natural analogues. However, a minimal effect was seen in most and, more importantly, this is supported by geological analogue studies (Karnland and Bigersson, 2006; Savage and Authur, 2012; Allard and Calas, 2009; ANDRA, 2005; Sellin and Leupin, 2013; Zhang et al., 2007; Liu et al., 2012). Most previous studies showed limited effects under GDF conditions with reversibility of dehydration, maintenance of swelling properties, stability over geological heating cycles, and some studies showing small enhancements in sorption properties (Couture, 1985; Pusch, 1983; Kamei et al., 1999; Hansen et al., 2012; Hicks et al., 2009; Wersin et al., 2007; Noyan et al., 2006 and Zhu et al., 2016).
If the clay buffer material chosen can be shown to endure higher temperatures (≈150 °C) from HLW whilst maintaining or enhancing key advantageous properties (associated with montmorillonite fraction), closer packing of heat generating waste canisters may be possible, allowing a smaller GDF to be constructed.
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2.3.2. Irradiation Irradiation in the form of ionizing α, β, and γ-irradiations as well as localised amount of heavy ion (elastic) bombardment would be experienced by mineral phases present in the buffer material used for a GDF. β radiation would initially dominate (500 years) within the wastes (self-shielded) and then α-irradiation would predominate. γ-irradiation would be present throughout the disposal period and then α-irradiation would become the main cause of damage in the buffer after canister failure (≈1000 years) (Allard and Calas, 2009; Ewing et al., 2000; Pusch, 1994; Ewing et al., 1995). These ionising radiations cause point defects and some radiolysis within minerals in which electron holes are formed as electronic defects causing an increase in surface charge of materials such as clays (Allard and Calas, 2009; Pattrick et al., 2013; Bower et al., 2016). Amorphisation has also be shown to occur through heavy ion atomic displacements; the effects are dependent on the type and dose of radiation: α-irradiation is highly damaging (energetic) but only penetrates to a depth of ≈20 μm in clays (high linear energy transfer) with most damage seen at the end of the damage track from recoil ions, and β and γ-irradiation have shown damage to mm and cm depths respectively (Allard and Calas, 2009; Allard et al., 2013; Fourdrin et al., 2010; Nasdala et al., 2006; Atkins et al., 2018).
Specifically the impact of α- and γ-irradiations on the clay buffer material are investigated with respect the physical and chemical properties of the clays, firstly, to examine the effects on the clay under conditions expected after initial storage and then to explore the effects expected after canister breakdown under conditions resembling a GDF environment.
2.3.2.1. γ-irradiation γ-irradiation would be one of the primary challenges faced by the engineered clay barrier from the heat generating radioactive wastes as γ-rays can penetrate through the canister and affect surrounding material (Allard et al., 2013; Sellin and Leupin, 2013). The γ-irradiation dose received is dependent on the composition of the waste; over the first 1000 years, before estimated canister decay, dose rates up to 72 Gy h-1 are estimated at the canister surface, giving total doses up to 500 MGy (Stroes- Gascoyne et al., 1994; Allard and Calas, 2009; SKB, 2006). In clays the effect of γ-irradiation has been shown to manifest as lattice dislocation and charge separation 57
(charge defects) resulting in an increased surface potential from the interactions of highly energetic electrons. These effects are shown through enhanced colloid stability, 3+ H2 production (radiolysis), and partial reduction of structural Fe (Holmboe et al. 2009; Kunze et al., 2008; Grambow et al., 2014; Allard et al., 2012; Gournis et al., 2001; Ewing et al., 1995).
Past studies on clay minerals showed only small changes on a macro-scale even at high doses, with many of the experiments demonstrating that heating after γ-irradiation mitigated some of the effects expected through annealing of defects formed (Sorieul et al., 2005; Allard et al., 2012; Huang and Chen, 2004; Negron et al., 2002; Plötze, 2003; Pusch et al., 1992). The effect of γ-irradiation after heating of the clay was shown in Chapter 4, exemplifying the initial storage challenges that would be present within a GDF environment. Further studies (Chapter 6) looked at how γ-irradiation effects may be influential to sorption of redox active contaminants after canister failure.
2.3.2.2. α-irradiation After the primary containment of HLW corrodes (1000 years), α-irradiation from longer lived radionuclides in the form of actinides (U, Np, Pu, Am) and fission products (I, Tc, Cs) in the HLW, that generate α-particles, would become the dominate form of radiation (Holton et al., 2012; NDA, 2010; Ewing, 2001). The α-damage is caused by α- particles (4He2+ ions, ≈5 MeV) and recoil of actinide nuclei (70-100 keV) quantified in materials using the measurement of displacements per atom (dpa), in which the energy of the particles, material density and crystallite size contribute to the displacements observed. A high linear energy transfer pathway of α-particles (20 μm) limits the damage seen to localised regions, with amorphisation doses in smectites shown to be as low as 0.13–0.16 dpa (Farnan et al., 2007; Bower et al., 2015; Bower et al., 2016; Lu et al., 2013; Lieser and Kratz, 2008; Sorieul et al., 2008).
Although effects are local to the source of the α-emitters, past studies have shown amorphisation from ionizing radiation and α-recoil nuclei to cause vacancies (Frenkel defects), lattice and structural distortions, activation of redox active elements, and radiolysis which has been shown to be the foremost cause of damage (Ewing, 2001; Ewing et al., 1995; Clozel et al., 1994; Patrick et al. 2013; Allard and Calas, 2009;
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Suckow, 2009; Sorieul et al., 2008). If sorption-desorption cycles were to occur then large amounts of damage could propagate through the buffer material, although previous studies have shown clay to have a high affinity for actinides which are the main source of α-emitters in the waste (Kitamura and Takase, 2016; Jonsson, 2012; Sorieul et al., 2008; Gu et al., 2001). On a geological timescale, natural analogue studies containing radioactive inclusions provide evidence of the effect of α-irradiation in materials related to a GDF, showing heating to anneal ionisation damage over time, but with structural defects remaining (Allard and Calas, 2009; Weber et al. 1997; Bower et al., 2015; Pattrick et al., 2013; Ewing et al., 1995). Important advantageous properties such as high swelling capacity, high sorption capacity, and low solubility must be maintained after α-irradiation to make sure the buffer material can still competently complete its safety function (Ewing et al., 2000; Pusch, 1994; Wilson et al., 2011; Petit et al., 1987; Weber et al., 2000). The effect of HLW on the buffer material after predicted canister breach is examined through measurements on samples with and without prior heat treatment (120 °C) before and after α-irradiation (5 MeV), to give a full characterisation of the expected physical and chemical properties of the clay buffer under condition relevant to a GDF (chapter 5).
2.3.3. Groundwater Infiltration A planned initial HLW repository in the UK involves dry installation of the buffer material (NDA, 2010; Holton et al., 2012), allowing for slow groundwater infiltration and resaturation of the buffer causing swelling of the clays (montmorillonite), limiting further ground water flow, stabilising the waste canisters, and filling cracks and gaps around the canister and host geology, making diffusion the predominant form of transport from the canister and inhibiting any advection flow that could have occurred (Hicks et al., 2009; Missana et al., 2008; Wilson et al., 2011). The ECB plays two main roles in a GDF safety case. First, it minimizes canister decay as the buffer becomes saturated and swells, thus decreasing groundwater flow around the canister for several hundred years. Secondly, after container degradation, the saturated buffer is significant in the retardation of radionuclides released from the waste through cation exchange on the surface, at edge sites, and within interlayers of the clay buffer (Wilson et al., 2011; Bernier et al., 2007; Holton et al., 2012).
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2.3.3.1. Cation Exchange Several factors, varying largely depending on composition, influence the cation exchange capacity (CEC) of clays; the quantity of negatively charged sites retaining cations in the interlayer and on the surface, with the highest value of 120 meq/ 100 g seen for pure montmorillonite (Summer and Miller, 1996; Karland, 2010; Hicks et al., 2009). The uptake of smaller, higher charged cations within clays follows the + + + 2+ 2+ + Hofmeister series: Li 2.3.3.2. Contaminant Geochemistry Environmentally mobile and radiotoxic radionuclides (i.e. Np, U, and Tc) and contaminants (i.e. Cr, Co, and Se) at nuclear sites, such as GDFs, can be both cationic and anionic in nature depending on the oxidation state and aqueous state formed by the mobile phase (Riley and Zachara, 1992; Greathouse and Cygan, 2006; Brookshaw et al., 2015; Sellin and Leupin, 2013; Ahn and Apted, 2010). These contaminants (radioactive and not) are most likely to become mobile in groundwater (Dozol and Hagemann, 1993), and therefore a comprehensive knowledge of the behaviour of mobile phases, relevant to a GDF environment prior to and after canister decay, and the effectiveness of the ECB to sequester and retard these phases is paramount to a safety case for geological disposal of radioactive wastes (Sylwester et al., 2000; Sellin 60 and Leupin, 2013; Bernier et al., 2007; Holton et al., 2012; Morris et al., 2011; Wilson et al., 2011). Two examples of mobile anionic and cationic species at nuclear sites are Cr(VI) and 2- 2+ U(VI) as CrO4 and UO2 respectively (Barnett et al., 2002; Pabalan and Turner, 1997; Riley and Zachara, 1992; Beaumont et al., 2008). A brief overview of uranium and chromium geochemistry in relation to nuclear disposal sites is included below with further detail included in an in-depth introduction in the relevant research chapter (6). 2.3.3.2.1. Radionuclide Geochemistry – Uranium Uranium is a long-lived, multiple oxidation state actinide, with a number of daughter products, making up ≈95% of the spent nuclear fuel in the UK (Barnett et al., 2002; Pabalan and Turner, 1997; Pourcelot et al., 2011; Hudson et al., 1999; Duff et al., 1997; Giaquinta et al., 1997). Open cast mining in Canada, Australia, Kazakhstan, Niger, Russia and Namibia produces the majority of uranium ores extracted for nuclear power generation with a number of primordial formed radioisotopes present: 234U 5 235 8 238 9 (t1/2=2.7X10 yr), U (t1/2=7.13X10 yr), and U (t1/2=4.56x10 yr). Subsequent enrichment is required to increase 235U concentration from 0.7% to 3-5% for use as nuclear fuel, creating large amounts of depleted uranium (Nier, 1939; Choppin, 2007; Wilson, 1996; Sharrad et al. 2011). Unstable nuclides, such as the isotopes of uranium, undergo spontaneous emission of particles or photons termed radioactive decay. Emissions occur through α or β decay, γ emission, proton emission, neutron emission, cluster radioactivity and spontaneous fission (random splitting of atoms), summarised below in Table 2.2. The main radioactive decays are α or β decays, usually accompanied by simultaneous γ-emission from the excited nuclei; the four key radioactive decay series are the thorium series, uranium series, actinium series, and neptunium series (artificial), all of which end in a stable lead isotope or bismuth in the case of the neptunium decay (Geibert, 2008; Kónya and Nagy, 2012; Choppin et al., 2013). Table 2.2. Key modes of decay showing changes in charge (Z), neutron number (N) and atomic number (A) (Ojovan and Lee, 2014). 61 Decay Mode ΔZ ΔN ΔA α-decay -2 -2 -4 β-decay ±1 ±1 0 γ-emission 0 0 0 An overview of the two main uranium decay chains 238U (uranium series) and 235U (U- Ac series), key long-lived α-emitters in regards to nuclear disposal over geological timescales (106 yrs), are summarised in Figure 2.8 and Figure 2.9, showing the expected daughter products that would also be present in the waste after disposal (Molinari and Snodgrass, 1990; Kónya and Nagy, 2012; ANL, 2005; Krishnaswami and Cochran, 2008). Figure 2.8. 238U radioactive decay series (adapted from Kónya and Nagy, 2012; Sharrad et al., 2011; Molinari and Snodgrass, 1990). 62 Figure 2.9. 235U radioactive decay series (adapted from Kónya and Nagy, 2012; Sharrad et al., 2011; Molinari and Snodgrass, 1990). Uranium has two principal oxidations states within the natural environment, mobile U(VI) under oxidising conditions and insoluble U(IV) under anoxic reducing conditions (Lloyd and Renshaw, 2005; Porcelli, 2008; Silva and Nitsche, 1995). After canister failure, the mobility of uranium within a GDF would be controlled through interactions on mineral surfaces. Mitigation of migration is achieved on minerals through adsorption at edge sites, interactions in the interlayer, and precipitation of U(VI) or reduction to U(IV) phases on surfaces (Langmuir, 1978; Williamson et al., 2014; Choppin, 2007; Barnett et al., 2002; Dent et al., 1992; Sellin and Leupin, 2013; Wilkins et al., 2007; Greathouse and Cygan, 2005). U(VI) mobility is influenced by phosphate, carbonate, and pH (Li and Kaplan, 2012; Waite et al., 1994; Cheng et al., 2012), with redox cycling allowing possibilities for re-mobilisation of U(VI) and transport in the environment (Massey et al., 2014; Brookshaw et al., 2015). At low carbonate concentrations, hydroxides of uranium often form, generally at a lower pH. As the pH increases and carbonate concentrations rise above 10-2 M, a reduction in sorption to mineral surfaces under ambient conditions is observed with the formation of stable uranyl carbonato complexes, exemplified in the speciation diagram in Figure 2.10 63 (Kobets et al., 2012; Brookshaw et al., 2015; Barnett et al., 2000; Greathouse and Cygan, 2006; Bargar et al., 1999). Figure 2.10. Speciation of U(VI) at 1 ppm and dissolved carbonate concentration(log, dashed line) as a function of pH, in an open system using MINTEQA2 calculations at log Pco2=-3.5 in 0.1 M NaNO3. Reprinted from (Barnett et al. (2000). The dominant U(VI) species found in contaminated groundwater systems is the 2+ hexavalent aqueous uranyl ion (UO2 ) (Greathouse and Cygan, 2006; Langmuir, 1978; Merkel and Hasche-Berger, 2008). Under low to modest pH ranges (>pH10), adsorption of U(VI) can occur efficiently through interactions at the mineral surface sites via adsorption onto minerals at edge sites, precipitation of U(VI) phases on the mineral surface, reductive precipitation to U(IV) on the mineral surface, incorporation into mineral structures or co-precipitation (Giaquinta et al., 1997; Hudson et al., 1999; McKinley et al., 1995; Wasserman et al., 1997; Catalano and Brown, 2004; Shoesmith, 2018; Liger et al., 1999; Payne et al., 1994; Duff et al., 2002; Nico et al., 2009; Stewart et al., 2009; Ilton et al., 2010; Marshall et al., 2014). Fe(II)-bearing minerals have shown high affinities for U(VI) in solution and within minerals facilitating reduction of U(VI) to insoluble U(IV) phases such as uraninite or coffinite (Neumann et al., 2015; Dreissig et al., 2010; Ilton et al., 2006; Skomurski et al., 2011; Duff et al., 2001; Jeon et al., 2005). 64 2.3.3.2.2. Contaminant Geochemistry – Chromium Chromium, is a major contaminant generated from industrial processes (i.e. leather tanning), farming (fishing), energy generation, and nuclear operations (Hanford). It is highly toxic and dangerous if leached into water supplies from contaminated areas (Riley and Zachara, 1992; Beaumont et al., 2008; Khan et al., 1995; Jemima et al., 2018; Dhal et al., 2013; Cheng et al., 2012). Chromium has two common environmental 2- states; mobile Cr(VI) as an anionic species (CrO4 ) under oxic conditions and insoluble Cr(III) as (Cr2O3 or Cr(OH)3) under reducing conditions, with mobility governed primarily through adsorption and solubility of chromium species present (Bishop et al., 2014; Richard and Bourg, 1991; Bartlett and James, 1979). Cr(VI) interactions with clay minerals may help predict future behaviour of other pertinent oxyanions present at geological disposal sites, such as Tc(VIII) and I(V), that are all redox active species relevant to GDF storage (Bower, 2015; Qafoku et al., 2017). A number of previous studies showed adsorption of heavy metal contaminants onto minerals proceeded through precipitation and co-precipitation on mineral surfaces or through ion exchange or organic binding on modified minerals (Arnfalk et al., 1994; Bishop et al., 2014; Jemima et al., 2018; Rai et al., 1989; Qafoku et al., 2015). Anionic Cr(VI) species show favourable sorption in high Fe saponite (75%) but lower sorption in bentonites (<20%). Sorption was shown to require a redox active Fe(II) portion of the mineral, converted either from chemical treatments or microbial actions, to allow conversion of Cr(VI) to a cationic Cr(III) species, for which clay minerals have a high affinity (Taylor et al., 2000; Pettine et al., 1998; Chon et al., 2006; Jung et al., 2007; Bishop et al., 2014; Parthasarathy et al., 2007; Qafoku et al., 2017; Chalermyanon et al., 2008). In relation to a GDF site, the effect of γ-irradiation may play key role in the behaviour of minerals towards radionuclide and contaminants, through an increased surface potential and small alterations in structural Fe (Allard et al., 2012; Holmboe et al., 2011; Missana et al., 2008; Kunze et al., 2008). This effect is examined later in a 2- research chapter (6) on a high and low Fe clay, with both an anionic (CrO4 ) and 2+ cationic (UO2 ) species, under conditions relatable to a GDF. 65 2.3.4. Corrosion products The formation of corrosion products at the canister-ECB interface could have large implications for protection of the waste canister both initially, and on sequestration of radionuclides after containment failure. Within the studies undertaken (Chapter 4 and 6) a set of higher Fe (≈30%) nontronites, NAu-1 and NAu-2, are used as pseudo- alteration products of Fe corrosion probing the effect of possible enhanced Fe content within the closely surrounding buffer material. Transformation of clay minerals through interactions with Fe-based canisters through alteration of smectites, affecting the buffer up to distances of 10 mm, has been documented in a number of studies. Fe(II) incorporation and sorption causes reduction of structural Fe(III), forming trioctahedral phases such as saponite 2+ 3+ 2+ (Ca0.1Na0.1Mg2.25Fe 0.75Si3AlO10(OH)2.4(H2O)), magnetite (Fe 2Fe O4), odinite 3+ 2+ 2+ (Fe 0.7Mg0.7Al0.5Fe 0.3Ti0.1Mn 0.1Si1.8Al0.2O5(OH)4), and cronstedtite 2+ 3+ 3+ (Fe 2Fe ((Si,Fe )2O5)(OH)4) and Fe rich chlorites (i.e. 2+ 3+ Chamosite (Fe ,Mg,Al,Fe )6(Si,Al)4O10(OH,O)8 or berthierine 2+ 3+ Fe 1.5AlFe 0.2Mg0.2Si1.1Al0.9O5(OH)2) as well as Fe-rich smectite phases (i.e. notronite), which show a reduction in swelling (interlayer space) from an increase in layer charge but an increased affinity for higher charge cationic radionuclides (Svensson, 2015; Svensson and Hansen, 2013; Lantenois et al., 2005; Lanson et al., 2011; Gaudin et al., 2009; Jodin-Caumon et al., 2010; Jodin-Caumon et al., 2012; Wilson et al., 2011; Milodowski et al., 2009; Milodowski et al., 2009). Iron content influences behaviour through, for example, providing electron transport pathways and influencing processes such as radiolysis through charge trapping within the structure, increasing the redox potential of buffer allowing for a high possibility of reductive precipitation reactions or incorporation of contaminants (Latta et al., 2017; Fourdrin et al., 2013; Lainé et al., 2017; Skomurski et al., 2011; Li and Kaplan, 2012; Jeon et al., 2005; Gorski et al., 2013; Neumann et al., 2015; Liger et al., 1999; Du et al., 2011). 2.4. References Ahn, J. and Apted, M.J. (2010) Geological Repository Systems for Safe Disposal of Spent Nuclear Fuels and Radioactive Waste. Woodhead Publishing Limited 1, 3-28. ISBN: 978-1-84569-542-2. 66 Allard, T. Balan, E. and Calas, G. (2013) Chapter 4 - Radiation Effects on Clay Minerals, in: Bergaya, F., Lagaly, G. (Eds.), Developments in Clay Science. Elsevier, 127-138. Allard, T. Balan, E. Calas, G. Fourdrin, C. Morichon, E. and Sorieul, S. (2012) Radiation- induced defects in clay minerals: A review. Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms 277, 112-120. Allard, T. and Calas, G. (2009) Radiation effects on clay mineral properties. Applied Clay Science 43, 143-149. Andra (2005) Andra Research on the Geological Disposal of High-Level Long-Lived Radioactive Waste: Results and Perspectives. Andra, France, 1-40. Argonne National Lab (ANL), (2005) Natural Decay Series:Uranium, Radium, and Thorium, Human Health Fact Sheet. DOE, USA, 1-4. Arnfalk, P. Wasay, S.A. and Tokunaga, S. (1996) A comparative study of Cd, Cr(III), Cr(VI), Hg, and Pb uptake by minerals and soil materials. Water, Air, and Soil Pollution Springer 87, 131-148. Atkins, P. De Paula, J. and Keeler, J. (2018) Atkins' physical chemistry. Oxford university press. Baldwin, T. Chapman, N. and Neall, F. 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Clay Source Repository Clays The smectite clays were obtained from the Clay Mineral Society (http://www.clays.org/) and changes in structure and chemistry mapped under different conditions such as: cation exchange, heating, irradiation (α, γ), groundwater infiltration, and radionuclide/contaminant interactions (U and Cr). The number of clays studied was reduced over time to a set of two reference clays. 3.1.1.1. Montmorillonites – SWy-1, STx-1b, SAz-2, SCa-3: Montmorillonite has the general formula (Na,Ca)0.3Al1.67Mg0.33Si4O10(OH)2.n(H2O), with the negative charge residing on the octahedral sheet (Stanković et al., 2011; Grim et al., 1978). Montmorillonite contains low quantities of Fe and was representative of the initial bentonite backfill composition (>75%), prior to canister degradation (Posiva, 2009). Montmorillonite contains small quantities of accessory minerals and interlayer cations were altered through homo-ionisation (Na+, Ca2+, K+, Mg2+) in this study. "Cheto" or Arizonian Montmorillonite SAz-2: Originates from the Bidahochi formation (Bidahochi lake, Arizona), formed in the Pliocene period (5.333 million to 2.58 million years before present). Texas Montmorillonite STx-1b: Originates from the Manning formation, Texas, along with sandstone, formed in the Eocene period (56 to 33.9 million years ago) (Castellini et al., 2017). Wyoming Montmorillonite (Na) SWy-1: Originates from the Newcastle formation, Wyoming, formed in the cretaceous period (145-66 million years ago). This Na-Montmorillonite (or Wyoming bentonite) has been used largely in GDF buffer material research. Otay Montmorillonite SCa-3: Originates from Otay, California (region of high Feldspar content) 84 3.1.1.2. Beidellite - SBld-1: Beidellite has the general formula Na0.5Al2(Si3.5Al0.5)O10(OH)2.n(H2O) with charge located in the tetrahedral sites. It has a low Mg content, some Fe, and a higher amount of K in the interlayer (Post et al., 1997). Homo-ionisation with other counter-ions (Na+, Ca2+, K+, Mg2+) was challenging so only the Ca2+ form was used in this study. Beidellite SBld-1: Originates from Idaho, and was formed as an alteration product in hydrothermal mineral deposits (especially porphyry Cu-Mo systems). 3.1.1.3. Nontronite - NAu-1 NAu-2: 3+ Nontronite has the general formula Na0.3Fe 2(Si,Al)4O10(OH)2.n(H2O), with a similar structure to montmorillonite, but containing high amounts of Fe and the charge within the tetrahedral layer (Keeling et al., 2000). Nontronite represents higher Fe clays formed after Fe intrusion from breakdown of steel/Fe metal canisters. Nontronite contains small amounts of accessory minerals. Interlayer cations were altered through homo-ionisation (Na+, Ca2+, K+, Mg2+). Nontronite NAu-1 and NAu-2: Originate from South Australia deposits with a Fe content of ≈30% and charge located in the tetrahedral sheet (Keeling et al., 2000). 3.1.1.4. Previous Clay Data: A summary of the source, chemical composition, formula, and key characteristics of the seven source clays is given in Table 3.1. 85 Table 3.1. Summary of data on clays used in this work, adapt from Clay Mineral Society (1979). Clay Country Time of Chemical Composition Cation Surface Formula IR data Structure of collection (%) Exchange Area collection (date) Capacity (m2/g) (meq/100g) 3+ SWy-1 Wyoming, 03.09.72 SiO2: 62.9, Al2O3: 19.6, 76.4 31.8 +/- (Ca0.12 Na0.32 K0.05) [Al3.01 Moderate Fe - Interlayer -1 USA TiO2: 0.090, Fe2O3:3.35, 0.2 Fe(III)0.41 Mn0.01 Mg0.54 855 cm . Quartz charge -0.55 FeO: 0.32, MnO: 0.006, Ti0.02][Si7.89Al0.02]O20(OH)4 - 780, 800,698, Octahedral - MgO: 3.05, CaO: 1.68, 400, and 373 cm- 0.53 1 Na2O: 1.53, K2O: 0.53, F: . Trace Tetrahedral- 0.111, P2O5: 0.049, S: carbonate - 1425 0.02 0.05, Organic C - 1.59, cm-1. Inorganic C - 4.47, CO2: 1.33. STx-1b Texas, 17.09.72 SiO2: 70.1, Al2O3: 16.0, 84.4 83.8 +/- (Ca0.27 Na0.04 K0.01)[Al2.41 Low Fe. Quartz – Interlayer -1 USA TiO2: 0.22, Fe2O3: 0.65, 0.2 Fe(III)0.09Mg0.71Ti0.03][Si8] 697 cm . Si charge -0.68 -1 FeO: 0.15, MnO: 0.009, O20(OH)4 phase 790 cm . Octahedral - MgO: 3.69, CaO: 1.59, Trace carbonate- .0.68 -1 Na2O: 0.27, K2O: 0.078, 1400 cm Tetrahedral - F: 0.084, P2O5: 0.026, S: 0.00 0.04, Organic C: 3.32, Inorganic C: 3.22, CO2: 0.16. SAz-2 Arizona, 08.05.73 SiO2: 60.4, Al2O3: 17.6, 120 97.4 +/- Ca0.39 Na0.36 K0.02)[Al2.71 Low oct. Fe. Si Interlayer -1 USA TiO2: 0.24, Fe2O3: 1.42, 0.5 Mg1.11 Fe(III)0.12 Mn0.01 phase 790 cm . charge -1.08 FeO: 0.08, MnO: 0.099, Ti0.03][Si8]O20(OH)4 Octahedral - MgO: 6.46, CaO: 2.82, 1.08 Na2O: 0.063, K2O: 0.19, Tetrahedral - F: 0.287, P2O5: 0.020. 0.00 Organic C - 7.54, 86 Inorganic C – 2.37 SCa-3 California, - SiO2: 52.8, Al2O3: 15.7, - - Mg0.45 Ca0.15 Na0.26 K0.01) - Interlayer USA TiO2: 0.181, Fe2O3: 1.06, [Al2.55 Fe(III)0.12 Mg1.31 charge -1.48 FeO: <0.10, MnO: 0.03, Ti0.02][Si7.81Al0.19]O20(OH)4 Octahedral - MgO: 7.98, CaO: 0.95, 1.29 Na2O: 0.92, K2O: 0.03, Tetrahedral - P2O5: 0.02, Carbon: 21.2. 0.19 SBld-1 Idaho, - - - Si3.77, Al(IV)0.228, - - USA - Al(VI)1.786, Fe(III)0.104, Mg(VI)0.046 Mn0.001Ti0.048 Na0.012 K0.159 NAu-1 South - SiO2: 53.33, Al2O3: 10.22, - - (M+1.0)[Si7.00 Al1.00][Al0.58 - - Australia Fe2O3: 34.19, MgO: 0.27, Fe3.38 Mg0.05] CaO: 3.47, Na2O: 0.08, K2O: 0.03. NAu-2 South - SiO2: 56.99, Al2O3: 3.4, - - (M+0.97)[Si7.57 Al0.01 - - Australia Fe2O3: 37.42, MgO: 0.34, Fe0.42][Al0.52 Fe3.32 Mg0.7] CaO: 2.67, Na2O: 0.11, O20(OH)4 K2O: 0.02. 87 3.1.2. Chemicals 3.1.2.1. Chemicals: All chemicals were prepared as stock solutions in deionised water (DIW). Chemicals used were anhydrous sodium acetate, anhydrous calcium chloride, calcium acetate mono-hydrate, anhydrous magnesium chloride, magnesium acetate tetra-hydrate, ferrozine, ammonium acetate, hydroxyl-ammonium hydrochloride, ammonium hydroxide, sodium nitrate, sodium silicate, magnesium sulfate hepta-hydrate, magnesium nitrate hexahydrate, calcium carbonate, sodium hydroxide, and iron sulphate (Sigma-Aldrich, >98% purity), anhydrous sodium chloride, potassium dichromate, and sodium bicarbonate (Fisher Scientific, >99% purity), anhydrous potassium chloride (Emsure, >99% purity), anhydrous potassium acetate (Alfa Aesar, >98% purity). Acids and bases used were ACS reagent grade hydrochloric acid (37%), ACS reagent grade nitric acid (70%), and analytical grade nitric acid (70%) (Sigma Aldrich, >98% purity). 3.1.2.2. Radioactive Isotopes - Uranium A uranyl chloride solution (17,200 ppm, 0.072 moles, RB132KM6: University of Manchester), was used to make up a uranyl sub-stock solution (3010 ppm, 0.63 M, 1.2 x 10-2 moles). 3.2. Methods 3.2.1. Size Fractionation Clays were milled in a TEMA ball mill and sieved to obtain a size fractions of <53 μm (clay and silt-sized particles). 3.2.2. Homoionisation Homoionisation was achieved with the relevant chloride and acetate solutions (i.e. Na, Mg, K, and Ca). For example, a 2 M solution of sodium chloride (2.34 g) in deionised water (20 mL) was prepared and left to equilibrate (2 hours). Clay (0.5 g) was added to the solution with shaking (Flow Laboratories DSG Titertek 1 Microplate Shaker, speed 10, 48 hrs). The solution was centrifuged (Sciquip, 4500 rpm, 10 minutes) to separate the clay and the supernatant decanted. The clay was washed twice with 1 M sodium chloride (40 mL, 2.34 g), twice with sodium acetate (40 mL, 3.29 g), and again with 88 sodium chloride (20 mL, 1.17), re-suspending prior to each centrifugation (4500 rpm, 10 minutes) and the supernatant was decanted after each wash. A final wash with a water ethanol mix (20 mL, 50:50) and final centrifugation (4500 rpm, 10 minutes) resulted in a wet clay paste, which was dried in a desiccator under controlled humidity (using a saturated MgCl2 solution) and then sealed in glass vials. 3.2.3. Clay Pellets Two sizes of clay pellets were pressed with a manual 15 ton hydraulic press (Specac), with a pellet die made from stainless steel (Lightpath Optical (UK) Ltd): (i) 13 mm pellets, 300 mg of clay, 9 tonnes of pressure for 30 seconds; and (ii) 8 mm pellets, 120 mg of clay, 5 tonnes for 30 seconds. Figure 3.1. Pellet press used in experiments and a 13 mm pellet of STx-1b (white) and NAu-1 (green). 3.2.4. Heating Clay pellets (0.3 g or 0.12 g) were heated over a range of temperatures (90, 160, 500, 1000 °C). NAu-1 and STx-1b pellets (high/low iron, 0.3 g), before and after homoionisation with Na, Mg, K or Ca, were heated (90, 160, 500 °C) for γ-irradiation studies. A model montmorillonite sample set (starting material (SM) and after homoionisation with Mg and Na), STx-1b (0.3 g), was used in α-irradiation studies (5 MeV He2+ particles) with prior heating (120 °C). A Gallenkamp hot box oven was used for temperatures <160 °C, and for measurement of water content. A Carbolite furnace was used for heating to temperatures above 160 °C for weight loss on ignition. 3.2.5. γ-irradiation Figure 3.2 shows pellets (0.3 g) and clay powders (0.5 or 1.0 g) before and after γ-irradiation to a range of total doses (0.2-5 MGy). Clay powders, (1.0 g, STx-1b, NAu-1, 89 SCa-3, SWy-1, NAu-2, SAz-2, and SBld-1), pellets (0.12g; SWy-1, NAu-2, SCa-3, SAz-2, and SBld-1; source clays) and homo-ionised pellets (0.3 g; NAu-1 and STx-1b; SM, Na, Mg, K, Ca) were flame sealed in vacuum evacuated (≈3x10-3 mbar) glass tubes (15 mm or 10 mm OD) separated by glass wool, before and after heating (90, 160, 500 °C; 48 hrs), and prior to γ-irradiation (5 MGy). One sample of SWy-1 was not evacuated to determine the effect of γ-irradiation under aerobic versus anaerobic conditions. Figure 3.2. Example of pellets (natural clays and treated sets) prior to γ-irradiation (left) and clay powders after γ-irradiation (right). γ-irradiation was carried out at the Dalton Cumbrian Facility (DCF, University of Manchester) on a Foss Therapy Services Model 812 60Co γ-irradiator (Leay at al., 2014) (Figure 3.3). A range of irradiation times were used to give absorbed dose rates from 4 Gy/min up to 450 Gy/min, determined by Fricke dosimetry (Leay at al., 2015). Figure 3.3. Foss Therapy Services Model 812 60Co γ-irradiator at DCF 3.2.6. α-irradiation α-irradiation was carried out at DCF on clay pellets (0.3 g) and smaller thinned samples to give doses from 0.1–1.5 dpa. 90 3.2.6.1. Bulk Clays for XRD and XAS Analysis – Whole Pellets A set of clay pellets (STx-1b, SWy-1, SCa-3, SBld-1, NAu-1, NAu-2; 0.3 g, 13 mm, 2 g cm-3) were mounted on individual glass slides and a circle (0.5 cm diameter) was 2+ cut and polished (100 μm) before α-irradiation (5 MeV 4He particles, 204 mins). Sample damage was estimated for each sample using an approximate circular Gaussian profile and SRIM modelling outlined below (explained in Chapter 5 SI). Samples were measured using XAS and XRD. 3.2.6.2. Homo-ionised STx-1b Samples for Tomography, XAS and XRD analysis A set of homo-ionised STx-1b samples (SM, Na, and Mg), without and with heating 2+ (120 °C, 48 hrs), were mounted as above for α-irradiation (5 MeV 4He particles, 204 mins), but prior to α-irradiation the thinned sample areas (100 μm thick; SM, Na, and Mg; heated and unheated) were separated from the glass slides. Samples were mounted on pins using kapton tape for analysis through XCT, XRD and XAS (Figure 3.4). The α-irradiation was undertaken on these samples as a group mounted on a marked glass slide next to a quartz screen for beam alignment (Figure 3.4), with the beam dispersed evenly over the samples to give an approximate α-dose of 0.15 dpa across all the samples (explained in Chapter 5 SI). Figure 3.4. α-irradiation homo-ionised samples on a glass slide (before and after α-irradiation), samples mounted on a pin and an SCa-3 pellet sample before and after α-irradiation 3.2.6.3. α-irradiation Set-up and Dose Estimation The α-irradiation was carried out at the DCF using a NEC 15SDH-4 tandem Pelletron ion accelerator (5 MeV) with a high current Toroidal Volume Ion Source (TORVIS) (Leay et al., 2015; Bower et al., 2015) (Figure 3.5). A quartz screen was used to align the beam (∼95% of the total ion flux was contained within a 1 cm2 area) and number of collisions 91 16 2+ (≈1x10 4He particles) was estimated using FWHM equation, time (204 mins), and current (238 nA). This circular Gaussian profile and SRIM modelling gave approximate displacements per atom (dpa) for each of the sample sets (0.10-1.5 dpa). A temperature of 75 °C was measured in beam (thermocouple). Figure 3.5. NEC 15SDH-4 tandem Pelletron ion accelerator at DCF; accelerator hall and target station. 3.2.7. Colloidal Suspensions Preparation of colloid suspensions was adapted from Bouby et al. (2011) to remove particles of a size greater than 500 nm. Each clay sample (1.0 g) was added to DIW (100 mL) and stirred (7 days). The resulting solutions were pipetted into centrifuge tubes (50 mL), and centrifuged (BOECO Germany C-29A centrifuge, 4000 rpm, 11 minutes), decanting the supernatant. Clay was re-suspended in DIW (50 mL) and centrifuged (4000 rpm, 11 min) decanting the supernatant. The process was repeated two more times and final supernatant kept as stock colloid solution. Colloidal suspensions of homo-ionised STx-1b clay samples (SM, Ca, K) before and after γ-irradiation (5 MGy), and of NAu-1, were prepared. 3.2.8. Sorption Experiments 3.2.8.1. Artificial groundwater (AGW) Artificial groundwater (AGW) was prepared (Wilkins et al., 2007) and used as a solution (5 mM and 0.5 mM carbonate) for sorption experiments (Chapter 6). Briefly, AGW (5 mM carbonate) was prepared using NaHCO3 (0.2424 g), NaNO3 (0.0275 g), CaCO3 (0.1672 g), MgCl2.6H2O (0.081 g), MgSO4.7H2O (0.0976 g), Na2SiO3 (0.0829 g), and NaCl 92 (0.0094 g) in DIW (1 L, pH 7.38). Lower carbonate AGW (0.5 mM carbonate, pH 7.36, LCAGW) used reduced amounts of carbonate added through NaHCO3 (0.03 g) and CaCO3 (0.017 g). Solutions were de-gassed and used in an anaerobic glove box or on the bench top under argon. 3.2.8.2. Uranyl sorption Uranyl chloride (3.49 mL, 17,200 ppm, 0.072 moles, RB132KM6: University of Manchester) was added to a weak HCl solution (16.51 mL, 0.001 M) to give a uranyl sub-stock solution (20 mL, 3010 ppm, 0.63 M, 1.2 x 10-2 moles). Under anaerobic conditions, clay (0.1 g; STx-1b, NAu-1, STx-1b γ-irradiated, and NAu-1 γ-irradiated) was added to AGW (9.9 mL, 5 mM carbonate) or LCAGW (9.9 mL, 0.5 mM carbonate) in microcosms and left to equilibrate (48 hrs). The uranyl sub-stock solution (1 mL, 3010 ppm, 0.63 M, 1.2 x 10-2 moles) was added to clay suspensions (9.9 mL, AC) to give experimental U(VI) solutions (10 mL, 30.1 ppm, 1.26 x 10-2 M, pH 8.16-8.88). Samples were sealed and sampling points (0.2 mL) were taken periodically (1 hr, 4 hr, 24 hrs, 1 wk, 3 wks, 5 wks, 6 wks, 11 wks, 24 wks, 38 wks and 51 wks). The experiment was conducted in triplicate. Samples were periodically removed (0.2 mL) and centrifuged (Sigma 1-14 Microfuge, 14000 rpm, 5 mins), the solution phase was extracted, acidified (0.02 mL, 8 M HNO3), and diluted with DIW (≈10 mL, ≈30 ppb) for ICP-MS analysis. Solid samples were prepared anaerobically; separated via centrifugation (10000 rpm, 10 mins), the solution was decanted, and solid extracted (≈2000—3000 ppm U on solid). Samples were used as slurries in XAS holders (micro- centrifuge tubes), sandwiched between wax to give an evenly distributed sample before flash freezing (-80 °C). 3.2.8.3. Chromate sorption Potassium dichromate (0.019 g, 6.46 x 10-5 moles) was added to DIW (25 mL) to give a chromate stock solution (25 mL, Cr: 6.7 ppm, 5.16 x 10-3 M, 1.29 x 10-4 moles). Clay (0.5 g, STx-1b, NAu-1, STx-1b γ-irradiated, and NAu-1 γ-irradiated) was suspended in AGW (9.95 mL) anaerobically and left to equilibrate (48 hrs). The chromium stock solution (0.05 mL, 5.16 x 10-3 M) was spiked into the clay solutions (9.95 mL) to give Cr(VI) anaerobic experimental solutions (10 mL, 5.16 x 10-5 M, pH 8.61-8.95). Periodic 93 sampling points (0.2 mL) were taken (1 hr, 4 hr, 24 hrs, 1 wk, 3 wks, 5 wks, 6 wks, 11 wks, 24 wks, 38 wks and 51 wks). Samples (0.2 mL) were taken anaerobically (argon) and solution separated through micro-centrifugation (14000 rpm, 5 mins). Solution phase was extracted, and acidified (2% HNO3, 9.8 mL) for ICP-MS analysis (≈1.5–3 ppb). 3.3. Clay Characterisation 3.3.1. Major Element Analysis 3.3.1.1. Weight Loss on Ignition (WLOI) WLOI was carried out, following the method used by Wang et al. (2012), on the series of clay starting materials (2.00 g) and pellets (≈300 mg). Clay (2.00 g) was weighed in a pre-weighed crucible, heated in an oven (110 °C, 24 hours) to dryness, then re-weighed to give free water loss. The clay was then heated to 400 °C (ramp rate 1.5 °C/ min, 16 hours), and weighed after cooling in a desiccator to give organic carbon content. The clay was heated to 950 °C (ramp rate 5 °C/ min, 8 hours), and weighed after cooling in a desiccator to give inorganic carbon content. 3.3.1.2. Thermogravimetric analysis (TGA) Clays (≈15 mg) were analysed on a Metter Toledo TGA/DSC 1 STAR System under an air purge atmosphere and at atmospheric pressure (0–1000 °C, air, 10 °C/ min). 3.3.1.3. X-ray fluorescence (XRF) Major element analyses were obtained (Chapters 4, 5 and 6) on pressed pellets (2 hrs, 15 g; 12 g sample and 3 g wax binder) with a wavelength dispersive PANanalytical Axios Sequential X-ray Fluorescence Spectrometer using the standard glass AUSMON (B255). XRF is a non-destructive analysis method, involving measurement of emission of characteristic secondary X-rays from a material that has been excited by bombardment with high-energy X-rays (Glocker and Schreiber, 1928; Atkins et al., 2018). 3.3.2. X-ray Diffraction (XRD) Two XRD instruments were used for this work and silicon (Sigma-Aldrich, 99%) was used as an internal standard. A Philips Xpert Modular Powder Diffractometer (MPD) 94 and a Bruker D8 Advance with a lynxeye detector (Figure 3.6), both with a Cu Kα source (0.154 nm), were used. Clay pellets (13 mm) were fixed to the sample holder (Kapton tape or Blu-tack) giving identical flat surface for each measurement (2휃 range from 4o to 60o, step size of 0.0197, tension of 40 kV and current of 30 mA). Powder samples (≈0.2 g) were ground thoroughly under amyl acetate using a pestle and mortar prior to apply a thin layer evenly to the sample holders. XRD data were analysed using the search-match routine on EVA (Bruker). Figure 3.6. Bruker D8 advanced diffractometer (University of Manchester). Powder XRD is a non-destructive analytical technique used to identify the crystalline components of solids through diffraction of X-rays by the lattice planes of atoms within materials. Incident X-rays are reflected from the parallel planes of atoms within the sample and their orientation distinguished through Miller indices (a, b, c; e.g. d001). Constructively interfering radiation reflecting from the sample forms a typical XRD scattering plot (Rutherford, 1911; Moore and Reynolds, 1989; Atkins et al., 2018). Diffraction patterns show bulk properties of structures, 1011–1012 unit cells or billions of crystals are averaged to give the diffraction patterns observed (Moore and Reynolds, 1989). Bragg (1913) described the relationship between the wavelength of the incident beam (λ), the angle of incident beam entry and parallel planes of atoms causing the diffraction (휃), and the distance between these planes (d) (Figure 3.7). 푛휆 = 2푑 푠𝑖푛 휃 Equation 2.1. Bragg’s law; n is an integer related to the order of reflection, λ is the wavelength of the incident X-ray (nm), d is the d-spacing (nm) and θ is the angle of incidence (°) (Atkins et al., 2009). 95 Figure 3.7. Reprint from Atkins et al. (2009) showing Bragg’s law interactions within a crystal lattice. The d-spacing (Å) gives information about the distance between the layers (and planes) found in the crystal lattice (equation 2.1). This technique cannot be used to: (i) identify amorphous materials due to broad peaks; (ii) quantify elemental composition; and (iii) identify phases present at <5% by mass (Moore and Reynolds, 1997). 3.3.3. Infra-red Spectroscopy (IR) Powder IR measurements were collected using a ID5 ATR Infra-red spectrometer (Nicolet iS5, 4000-670 cm-1, 16 scans, RT), under atmospheric conditions (298 K) to quantify water, hydroxyl loss, and structurally bound oxygen atoms (i.e. Si–O, Fe–O) after heating (25, 90, 160, and 500 °C) and γ-irradiation (5 MGy) experiments. IR spectroscopy is associated with molecular vibrational frequencies in the infrared region of the electromagnetic spectrum (4000–40 cm-1) and can be used to identify functional groups (i.e. OH or free water) through the relationship in equation 2.2 (Larkin, 2018). 휈 1 휈̅ = = 푐 휆 (휂) Equation 2.2. Showing the relationship between wavelength (λ), frequency (ν), and wavenumbers (ύ) where c is the speed of light and η the refractive index of the medium it (Larkin, 2018). Vibrations and rotations cause a net change in the dipole moment that result in bonds stretching or bending, characterised by frequency (dependent on vectors, dispersion curves), intensity (polar character), and band shape unique to each molecule (Atkins et al., 2009: Balan et al., 2017; Larkin et al., 2018). 96 In clays, fundamental stretching and bending vibrations relate to the primary functional groups present; OH stretching and bending modes (3700–3500 and 950–600 cm-1), effected by octahedral associated groups and tetrahedral charge distribution and Si–O/ Al–O stretching (1200–700 cm-1) and bending modes (600–400 cm-1) (Gates, 2005; Madejová et al., 2017). 3.3.4 Cation Exchange Capacity (CEC), Sorption Properties, and Extractable Fe2+ Measurements Measurements were carried out on each of the standard clays (STx-1b, NAu-1, SWy-1, SCa-3, SAz-2, SBld-1, NAu-2) and a set of homo-ionised montmorillonites (SM, Na, Mg, K, Ca) before and after γ-irradiation (5 MGy). 3.3.4.1. Cation Exchange Capacity The procedure was adapted from Summer and Miller (1996). A saturating solution of NH4OAc (77 g, 1M) in DIW (800 mL) was pH balanced (Mettler Toledo) using NH4OH (1M) and acetic acid (10%) to pH 7, the solution was diluted (DIW) to give a NH4OAc saturating solution (1 L, 1M) (Van Reeuwijk, 2002). Clay (5 g) was placed into a polypropylene centrifuge tube (50 ml) and NH4OAc saturating solution (25 mL, 1 M) was added. The sample was shaken for 2 hours and additional NH4OAc (25 mL, 1 M) was added and sample left overnight (24 hrs). The supernatant was filtered through a Whatman Filter paper (0.22 μm) into a Buchner volumetric flask (200 mL) and the theoretical CEC was calculated indirectly by measuring the cations (Na+, Mg2+, K+, Ca2+, Fe2+/3+, Al3+, Si4+) present in the extract. 3.3.4.2. Inductively Coupled Plasma–Atomic Emission Spectroscopy (ICP- AES) and Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) ICP-AES analysis was used to calculate the CEC and rough colloid concentration (ratio of Al to Mg ions; Laaksoharju and Wold, 2005) using a Perkin-Elmer Optima 5300 dual view ICP-AES to analyse cations present in extractant (Na+, Mg2+, K+, Ca2+, Fe2+/3+, Al3+, Si4+ - ppm concentrations). An Agilent 7700x ICP-MS in a clean room (class 1000) was used to measure chromium (≈3 ppb) and uranium (≈30 ppb). Both techniques required the removal of larger particles (<0.45 μm). ICP-AES required total dissolved solids to be less than 10,000 ppm (or 1-5%), typically giving ppm concentrations (but possible to reach ranges of 10–100 ppb). ICP-MS required the total dissolved solid to be less than 97 1000 ppm (<0.1%), typically giving ppb concentrations (down to 0.01–0.1 ppb). Up to seven ions can be measure in solution simultaneously, across a broad range of concentrations (Burrows et al., 2017; Mendham et al., 2000; Nölte et al., 2003). ICP-AES and ICP-MS both use a nebuliser to convert liquid samples into an aerosol mist before introducing into an Ar plasma stream. Plasma is created by ionising gaseous Ar atoms with a magnetic field, triggering a cascade reaction (through collisions). When samples are injected to the plasma, electrons are lost through interactions with the mobile Ar+ ions. ICP-AES excites atoms causing the emission of photons at given wavelengths as they return to ground state, each element emitting energy at a specific wavelength. The intensity of the energy emitted at the element specific wavelength is proportional to the amount of that element present. ICP-MS is operated under high vacuum, passing ions through a quadrupole with an alternating voltage allowing ions of specific charge/mass to reach the detector at each voltage and giving a mass spectrum that can be compared to known standards. 3.3.4.3. Ferrozine Assay Clay (0.2 g) was added to HCl (5 mL, 0.5 M, ≈pH 0.5) and Fe extracted at room temperature over 1 hr (Lovley and Phillips, 1987). The sample (5 mL) was centrifuged (3000 rpm, 5 mins) and a solution extract (0.1 mL) was added to deionised water (3.0 mL), ferrozine reagent (0.3 mL, 1 g L-1 ferrozine in 50 mM HEPES buffer, pH 7.0), and left to stand for 10 minutes before measurement (Stookey, 1970; Violler et al., 2000). 3.3.4.4. Ultraviolet–visible Spectroscopy (UV-Vis) Extractable Fe2+ was measured using the ferrozine assay on a UV-1800 Shimadzu UV spectrophotometer at 562 nm to determine the content of extractable Fe(II) from clay minerals before and after γ-irradiation (5 MGy) (Stookey, 1970; Viollier et al., 2000). UV-Vis spectroscopy is a quantitative method that involves the absorption of ultraviolet (190–400 nm) and visible (400–800 nm) electromagnetic radiation by molecules allowing the determination of concentration (Gauglitz and Moore, 2014; RSC 2009; Atkins et al., 2018). The intensity of light passing through both a reference cell (Io) and the sample cell (I) is measured over a range of wavelengths allowing concentration to be ascertained from the Beer-Lambert Law with the use of standards and a known molar absorption coefficient (Equation 2.3 and Equation 2.4). 98 퐼0 퐴 = 푙표푔 10 퐼 Equation 2.3. Absorbance of light through sample 퐴 = 휀푐푙 Equation 2.4. Beer-Lambert law; A=absorbance, l= path length, c=concentration (mol dm-3), and ε=molar extinction coefficient (dm3 mol-1 cm-1). 3.3.5. Electron Paramagnetic Resonance spectroscopy (EPR) EPR spectroscopy was used analyse radiation induced defects present in heat treated (25, 90, 160 and 500 °C) and γ-irradiated (5 MGy) clay samples (0.2 g). A Bruker EMX Micro X-band EPR Spectrometer (Figure 3.8) was run at a frequency of 9.5 GHz with a 1 Tesla electromagnet (modulation amplitude = 5 G, microwave power = 2 mW, attenuation = 20 dB, receiver gain = 30 dB, centre field = 2500 G, and sweep width = 5000 G and at 298 K). A strong pitch standard was used for calibration (g = 2.0026, frequency = 9.858527 GHz) and the values (g) of the main spectral features were calculated and plotted up in Origin Pro for comparison with previously published papers (Allard and Calas, 2009; Allard et al., 2012). Figure 3.8. Bruker EMX Micro X-band EPR Spectrometer (at the Photon Science Institute, University of Manchester. EPR spectroscopy is used to study molecules or atoms with unpaired electrons. EPR spectroscopy depends on the quantification of angular momenta of one or more unpaired electrons that reside within a structure and can interact with an electromagnetic field (electronic and magnetic field perpendicular to one another) giving magnetic dipole moment changes. The excitement of electron spins within samples through absorption of a photon of correct energy (hν) corresponds to a 99 change in the electronic Zeeman level from a alteration in spin orientation and is related through the EPR resonance condition giving the separation between spin states (Weil and Bolton, 2007; Kevan, 1997; Wertz, 2012) ℎ휈 = 푔푒휇퐵퐵푂 Equation 2.5. Transitions between electronic Zeeman levels; where h is Planck’s constant -34 -1 (6.62607x10 J s), ν is the frequency (s ), ge is the free-electron Zeeman factor (2.0023), 휇퐵 is the Bohr –24 Magneton (9.274 × 10 J/T) and B0 is the magnetic field strength (T) (Atkins et al., 2009; Weil and Bolton, 2007). Varying the photon frequency (ν) of the incident beam on the sample at a constant magnetic field constant (or vice versa) gives information on the environment of the unpaired electron; electronic structure, geometric structure, physical properties, reactivity, and dynamics in clay minerals and other paramagnetic materials (Weil and Bolton, 2007; Atkins et al., 2009) 3.3.6. X-ray Computed Tomography (XCT) XCT was used to give a 3D representation of clay structure prior to and after α-irradiation, giving a detailed structural representation of physical changes in mineral structure (particle size (nm) to cracking (>µm)). XCT was carried out on thinned sectioned clays (≈100 µm) mounted on the end of pins. XCT measurements were carried out on a Zeiss VersaXRM-520 system (1.5 s exposure, 3 μm per pixel, 60 kV, 5 W, 4 x objective lens, under and atmospheric environment), with source and detector positions maintained, at the Manchester X-ray Imaging Facility (Figure 3.9). AvizoFire 7.1 software (FEI: Thermo Fisher Scientific) was used to compute the 3D X-ray images. A comparison of pore density before and after α-irradiation was achieved through averaging of a sub-volume as a 2D schematic and plotting as a 1D damage vs depth graph in MATLAB. Crack propagation was also followed through visualisation of damage in imaged cross sections taken at points traversing each sample. 100 Figure 3.9. Zeiss Xradia Versa 520 XCT system (at the MXIF facility) XCT is a non-destructive technique for visualizing interior features within solid objects and 3D geometries and properties of samples. XCT has been used: (i) in radioactive waste processing (Primrose, 2015); (ii) in mineral transportation studies (Tan and Dong, 2015); and (iii) in the medical industry (Kalender, 2006; Hampel, 2015). In 3D XCT, X-rays are directed through an object at multiple orientations (0 to 180 o) and the decrease along each linear X-ray path is measured. Attenuation of the material is dependent on the density and atomic number, discriminating structures through differences in attenuation and allowing phase identification (Hampel, 2015; Salvo et al., 2003; Salvo et al., 2010). Beer-Lambert law is used to describe intensity reduction as a function of X-ray energy (l/l0), path length (x), and a material linear attenuation coefficient (μ) summed for multiple materials studied (Ketcham and Carlson, 2001). Multiple scans are combined to give 3D projections (Figure 3.10). ∑ −휇푥 퐼 = 퐼0푒푥푝 Equation 2.6. Beer-Lambert law in relation to XCT 101 Figure 3.10. Reprint from Salvo et al. (2010); generalised overview of XCT. 3.3.7. Colloidal Measurements Particle size analysis (photon correlation spectroscopy), zeta potential measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to examine changes to the clay colloids before and after γ-irradiation. 3.3.7.1. Atomic Force Microscopy (AFM) AFM was carried out using a Bruker Dimension 3100 (equipped with Nanoscope IV controller) at the Institute for Nuclear Waste Disposal (KIT). Particle size distribution, particle deposition/detachment at varying pH values (5 and 9) and force measurements were taken allowing the effects of γ-irradiation (5 MGy) on the clays to be examined. AFM allows depth characterisation of mineral surface features, building images (nanometre resolution) up based on the interaction of a examine (cantilever) with atomic forces such as van der Waals, electrostatic, hydrophobic, hydrophilic, and capillary interactions on the mineral surface. Topographic images produced relate to the height of features and measured deflection towards or away from the surface (attractive or repulsive) using incident laser beam and position-sensitive photo diode (PSPD) (Kaupp et al., 2006; Hunger et al., 2000). AFM can be run in contact (scraped 102 along surface), non-contact (vibrated) and tapping mode (oscillated) (Binnig et al., 1986). 3.3.7.2. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) Colloids were mounted dry on carbon plates and imaged on a Phillips XL30 ESEM-FEG (15 kV) with a Princeton Gamma Tech EDX system or on a FEI QUANTA 650 FEG environmental scanning electron microscope (nm resolution at 30 kV) at the Institute for Nuclear Waste Disposal (INE, Karlsruhe) to give morphology and size. SEM-EDS has been used routinely in environmental mineralogy studies (Gottlieb et al., 2000). Colloid samples were analysed using a highly focused, raster scanning electron beam to give high resolution images, predominantly from backscattered electrons (difference in atomic number) and secondary electrons (topography and morphology). EDS measurements on the characteristic X-rays produced allowed the quantitative and qualitative analysis of elements in the sample (Stokes, 2008; Davis, 2009). 3.3.7.3. Zeta Potential The surface charge on the colloidal particles in solution was measured using a Melvern Zetasizer and a Brookhaven ZetaPlus (NanoBrook Omni) zeta potential analyser using a Smoluchowski model for calculations. Zeta potential (ζ) is the average electrokinetic potential at the surface of a mobile charged particle, in the electronic double layer (at the zone of shear), immersed in a conducting liquid (Zasoski et al., 2008). Factors influencing zeta potential in colloids are ionisation of surface groups, surface co-ordinated materials and charges on the crystal structure, an effect of γ-irradiation (Allard and Calas, 2009; Holmboe et al., 2009; Hunter, 1988). Zeta potential can be calculated from Equation 2.7 using electrophoretic mobility (ue) 휈 from measured velocity (ν ) over the applied field ( 푒), the permittivity of the vacuum e 퐸 (ε0), dielectric constant of water (D), viscosity of water (η), and the Debye Hückel term from κ (mixture of terms) and a (particle radius) (Zasoski et al., 2008; Hunter 2001). 103 2ε표퐷ζ 푢 = 푓(κ, 푎) 푒 3η Equation 2.7. Calculation of Zeta potential using electrophoretic mobility measurements (Hunter, 1981) This equation can be simplified to the Smoluchowski equation (Equation 2.8) if the particle is large with respect to the double layer i.e. f(κ,a)= 1.5; an approximation used in the measurements within chapter 4. ε표퐷ζ 푢 = 푒 η Equation 2.8. Smoluchowski Equation (Zasoski et al., 2008) 3.3.7.4. Particle size analysis Particle size analysis (photon correlation spectroscopy, PCS) was carried out on a Malvern Mastersizer 3000 using a laser diffraction system (size range <50 nm–3.5 mm) and a Malvern NanoSight NS300. PCS uses Brownian motion of particles in solution (Hida, 1980) encompassing random motion due to collisions from the thermally active surrounding medium. Light from a laser is scattered by the sample and scattered light is collected at detectors allowing the speed of Brownian motion to be calculated. The speed is affected by the molecular weight, shape, and size (size range 10–2000 nm) of the particles present and through the reflective indices of the particles and solvent (Schmitz, 1990). The factors are related through the Stokes-Einstein equation used to estimate the particle size distribution (Equation 2.9). 푘푇 퐷 = 6휋휂푅 Equation 2.9. Stoke-Einstein equation (Edward, 1970); using the diffusion coefficient (D), Boltzmann constant (k), absolute temperature (T), viscosity of solution (η), and the hydrodynamic radius (R) of the particle. 3.3.8. Synchrotron Measurements Synchrotron measurements were made at the Diamond Light source on beam lines I18, B18 and I20. XRD and X-ray adsorption spectroscopy (XAS) studies on montmorillonites (STx-1b, SCa-3, SBld-1, and SWy-1) and nontronites (NAu-1 and 104 NAu-2) before and after γ- and α-irradiation were undertaken, with subsequent studies on sorption of uranium on clays (STx-1b and NAu-1) after γ-irradiation (5 MGy). XRD studies were predominantly conducted on the d001 basal spacing (6.7 keV) with higher energy scans (12 KeV) showing longer range ordering within the minerals. XAS studies at the Ca-, K-, and Fe K-edge and U L3-edge were undertaken to examine oxidation state and geometry of species present in clay mineral samples. Electromagnetic radiation is generated by a synchrotron through the constraint of charged particles (electrons and positrons) to a circular trajectory, commonly known as synchrotron radiation, releasing high intensity radiation over a broad spectral range (Gerson et al., 2006). Particles were initially accelerated before entering a booster ring, were partiles are further accelerated before injection to the main storage ring, under ultra-high vac (>10-10 torr) using bending magnets to steer the beam (keep in a closed loop) and further accelerated using wigglers or undulators producing elliptically polarised radiation (Gerson et al., 2006; Bunker, 2010). Beamlines were comprised of many components focusing the synchrotron radiation and selecting desired wavelengths for analysis using apparatus such as a monochromator and mirrors, measuring the outcome using a photon detector system such as a Si/Ge detector for XAS or a CDD detector for XRD (Van Bokhoven and Lamberti, 2016; Bunker, 2010). 3.3.8.1. XRD measurements High resolution XRD and μXRD was undertaken on beamline I18 (Figure 3.11) at the Diamond Light Source supplying synchrotron radiation (6700 eV), using a CCD detector (3 μm spot size), LaB6 standard (peaks at 16 KeV, 0.7749 Å; and 6.7 KeV, 1.8505 Å), and DAWN processing software for the analysis of data (Basham et al., 2015; Bower et al., 2015). Clay pellet samples (13 mm) were mounted on glass slides and thinned (100 μm) before mounting on larger metal sample holders for XRD analysis, or smaller samples (≈50–100 μm, α-irradiated) were fixed on kapton tape directly and attached to the same sample holders. 105 Figure 3.11. Beamline I18 XRD set up (Diamond light source, UK). 3.3.8.2. XAS analysis XANES and EXAFS data was collected on multiple beamlines the Diamond Light Source: I18 – Energy range 2.05–20.5 keV, Flux 2x1012 photons/s, beam size 2–7(H) x 1.8(V) μm, and spatial resolution of 2x2 µm, using a Si(111) detector with aluminium attenuation to give >10,000 counts total (Mosselmans et al., 2016). A Fe metal foil standard (Fe) and calcium oxide standard (CaO) were used for calibration and comparison purposes. Fe K-edge data was collected collect in transmission mode and Ca and K K-edge data was collected in fluorescence mode at a number of points across a number of clay pellet samples (NAu-2, SCa-3, Bld-1, SWy-1 and STx-1b) to examine the effect of heating, γ-irradiation and α-irradiation on oxidation state if structural (Fe) and interlayer species (Ca, K). Samples were mounted on metal XAS sample holders either as clay pellets sandwiched in kapton tape, clay pellets mounted on glass slides or as smaller fragments on kapton tape under atmospheric conditions (298 K). B18 – Energy range 2.05–35 keV, Flux 5x1011 photons/s, and beam size 200 (H) x 250 (V) μm, using a Si(111) detector (2.05–20 keV) or Si(311) detector (4–35 keV). Fe samples were diluted with cellulose to give the necessary absorption coefficient for the Fe K-edge (Dent et al., 2009). A Fe metal foil standard (Fe) and calcium oxide standard (CaO) were used for calibration and comparison purposes. Fe K-edge data was collected in transmission mode and K and Ca K-edge X-ray absorption spectra were measured in fluorescence mode, mapping a number of spots 106 on samples that allowed the detailed analysis of oxidation state and co-ordination geometry of both structural (Fe) and interlayer cations (Ca2+, K+) with regards to heating, γ-irradiation and α-irradiation. Samples were mounted on metal XAS samples holders as pellets held between kapton tape under ambient conditions (298 K). I20 - energy range 4–34 keV, Flux > 1012 photons/s, and beam size 400 (H) x 300 (V) μm, using a Si(111) detector (4–20 keV) or Si(311) detector (7–34 keV) (Diaz-Moreno et al., 2018). Samples were diluted with Y foil to reduce elastic scattering and reach the necessary absorption coefficient within samples (>200 KHz total counts). A Y foil was used a standard for calibration before and after measurement’s to examine any changes in the beam and a U(VI) schoepite, U(IV) uraninite and U(IV) silicate were used as calibration standards for data obtained. U L3-edge data was collected in fluorescence mode, mapping a number of points across the sample to generate an average U profile for samples before and after γ-irradiation, probing the oxidation state changes and co-ordination geometry of uranium sorbed in the clays samples. Samples were measured as pastes sandwiched between wax in XAS sample holders and samples were run under vacuum at liquid nitrogen temperature (77 K). XAS is based on the photoelectric effect using incident X-ray photon wavelengths on the order of Angstroms (0.1 nm), with energy ranges spanning from a few KeV (hard) down to hundreds of eV (soft). XAS gives information on the local structure of multi- component materials, for example probing reactive sites on clay mineral surfaces (Bunker, 2010; Stumpf et al., 2004; Nachtegaal et al., 2016). The excitation of photoelectrons from a target material causes interferences between surrounding atoms, constructively and destructively, proportional to the X-ray absorption coefficient, μ(E). XAS measures the linear absorption coefficient, μ(E), giving a spectrum that is composed two key fragments; the X-ray absorption near edge structure (XANES) centred around the a main edge specific to an element and extends up to 50 eV past the edge (excitation and adsorption of electrons), and extended X-ray absorption fine structure (EXAFS) after the edge related to photo-electron oscillations and vibrations 107 ranging as far as 1000 eV past the edge. A spectrum is generated from measurements of μ(E) taken over a range of X-ray energies that cause adsorption and subsequent excitation (and ejection) of core electrons. The μ(E) can be calculated directly from the fraction of a beam radiation absorbed per unit thickness of absorber (transmission) or indirectly from variation in the specific fluorescence emission lines generated by electrons filling core orbitals holes or ejected electrons as the hole is filled (Auger electrons) giving information the local structure of the bulk material (Gerson et al., 2006; Bunker, 2010; Koningsberger and Prins, 1998). The linear adsorption coefficient (μ(E)) is specific for each crystalline solid (element specific) and related to the adsorption cross section (σ), number of chemical elements (per unit cell) and volume (unit volume) e.g. the density (ρ) shown in equation 2.10 and described according to Femi’s golden rule comparing the initial state, interaction states and final state of the X-rays. 휌푚푍4 휇 = 퐴퐸3 Equation 2.10. Absorption coefficient is dependent on atomic mass (Z), sample density (ρ), X-ray energy (E), and atomic mass (A) (Wilke, 2018). In transmission mode Bouguer’s Law gives the absorption coefficient at each X-ray energy giving the attenuation of the X-ray flux (I) measured as a function of the photon energy and sample thickness (d) shown in Equation 2.11. In fluorescence measurements intensity is proportional to the absorption coefficient μ, measured through incident flux (Io) and fluorescence X-rays, approximated by Equation 2.12, and using a detector angle (normally 90o) and sample angle (normally 45o) to minimise elastic scattering effects (exploiting polarization) that can cause unwanted noise in the measurements (Joly et al., 2016; Bunker, 2010, Calvin, 2013, Gerson et al., 2006). 퐼표 퐼 = 퐼 e−휇(퐸)푑 or 휇(퐸)푑 = 푙푛 ( ) 0 퐼 Equation 2.11. Adsorption measurements in transmission mode; μ(E) is the linear energy coefficient, d is the sample thickness and I is the intensity initially (Io) and after interaction with sample (I). 퐼 ≈ 휇(퐸)퐼0 Equation 2.12. Relationship between adsorption measurements taken in fluorescence mode. 108 XAS is an element specific tool (to the absorbing atom) exploiting the characteristic binding energy of electrons in elements, fine tuning of the incident X-ray beam to target specific electron transitions. These are known as adsorption edges showing a strong increase in adsorption intensity at specific energies where electron binding energies are met and electrons are ejected (Ravel, 2016; Gerson et al., 2006). Adsorption edges are named using the electron orbital principle quantum number; K-edge relates to 1s electrons, L-edge to 2s or 2p electrons, and M-edge to 3s, 3p, 3d. Pre-edge features (before the edge), from quadrupole contributions, may be observed if insufficient energy is absorbed by inner-shell electrons to move to higher unoccupied orbitals such as those seen in the pre-edge region of clay Fe XANES from the forbidden 1s-3d electronic transition related to octahedrally or tetrahedral co-ordinated Fe (Figure 3.12) in clays. 1s-3d transition Oh and Td Figure 3.12. Example of a pre-edge feature in NAu-1 Clay from a 1s-3d transition. The EXAFS equation (Equation 2.13) is an extension of the photoelectron wavenumber (k) taking into account: the probability of absorption of a photoelectron (χ); the scattering probability of the photoelectron using a proportionality constant for spherical waves (f(k)) altering with the atomic number of the absorbing atom; the average distance from the absorbing atom (D) also shown as the interatomic distance (R); the number of multiple scattering paths (N); the phase shift term (cosine to sine) with an associated proportionality constant for the shift (δ(k)) for the photoelectron passing the potential of the absorbing and scattering atoms dependent on the atomic number of the absorber; the effect of incomplete overlap from the increased positive 2 charge on the nucleus covered through an amplitude reduction factor (푠0 ); the mean free path (λ(k)) considering other fates of the photoelectrons and limiting the range of the photoelectron to local environment (>10 Å negligible effects seen) giving EXAFS 109 sensitivity on a nm scale specific to each element; the adjustment for different environments of the absorbing atom (phases, static disorder, gradients in materials, and thermal disorder) through a mean square radial displacement factor (σ2) also called the Debye Waller Factor; all of which is summed over i to account for the multiple neighbour possibilities (Calvin, 2013; Koningsberger and Prins, 1998; Wilke, 2018). f(k), δ(k), and λ(k) can be calculated using computational methods such as FEFF. 2퐷푖 푓𝑖(푘) − 2 2 휒(푘) = 푠2 ∑ 푁 푒 휆(푘)푒2푘 휎푖 sin (2푘퐷 + 훿 (푘)) 0 𝑖 푘퐷2 𝑖 𝑖 𝑖 𝑖 Equation 2.13 . The EXAFS equation (Calvin, 2013). There are many advantages to the use of XAS; it is non-destructive so samples are maintained for multiple measurements at various time points, it allows for in-situ studies, it can be run under extreme temperatures and pressures and on small sample volumes and concentrations (Newville, 2005). Some draw backs to XAS analysis are that although element specific, elements close in atomic number (≈5 mass units) are hard to distinguish and may interfere if a number of elements in close relation are in the same sample. The co-ordination number of the elements is hard to define unequivocally (±1 or ≈25%) due to the counter dependence on a number of factors 2 2 such as the amplitude dampening factor (푠0 ) and the Debye Waller factor (σ ). The spectrums observed are an average so mixed-oxidation states within samples can be hard to ascertain and separate and the self-absorbance within minerals containing higher quantities of certain elements (e.g. Fe) can occur giving lower than expected values. Interatomic distances can also be obtained but are an average and show some dependence on other factors with values accurate to 0.02 Å (Ravel, 2016). 3.3.8.2.1. X-ray absorption near edge structure (XANES) The XANES region of the spectrum (Figure 3.13) includes pre-edge features, adsorption edges and post edges (up to 50 eV past edge) and can be used to determine oxidation state and geometries of different elemental species (i.e. Ca, K, Fe and U) through tuning X-ray intensity to target specific absorption edges within the elements of interest (Calvin, 2013). 110 XANES EXAFS Edge (E0) Pre-edge Near-edge Figure 3.13. A Ca-K edge XAS spectrum, highlighting the different sections of interest. The edge is the key feature of the XANES region, specific to each shell and element, related to a sharp rise in the linear adsorption coefficient (μ) and is representative of the binding energy of an electron in the target electron shell and softly linked to Eo (Joly et al., 2016). The final states of K- and L1-edges are p-states such as that shown in Figure 3.13 for a Ca k-edge 1s-3p transition (≈4035 eV) and the final states in L2- and L3- edges are a mixture of d- and s-character as seen for the U L3-edge 2p-3d transition (≈17176 eV) (Chapter 6). Differences shown in these distinct edge positions occur from changes in the oxidation state and site symmetry of the absorber and thus changes in charge density and bond length. Increases in the edge position energy with higher oxidation state atoms are shown due to increased effective charge and reduced bond lengths (Calvin, 2013; Bunker, 2010). For instance seen between the U(VI) phase schoepite and U(IV) phase uraninite (Figure 3.14), where U(VI) is shown to have a higher edge energy (4-8 eV) (Newsome et al., 2015) or in Fe XANES for Fe(II) vs Fe(III) (Wilke et al., 2001). If no changes are possible, such as in Ca or K K-edge XANES then any changes observed are linked to surrounding atoms. White line maximum is often linked to the edge and comprises the sharp feature at the top of the edge, influenced 111 by a number of factors such as oxidation state, covalency of the bonding, and local symmetry (Denecke, 2006). Figure 3.14. Uranium L3-edge XANES of schoepite (black) and uraninite (yellow). Pre-edge features are features at lower energies than the edge arising from transitions from core level to a lowest unoccupied level. Exemplified in the forbidden 1s-3d transition seen in Fe XANES (Figure 3.12), observed due to distortion of octahedral sites and often explainable through molecular orbital theory, indicating local site symmetry and orbital occupancy of the absorbing atom (Bunker, 2010; Gates, 2013). Strong oscillations after the edge (up to 50 eV), the near edge, are related the ionisation of core electrons within the structure which give 3d geometrical information on the absorbing atom from multiple scattering of atoms in the local vicinity linking closely with the co-ordination within the absorbing species (Gates, 2013). This is visualised in Figure 3.14 (black line) where uranyl co-ordination (U(VI)) gives a shoulder in the XANES (17190 eV) just after the edge from multiple scattering within the linear uranyl ion structure from short axial U=O bonds (Farges et al., 1992; Hudson et al., 1999) The analysis of XANES spectrum uses a reference spectra (standards) of a known value such as Fe foil (7112 eV) for Fe K-edge XANES or a standard without multiple oxidation states such as Y-foil for U L3-edge XANES. This process is known as fingerprinting and either uses theoretical standards (calculation of expected standard spectra) or 112 empirical standards (known samples) for comparison. Mixed oxidation states are determined through linear combination analysis (LCA) using a library of standards and allowing computer generation of linear combinations allowing relative amounts of constituents to be found (Calvin, 2013). 3.3.8.2.2. Extended X-ray absorption fine structure (EXAFS) EXAFS is routinely used to study clay mineralogy and reactions (Gates, 2013; Greathouse and Cygan, 2005; Denecke et al., 2003). EXAFS provides information on interatomic distances, the mean square deviations in these distances, coordination numbers of surrounding atoms around a central absorber, and the coordinating species (Ravel, 2016). EXAFS comprises the later part of the XAS spectrum (Figure 3.13) towards the higher energy region past the adsorption edge into a region called the post edge (from 50 eV up to 1000 eV), normally reducing in quality with distance from edge. Oscillations are measured in the EXAFS region as interference (constructive or destructive) from ejected electrons (X-rays) that propagate as spherical waves scattering off surrounding atoms (elastically) and backscattering electrons giving maxima and minima in the EXAFS oscillations measured. These oscillations give information on the number of backscattering atoms and the distance and characteristics of these atoms (Gerson et al., 2006). EXAFS are fully described through the EXAFS equation (Equation 2.13) and are measured as sinusoidal oscillations (due to a phase shift factor) in photoelectron wavenumber k (Å-1) plotted in k-space. This conversion to photoelectron wavenumber 1 ℎ is achieved through the relationship to photon energy; (where ħ = ) times the ħ 2휋 square route of the difference in X-ray incident energy (E), less the energy of the white line (Eo), giving the kinetic energy of the emitted photoelectron and relating this to the mass (me) of the photoelectron (Equation 2.14) (Calvin, 2013). 1 푘 = √2푚 (퐸 − 퐸 ) ħ 푒 0 Equation 2.14. Conversion of photon energy to photon wavenumber (Calvin, 2013). 113 A slow decrease in the absorption rate versus energy with superimposed oscillating structures is countered using k-weighing (to K2 or K3) to enhance the signal at higher energies and a Fourier transform of this data gives a pseudo-radial distribution function from component sinusoidal frequencies that is easier to interpret and can be modelled to give radial distances and co-ordination structures (Joly et al., 2016). The frequency of the oscillations is inversely proportional to the distance of the scatterer (R) with faster oscillations relating to longer path lengths. The amplitude is related to degeneracy of the signal (N) giving rise to larger peaks with increased amplitude and the shape giving the chemical identity using a structural model (bunker, 2010). EXAFS measurements are summed averages that give information on the average co-ordination environment of the absorbing species. 3.3.8.3. XAS data processing EXAFS data are fit using Athena and Artemus programs, part of the Demeter software package (Ravel and Newville, 2005), using FEFF calculations to give information on possible shells (from .cif files of known structures run in atoms). When fitting parameters that are chosen care must be taken to avoid false minima (from the oscillations), use only pertinent paths due to the limited number of parameters available, and to choose physical appropriate paths. Multiple scattering paths are dependent largely on angle and are normally excluded from fits; however, in higher 2+ o angled scattering paths like that of the UO2 ion (180 , Chapter 6) it must be considered (Koningsberger and Prins, 1998; Bunker, 2010). Realistic and defensible models are linked to other complimentary analytical techniques such as XRD or liquid phase analysis (ICP-MS). Background subtraction, normalisation and liner combination fitting was carried out in Athena and EXAFS analysis was carried out in Artemis using FEFF calculations (using data from known crystals structures) to give reasonable models for the co-ordination environment in each structure. Scan files were collected in transmission mode (lnI0/It) or fluorescence mode (FF/I0) as a function of energy. Multiple scans of the same sampling point (on a single sample) were merged to reduce the signal-to-noise ratio of the data. Each data set edge was shifted using a known standard, e.g. Fe metal (7112 eV) to account for beam drift. The U L3-edge data used a yttrium standard (K-edge 114 17038 eV). The edge (Eo) was selected for each average sample point, making use of the 1st derivative of the data (gradient measure) to show the point at which a maxima occurred (Figure 3.15). st Figure 3.15. 1 derivative of U L3-edge data; E0 is the point at which the gradient reaches a maximum. The pre-edge and the post-edge of the data is fit through linear regression to determine the normalization constant (Figure 3.16), effectively allowing background subtraction of the data, minimising any pre-edge noise and cutting data evenly in the post-edge region (starting ≈100 eV past the edge). This lessens the low frequency components of µ(E) in the post-edge and should give parallel pre- and post-edge lines in the normalised spectrum, with a spectrum that oscillates around µ(E) = 1 (EXAFS region) (Ravel, 2016). E0 Post-edge line Δμ0 Pre-edge line Figure 3.16. Showing a Fe K-edge XAS spectrum of montmorillonite, including the fitted pre- and post- 115 edge linear regression lines to allowing normalisation of the spectrum (Δμ0 is the measured jump at the absorption threshold energy E0) Visualising data in K-space (Å-1) using a higher K weighting (i.e. K3) allows the edge fit to be checked and should give a clear oscillating shape out to the scan K range limit. A Fourier transform of this data gives a pseudo-radial distribution function (in R-space, Figure 3.17) effectively giving the individual components around the central absorbing atom, representative of interatomic correlation (not phase shifted yet). In R-space the window used to fit the data (i.e. Hanning) and k-range to fit over (window of visualisation) must be chosen to allow relevant features to be resolved and so not to include more data than required. The data background subtraction (Rbkg) can be altered to cut out unrealistic features seen in R-space (structures at <1 Å), but not at the expense of the data and no bigger than approximately half of the interatomic distance to the nearest neighbour, for instance in Fe at ≈1 Å (Fe-O seen at ≈2.01 Å in clays) (Ravel, 2016; Calvin, 2013). Figure 3.17. Showing Ca k-edge data before (black) and after (red) γ-irradiation, plotted in k-space and the Fourier transform of the data in R-space (taken from chapter 4). 3.3.8.3.1. Linear Combination Analysis (LCA) LCA was used to determine the relative quantities of compounds present in a sample, and required the use of standards to compare XAS datasets within energy space (μ(E) vs. eV) or k-space (μ(E) vs. Å-1). Empirical standards for U(VI) and U(IV) (schoepite and uraninite) were used to giving a best match fit for the amount of each constituent. A large uncertainty can occur due to changes in data normalisation, energy alignment, background subtraction, different attenuation of measurements, and noise in the data (Calvin, 2013) 116 3.3.8.3.2. EXAFS The EXAFS equation (Equation 2.15) was used to sum the contributions seen form individual scattering atoms to give information on the co-ordination geometry of the absorbing species, computed and modelled in Artemis on k-space and R-space experimental data (Ravel and Newville, 2005). 2푅푖 푓𝑖(푘) − 2 2 휒(푘) = 푠2 ∑ 푁 푒 휆(푘)푒2푘 휎푖 sin (2푘푅 + 휙 (푘, 푅)) 0 𝑖 푘푅2 𝑖 𝑖 𝑖 𝑖 Equation 2.15. The EXAFS equation, phase shift; with theoretical (red) and calculated (blue) variables. Artemis EXAFS modelling in Artemis is based on the use empirical standards from crystallographic data of known compounds that related to the structure under study. Imported crystallographic information files (CIF) from the American Mineralogist Crystal Structure Database (AMCSD) were used to give potential paths, and phase shifts were accounted for through FEFF6 calculations (Rehr et al., 2009) using files generated in ATOMS (Dowty, 2006). These data files may require small changes prior to use in modelling complex structures, for example in uranium sorption onto a montmorillonite or a nontronite, modelling U-Si distances through the substitution of a uranium central atom into a nontronite structure (U92 instead of Fe56); however, most paths are obtainable from known crystal structures (i.e. schoepite and uraninite). The red values in Equation 2.15 are theoretically calculable parameters from known structure calculations (FEFF calculations), minimising the 휒2 fitting metric (damped least squares) as a function of the fit parameters to maximise the fit in (k). The blue 2 2 parameters are those to be fit, equating to N, 푠0 , 휎 , E0, and R. Each scattering path used from the FEFF calculations is parameterised and refined to obtain values for the all parameters to be fit. This is initially achievable for the first co-ordination shell through a simple shell fit. Complete fits become untenable for larger fitting models where the number of fitting parameters exceeds the number of independent points (Equation 2.16); this varies depending on k-space and r-space the fit is over and gives the degrees of freedom for the model (as tend to zero fit reliability decreases). 117 2(푘푚푎푥 − 푘푚𝑖푛)(푅푚푎푥 − 푅푚𝑖푛) 푁 = 𝑖푛푑 휋 Equation 2.16. Number of independent points often referred to as the Nyquist criterion (Calvin 2013). 2 The fitted parameters possible to obtain in Artemus are 푠0 , relating to the amplitude reduction factor, 휎2, better known as the Debye-Waller factor, N, the co-ordination number, E0, the adsorption edge energy and R the interatomic distance (Å). To reduce the number of fit parameters the co-ordination number is often set to an expected 2 value for each path, the same 푠0 and E0 are maintained over all scattering paths (within one fit) and R and 휎2 vary with each path. 2 푠0 is a good approximation of intrinsic losses from passive electron adjustment and shake-up processes (outer electrons) causing suppression of the EXAFS signal. 2 Ordinarily 푠0 should not be smaller than 0.7 or larger than 1.05, with a value of 1 showing expected co-ordination of the path has been met, deviations away from 1 show a poor fit to the co-ordination environment. Due to multiple paths included in most fits and the large number of manipulating parameters such as self-absorption 2 2 effects (low 푠0 values seen), over fitting and sample geometry, 푠0 is often hard to fit 2 accurately (within 20-30%) (Denecke, 2006; Ravel, 2016). Occasionally the 푠0 is restrained or set to allow physically plausible fits to be obtained, for instance in UO2 2 (uranyl-U(VI)) EXAFS where U-O axial bonds cause mis-fitting of the 푠0 due to the large amplitude of this shell. The Eo is the edge energy but related to the energy required to calculate the momentum of a photoelectron. The E0 (Figure 3.16) or ΔE0 should be close to 0, avoiding the possibility of a false minimum in the EXAFS oscillations, with values of ± 10 eV plausible. 휎2 also known as the mean square relative displacement is normally between 0.002 and 0.003 Å2, increasing with distance from the absorber (up to 0.02 Å2 for longer range scattering atoms), and should never be negative (or less than 0.001 Å2). It equates to the variance in the half path length (equation 2.17) and is the measure of average disorder from thermal and static disorder effects correcting for differences in experimental and theoretical χ(k). 118 휎2 = ⟨(푟 − ṝ)2⟩ Equation 2.17. Mean square relative displacement (휎2); standard deviation of the half path length (r). R is related to the average interatomic distance (bond length) between the absorber and scatterer, commonly referred to a D (half path length), and effected through thermal (thermal motion) and static disorder (shape). The values obtained for R should resemble the theoretical standard being used (± 0.1–0.2 Å), if larger it suggests the path chosen was inexpressible of the real structure and a different path standard should be used (Calvin, 2013). N is the co-ordination number linked to the degeneracy of each path or the number of 2 identical paths that occur from a single scattering species. It correlated with 푠0 as a function of k and is normally set in a fit to a reasonable assumption as the calculation of co-ordination number is poorly resolved in modelling and normally associated with large errors (± 1 co-ordination number). Multiple k-weights are used in EXAFS analysis to minimise the 휒2 within the fit, larger values of k-weighting (i.e. 3) show misfits in 휎2 and lower k-weighting (i.e. 1) show misfit in E0, giving enhanced sensitivity across the entire k-range. A number of fitting parameters are used to model EXAFS, with some parameters correlating with 2 2 2 others; 푠0 and 휎 , and N and 푠0 effect amplitude as a factor of k, and Eo effects R influencing the slope of phase (Bunker, 2010; Calvin, 2013). A low correlation between parameters is desirable, indicating a robustness of the parameterization but correlations are common due to the limit of floating parameters possible and this should be limited (0.95) (Ravel, 2016). A number error analyses are built into the EXAFS modelling software (Artemis) to back up the models postulated. The R-factor quantifies the fit mismatch over all measured points (N) using the squared sum of the differences between data and fit (Equation 2.18). 푁 2 ∑𝑖=1(푑푎푡푎𝑖 − 푓𝑖푡𝑖) 푅 = 푁 ∑𝑖=1(푑푎푡푎𝑖) Equation 2.18. R-factor - the mismatch between data and fit. 119 The R-factor is simply a measure of how well the two lines (data and fit) match one another (Table 3.2) and not the statistical analysis of the actual model used. Table 3.2. Showing the interpretation of R-factor values, quantifying fit mismatch. R-factor Interpretation <0.02 Good fit 0.02-0.05 Model has some detail wrong or low quality data 0.05-0.10 Flaw in the model or very low data quality >0.10 Model maybe fundamentally incorrect The R-factor is a useful metric for judging misfit, but not fit quality. To make sure fits are physically defensible another factor, chi-squared (χ2) is also used. It weights the closeness of the fitted function to the data by the unused information content with a smaller value implying a ‘better fit’ (equation 2.19). 푁 푖푛푑 2 (푑푎푡푎𝑖 − 푓𝑖푡𝑖) 휒2 = ∑ 휀2 𝑖=1 𝑖 Equation 2.19. Quantifying the fit mismatch using statistical quantity χ2. Where ε is the measurement uncertainty (calculated by Artemus) associated with 2 point i, Nind is the number of independent points over which i is taken and χ is the statistical uncertainty measurement (Calvin, 2013). To identify the statistical ‘goodness of fit’ the statistical uncertainty measurement (χ2) 2 is converted to reduced chi squared (휒푣 ) where Equation 2.19 is modified to account for the degrees of freedom (υ) and number of independent points (Nind) (Equation 2.20). 푁 2 푁𝑖푛푑 (푑푎푡푎𝑖 − 푓𝑖푡𝑖) 휒2 = ∑ 푣푁 휀2 𝑖=1 𝑖 2 Equation 2.20. Reduced chi squared (휒푣 ) accounting for degrees of freedom. 2 When 휒푣 is used with other measures of fit an assessment of the ‘goodness of fit’ and comparative validity of different fitting models can be achieved, where fitting paths (backscattering atoms) decreasing multiple measures of mismatch are included in the 2 fit. A 휒푣 of ≈1 shows the data is consistent with the model, although this is rarely seen 120 in EXAFS modelling due to the uncertainties associated with natural systems, so the lowest possible value is targeted (>50). 3.4. Safety A high standard of safety was maintained following the University of Manchester health and Safety policy (University of Manchester Health and Safety Unit). 3.4.1. Laboratory Safety All reactions involving chemicals, heating, or centrifugation were carried in the laboratory using correct PPE (lab, coat, safety specs, and gloves where appropriate), following university guidelines for safe practice and subsequent to the completion of a chemical risk assessment in accordance with the Control of Substances Hazardous to Health Regulations 2002 (COSHH). Safety training courses were undertaken prior to lab work, and strict protocols and operating procedures were in place following the University guidelines (www.healthandsafety.manchester.ac.uk) and in accordance with Health and safety at work act 1974 and Environmental Permitting Regulations 2010. 3.4.2. Radiological Hazards The work involved the use of ionising radiation for various measurements and the use of a radioactive isotope 238U. Ionising radiation can be hazardous to human health and therefore additional protocols were put in place, including additional radiation training, required monitoring (lab and machine), and adherence to the Ionising Radiation Regulations 1999. Machines are fit with sufficient shielding to give no external dose and have safety mechanisms to shut off sources if not properly contained. The use of radioactive sources involves the completion of a radiological risk assessment and experimental protocol form prior to the undertaking of work in which the key hazards and prevention procedures are outlined, and areas of work designated. 238U is an α-emitter, and the annual limit on intake (ALI) was minimised; a worst case scenario showed a dose of 0.001 ALI (0.13 Bq) through inhalation of the top stock (214 Bq/ mL, 1.65 ALI/ mL). 3.4.3. 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Molecular- to meso-scale effects of heat and subsquent gamma radiation on engineered clay barrier performance for radioactive waste disposal This chapter is presented as a manuscript submitted to Applied Geochemistry (29.03.2018), currently in review. Supporting information for this manuscript is included immediately succeeding this document. 129 Molecular- to meso-scale effects of heat and subsequent gamma radiation on engineered clay barrier performance for radioactive waste disposal A. P. Simsa,b, J. M. Devinea, W. R. Bowera, K. Morrisb, R. A. D. Pattrickb, R. Edgec, S. M. Pimblotta,c,d, A. J. Fieldinge, T. Schäferf,g, G. K. Darbhag,h, J. F. W. Mosselmansi, F. R. Livensa, C. I. Pearce*a,j a Centre for Radiochemistry Research, School of Chemistry, University of Manchester, M13 9PL UK ; b Research Centre for Radwaste Disposal and Williamson Research Centre, School of Earth and Environmental Sciences, University of Manchester, M13 9PL UK; c Dalton Cumbrian Facility, University of Manchester, M13 9PL UK; d Idaho National Laboratory, Idaho Falls, 83402, Idaho, USA; e School of Pharmacy and Biomolecular Sciences, Liverpool John Moores University, L3 3AF UK; f Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Germany; g Friedrich- Schiller-University Jena, Institute of Geosciences, Applied Geology, Burgweg 11, D-07749 Jena, Germany; h Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, West Bengal, India; i Diamond Light Source, Harwell, Oxford, UK; j Pacific Northwest National Laboratory, Richland, Washington, 99352, USA. 4.1 Abstract Safe containment of long-lived, heat-generating radioactive wastes is crucial for the future of nuclear power; a geological disposal facility (GDF) is the internationally- favoured option. The host rock for geological disposal may vary, but most countries are considering an engineered clay barrier to protect the waste container against corrosion and limit radionuclide release after container failure. The clay barrier must have the necessary physical and chemical properties to fulfil its safety functions, despite challenges presented by the evolving radioactive waste. Exposure to heat (100–160 °C) and gamma radiation (γ, doses up to 5 MGy) would be the primary challenges over the initial several thousand years of disposal, prior to ground water infiltration and container corrosion. Previous work has shown that macro-scale changes under these conditions are negligible but, given the temporal scale over which the clay barrier is expected to perform its safety functions, a fundamental understanding of the molecular scale effects is essential to underpin predictions of 130 barrier performance. Here, a multi-technique approach has been employed to investigate the molecular- to meso-scale response of smectite, the main component of typical clay barrier systems, with sequential heating (up to 160 °C) and γ-radiation in a Co-60 irradiator (up to 5 MGy). X-ray diffraction (XRD) confirmed limited changes to smectite structural order as a function of heating and irradiation; however, minor but significant changes to hydroxyl content (measured using infra-red (IR) spectroscopy), cation exchange capacity (CEC) and extractable Fe(II) were clearly observed. X-ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR) spectroscopy of the representative montmorillonite STx-1b after exposure to heat and γ-radiation showed an increase in interlayer disorder and the formation of electronic defects. Atomic force microscopy (AFM) and zeta potential measurements demonstrated that γ-radiation also increased the clay surface charge, resulting in more stable clay colloids. These effects have the potential to impact clay safety functions associated with radionuclide sorption to limit transport after container failure. This new understanding molecular- to meso-scale effects provides the opportunity to utilise a readily available natural clay resource and optimise composition for resistance to heat and γ-radiation. This has the potential to reduce costs associated with heat dissipation by, for example: (i) shortening storage times necessary for cooling prior to HLW canister disposal; and (ii) reducing distances required between canisters, ultimately allowing for a more compact, economical, and safer GDF to be constructed. Key Words: Geological disposal; radioactive waste; clay barrier; mineralogy; gamma radiation; spectroscopy; diffraction 4.2. Introduction Currently, nuclear power accounts for approximately 15% of global electricity generation, and is considered as an affordable, dependable, and sustainable source of energy (Curtis and Morris, 2012). A legacy of radioactive waste has been produced as a result of energy and weapons production, and the safe containment of long lived, highly radioactive waste is crucial for the future development of the nuclear industry (BERR, 2008). In many countries, geological disposal has been chosen as the preferred method for managing higher activity radioactive waste for the long term (DECC, 2014). All proposed geological disposal facilities (GDFs) incorporate a multi-barrier system to 131 maximise the time between containment failure and the final interaction of released radionuclides with the biosphere (Baldwin et al., 2008; Svenke and Nilsson, 1983). In many proposed GDFs, the heat-generating radioactive waste package would be surrounded by an engineered clay barrier, which is intended to swell and fill the gap between the waste package and the host geology, minimising groundwater flow to diffusive transport for at least several hundred years. The clay selected for this purpose would need to have the necessary physical and chemical properties to protect the waste canister against corrosion, and also limit the release of radionuclides after container failure, in spite of challenges from heat, irradiation, and infiltration of groundwater (Wilson et al., 2011). Long-term prediction of engineered barrier performance in response to the coupled effects of heat and radiation from radioactive waste is essential to the GDF safety case (Bernier et al., 2007, Holton et al., 2012). Bentonite is commonly chosen as the barrier material for proposed GDFs due to its stability over geological time scales, small pore size distribution, low hydraulic conductivity, high swelling properties, and high ion exchange and sorption capacity for radionuclides (Hicks et al., 2009). Regardless of its source, bentonite for geological disposal consists predominantly (>75%) of montmorillonite, which is a di-octahedral aluminium phyllosilicate from the smectite group (Posiva, 2009). Additionally, quartz, feldspar, kaolinite, sulphides, carbonates, sulphates and organic matter typically are present as minor accessory phases within the bentonite, but their content is minimized to avoid detrimental effects, with smectite content dominating bentonite behaviour (Ahn and Apted, 2010; Karnland, 2010; Stanković et al., 2011). Di-octahedral smectites, including montmorillonite, nontronite, and beidellite differ in terms of their chemical composition and site occupancy, with Fe2+/3+, Mg2+ and Al3+ predominantly substituting into octahedral sites, and Fe3+, Al3+ and Si4+ predominantly substituting into tetrahedral sites (Brigatti et al., 2011; Wilson, 1978; Wolters et al., 2009; Christidis, 2011). These substitutions result in weak negatively charged clay layers (Figure 4.1), held together by electrostatic attraction to charge-balancing cations of different size, charge, and polarizability, located in the interlayer space (Savage and Authur, 2012). Hydration of these interlayer cations causes the clay to swell, filling surrounding gaps and cracks, stabilizing the canister, and making diffusion the 132 predominant transport mechanism whilst the clay barrier remains intact (Nagra, 2002; Missana et al., 2008; Beattie and Williams, 2012; Sellin and Leupin, 2013; NEA, 2013). Substitution of ions into the octahedral and/or tetrahedral layers has a significant effect on clay layer charge and expansion properties in smectites (Savage and Authur, 2012; Gorski et al., 2013: NEA, 2013). In addition, clays that are high in iron (nontronites) show enhanced redox reactivity due to the presence of electron transfer pathways in the octahedral sheet, which vary depending on Fe site occupancy and valence state (Stucki et al., 1984; Gorski et al., 2013; Lantenois et al., 2005; Neumann et al., 2015; Rosso and Ilton, 2003). The quantity of negatively charged sites that retain cations through electrostatic interaction on the surface and in the interlayer is determined as the cation exchange capacity (CEC). The CEC can vary significantly depending on composition from, for example, 34 meq/ 100 g in Friedland Clay (mixed montmorillonite-illite) and 76 meq/ 100 g in MX-80 (sodium bentonite), up to a maximum of ≈120 meq/ 100 g for pure montmorillonite (Karland, 2010; Hicks et al., 2009). The uptake of smaller cations with higher charge is favoured in clays, following the Hofmeister series: Li+ < Na+ < K+ < 2+ 2+ + Ca < Mg < NH4 . Hydrated divalent cations expand the interlayer to ≈15 Å (Savage + + + and Authur, 2012; Panna et al., 2016), whereas large monovalent ions (K , Cs , NH4 ) have lower hydration energies and contract the interlayer to ≈12 Å, resulting in cation fixation (Pusch, 1994; Pusch and Yong, 2003; Sawhney, 1972; Comans and Hockley, 1992; Fuller et al., 2015). Depending on the situation, mono- and di-valent cations can have useful properties, which vary depending on environmental factors. For example Ca2+ substituted smectites show greater swelling pressure (fresh water), strength, and resistance to colloid formation, whereas Na+ substituted smectites have a weaker interlayer electrostatic interaction leading to higher sorption capacity (Savage and Authur, 2012; Panna et al., 2016; Segad et al., 2010; NEA, 2013). 4.2.1. Effect of heating on clay performance In the UK, a maximum external temperature of 100 °C is currently the limit for heat generating radioactive wastes canisters prior to disposal, although temperatures up to 120 °C are being considered (BERR, 2008; Hicks et al., 2009). The effect of heating to these temperatures on the hydration state of swelling clay minerals, such as 133 montmorillonite, has been studied using in-situ X-ray diffraction (XRD) to measure changes in the basal spacing d001, corresponding to changes in water occupancy in the interlayer space (Bergaya et al., 2006). Smectites with monovalent interlayer cations, e.g. Na+, are susceptible to dehydration and dehydroxylation at lower temperatures than those with divalent cations. The smaller monovalent cations have lower hydration energies so can migrate into hexagonal cavities in the tetrahedral sheet, and structural vacancies in the octahedral sheet, that are too small for divalent cations (Panna et al., 2016; Hall, 1980). Wang et al. (2012) monitored the change in d001 spacing for Na- montmorillonite, as a function of temperature. A water bi-layer (15.2 Å) was present at 25–40 °C, reducing to a monolayer (13.0 Å) at 80 °C, and a semi-layer (11.0 Å) at 140 °C, with no water present in the interlayer at temperatures greater than 170 °C. The interlayer in montmorillonite, where the charge is located predominantly in the octahedral layer, collapses to 9.5 Å at 450 °C due to dehydroxylation (Stanković et al., 2011; Villar, 2002). For smectites with a predominantly tetrahedral charged interlayer, e.g. beidellite, the charge is more localised and dehydroxylation causes the interlayer to collapse to 12.5 Å (Lantenois et al., 2008). Smectites are stable in the sub-surface environment, maintaining their advantageous properties of swelling and low hydraulic conductivity through heating-cooling cycles at temperatures of up to 80–100 °C over periods of millions of years (Pusch, 1983; Kamei et al., 1999; Hansen et al., 2012). Studies have shown that heat-induced processes have a limited effect on the performance of the engineered clay barrier at the temperatures predicted in a GDF environment with temperatures declining rapidly over the initial storage period (Couture, 1985; OECD/NEA, 2014). Heating saturated bentonite at 90 °C for one year does not significantly affect clay mineral composition, CEC or total surface area, although some calcite precipitation did occurred (Honty et al., 2012). Over shorter time periods (5 days), Towhata et al. (1998) observed reversible effects on smectite properties as a result of heating to 60, 100 and 200 °C under both wet and dry conditions. Furthermore, Bernier et al. (2007) and Yu et al. (2014) reported that, at temperatures of 80 °C, expansion of water in pores and in the clay interlayer caused a pressure increase, reducing the strength of the clays studied. The overall effect was to cause an increase in plasticity and swelling, accelerating fracture closure and reducing radionuclide migration around the canister, although a 134 rapid increase in temperature was shown to cause fractures in bentonite with an associated increase in surface area (OECD/NEA, 2005).Thermal alteration of montmorillonite (≈15.5 Å interlayer), can lead to the formation of layered silicates, such as illite (10 Å interlayer containing K+) or chlorite (14 Å interlayer containing Mg2+ and Fe2+/3+), that are highly charged, have a smaller interlayer spacing, and no longer swell (Dunoyer de Segonzac, 1970; Arthur et al., 2005; Karnland and Birgersson, 2006). The conditions within the GDF must therefore remain below the temperature at which the smectite-illite/chlorite transformation occurs (Posiva, 2009; Karnland and Birgersson, 2006; Holten et al., 2012). This thermally-induced transformation is also accelerated by high concentrations of K+ in the pore water (Wersin et al., 2006, Zhang et al., 2007; Liu et al., 2012). The transformation of smectite to illite, as shown by a disruption in short range order with uptake of K+ into interlayers (from pore water) and associated interlayer collapse, was demonstrated to occur over relatively short time periods (<1 year) at temperatures of 35–60 °C and at a high pH on a laboratory scale (Eberl et al., 1993). However, there is a great variability in transformation rates being published in the literature, with lab rates always being higher than natural analogue studies giving an uncertainty to the reported data (Cuadros and Linares, 1996; Haung et al., 1993; Pytte and Reynolds, 1989, Pytte, 1982). Despite this experimental evidence for shorter term alteration, under specific conditions, natural analogue studies suggest that clay mineral transformations over the longer scale of the GDF engineered barrier lifetime are very slow. This stability is exemplified through the slow illitization (<40%) observed at a natural analogue site at 70 °C over 60 million years in the Texan Peeler sediments at a depth of 2 km (Sellin and Leupin, 2013), possibly relevant to longer lived radio-isotopes. Experimental and modelling studies on clays exposed to higher temperatures of: (i) 100 °C over geological time scales (0.1–1 Ma) (Johnson et al., 1994a; SKB, 2006); and (ii) 130–150 °C over tens of years support this observation with only minimal changes and no significant illitization or loss of swelling properties (Wersin et al., 2006). Investigating changes in smectite structure using thermogravimetric analysis (TGA) showed dehydration at 100–125 °C, loss of organics at 345 °C, dehydroxylation at 400–750 °C, and inorganic structural breakdown at 800– 990 °C, resulting in the formation of cristobalite and other high-energy silicate phases (Panna et al., 2016). 135 The impact of heat from the waste form on clay barrier performance depends on the thermal conductivity of the clay, which is influenced by clay chemistry, density, water saturation, and the presence of impurities (Ikonen, 2005; Börgesson and Hernelind, 1999). Hicks et al. (2009) demonstrated that clay temperature remained below 100 °C even when the canister temperature was 160 °C. These steep thermal gradients expected in the clay would limit the amount of clay affected to that in the immediate vicinity of the canister, resulting in limited loss of swelling properties (Pusch and Yong, 2006; Delage et al., 2010). At higher temperatures (150−250 °C), reaction of the clay with steam resulted in rapid irreversible loss of swelling capacity and an increase in permeability and thus degradation of the barrier function (Couture, 1985). However, it is unlikely that these conditions would occur in a GDF at depths of >300 m as the boiling point of water would be greater than 200 °C, due to the increased hydrostatic pressure. In addition, dry installation of the engineered clay barrier before closure would limit the amount of water that is present in the early stages when the waste package is generating the most heat thus avoiding damage (Pusch and Yong, 2003). After inevitable waste canister failure, the principal safety function of the geo- engineered barrier is to limit the release of radionuclides. The montmorillonite component of the clay barrier adsorbs cationic contaminants through adsorption to negatively-charged sites on the surface, and through exchange with cations in the interlayer (Wu et al., 2011; Bhattacharyya and Gupta, 2012). Noyan et al. (2006) and Zhu et al. (2016) demonstrated that heating montmorillonite from temperatures of up to 300 °C, increased surface area, micro- and meso-porosity, and surface acidity which could lead to enhanced radionuclide sorption capacity if the canister were to fail at an early stage of storage. However, at higher temperatures (450–900 °C), oxidation may promote the release of redox active contaminants from the clay as well as the possibility for advection and clay decomposition (Zivica et al., 2015; Zhu et al., 2016). Clearly, as well as the impacts of elevated temperature on the barrier retention properties are complex and if closer packing of heat generating wastes in the sub- surface is to be explored, long term clay barrier performance at elevated temperatures (≈150 °C) must be investigated. 136 4.2.2. Effect of γ-irradiation on clay performance As well as heat, the engineered clay barrier would also be exposed to γ-radiation from the heat generating radioactive wastes in the waste package. The γ-irradiation dose received over the first 1000 years would depend on the composition of the waste; higher estimated dose rates at the canister surface of 52 Gy h-1 (Stroes-Gascoyne et al., 1994) and 72 Gy h-1 (Allard and Calas, 2009) give a total dose on the order of ≈500 MGy, whereas lower estimated dose rates of 0.5 Gy h-1 (SKB, 2006a) outside the canister give values of ≈5 MGy. In non-metallic solids, the effects of γ-irradiation are manifest as lattice dislocation (migration of cations) and charge separation resulting in an increase in surface potential. These charge separation effects occur through the generation of defect sites in the clay; these are oxygen radicals that have been deprotonated, and impart an overall negative charge to the clay structure. This can result in increased colloid stability, radiolysis (of water present in the interlayer), and changes in radionuclide affinities (Missana et al., 2008; Kunze et al., 2008: Gournis et al., 2001). For example, Holmboe et al. (2009) showed that γ-irradiation (0.54 kGy) enhanced the stability of Na-montmorillonite colloid dispersions, attributed to an increase in surface potential. The large specific surface area associated with colloids (particles in the size range of 1–1000 nm) results in a high charge density per unit mass, giving a strong affinity for specific radionuclides, and more importantly possible changes in desorption kinetics (Hunter et al., 2001; Wold, 2010; Norrfors et al., 2016). In radioactive waste disposal systems, both natural clay colloids and waste- or repository-derived clay colloids may enhance radionuclide transport; therefore, it is necessary to understand the conditions under which clay colloids are stabilised (Grambow et al., 2014; Kunze et al., 2008; Missana et al., 2008; Mori et al., 2003). The impact of γ-irradiation on clay barrier performance must be determined for a complete GDF safety case analysis and a review of the current literature is presented in Table 4.1. 137 1 Table 4.1. Effect of γ-irradiation on clay: review of studies reported in the literature. Material Radiation Conditions Findings Author Kaolinite γ-radiation (0.662 MeV, Localised decrease in crystallinity. Increase in specific surface area. Smaller Corbett et 1018–1022eV g-1), 137Cs particle size. Small changes in CEC. al., 1963 source Bentonite γ-radiation (3 x 108 Gy), No structural alteration. Release of carbon dioxide and hydrogen gas. Grauer, 1986 60Co source Radiolysis could lead to Fe3+ reduction within clay structure. Increase in CEC. Montmorillonite γ-radiation (1 MGy), 137Cs Thermo-luminescence (electronic energy storage) decreased with Fe content Coyne and Source (up to 50%). Increase in radiation induced defects (RID) at g = 2 by electron Banin, 1986 paramagnetic resonance (EPR) spectroscopy. Electronic energy from γ-radiation stored in structure, dissipated slowly over a 3 month period. Kaolinite Review of γ-radiation EPR signals at g = 4 corresponding to Fe3+ ions occupying two sites: (i) Jones et al., interlayer signal distorted by changes in hydroxyl orientation due to disorder; 1974 (ii) octahedral signal affected by degree of crystallinity. Stable defect centre at g = 2 corresponding to electron hole centre located on Si–O bond adjacent to 2+ - Mg octahedral or O2 ion trapped within lattice. Clay minerals Review of γ-radiation Characterization of general smectite EPR spectrum with characteristic defect Hall, 1980 peaks identified. Natural radiation-induced defect in clay due to background γ-radiation. Bentonite (MX-80) γ-radiation (400–4000 Almost no differences in mineral and chemical MX-80 composition over one Pusch et al., Gy/hr) 60Co source year. In proximity to Fe metal, Fe migration into clay more rapid under 1992 γ-irradiation. Smectite γ-radiation (2.5 x 106 R α-radiation causes displacement or amorphisation. γ-radiation produced small Ewing et al., 138 h-1) 6oCo/ 137Cs sources, amounts of damage through radiolysis and was shown to form bubbles on 1995 α-irradiation (5 MeV) HLW glasses. Clay more resilient to γ-radiation than α-irradiation. Boom Clay γ-radiation (397 TBq, 5 Fe and Ca increase/ Mg and Al decrease near hydroxyl groups. No obvious Noynaert et 60 MGy over 5 years) Co change in pore water and pH. Enrichment of CO2. Heating and radiation al., 1998 source produced small mineralogical transformations. Bentonite 60Co source (20 kGy h-1) RIDs increased with dose. Behaviour dependent on clay content versus Dies et al., accessory mineral content. Trapped charge corresponding to higher defect 1999 concentration in coarse fraction (thermo-luminescence). Na-montmorillonite γ-radiation (84–421 kGy) Partial (3%) Fe reduction (Mössbauer spectroscopy): Gournis et 6oCo source 퐹푒3+ + 퐻• ↔ 퐹푒2+ + 퐻+ al., 2000 Production of hydrogen radicals through radiolysis of interlayer water (≈1%): • • − + (퐻2푂)𝑖푛푡푒푟푙푎푦푒푟 → 퐻 , 푂퐻 , 푒푎푞, 퐻2, 퐻2푂2, 퐻3푂 Reversible H2 diffusion through octahedral vacancies. No measurable change in XRD. Na-montmorillonite γ-radiation (58.5 Gy Heating after radiation caused migration of small cations into tetrahedral Gournis et min-1) 6oCo source lattice and reduction of g≈4 defect (EPR). al., 2001 Montmorillonite γ-radiation (2 MGy) 60Co Small water loss due to radiolysis with increasing dose (IR). No change in long Negron et al., source range structural ordering (XRD). No change in spin centres (27Al and 29Si NMR). 2002 Bentonite γ-radiation (3.27 MGy) Limited alteration with heating (130 °C). Increase in CEC with preferential Pusch, 2002 uptake of Ca over Na. Increase in defects (EPR). kaolinite, γ-radiation (0.1–30 MGy) RID stabilisation determined by tetrahedral layer composition (Si>Al Pushkareva palygorskite, incorporation) and interlayer water content – higher disorder. A and B centres et al., 2002 montmorillonite (Si–O/Al–O) sensitive to water content. B’ centres not seen in montmorillonite. 139 Increase specific surface and change of solubility (Si4+ loss increased in montmorillonite). Montmorillonite, γ-radiation (1.1 MGy) Small change in CEC and decrease in crystallinity. Reduction of lattice Fe3+. Plötze et al., kaolinite, dickite, Assignment of EPR signals: (i) Fe3+ structural peak; (ii) broad Fe impurity; (iii) 2003 illite, smectites surface hydrated Fe3+; (iv) defects from electron holes on SiO and tetrahedrally substituted Al–O. Montmorillonite γ-radiation (13 kGy s-1, Orthorhombic native defect (g≈2) due to axial Si-O π orbital linked to Sorieul et al., 0.9 MGy–3.6 GGy) octahedral sheet. Second isotropic native defect due to in-plan Si–O π orbital 2005 in tetrahedral sheet. Defect t½ = ≈3,000 years, annealed at 500 °C. Saturation of defect centres above 3.6 GGy (EPR). Bentonite Review of γ-radiation Temperature independent attenuation of γ-radiation influenced by density SKB, 2006b and water content. Minimal impact of radiolysis at canister surface. Na- γ-radiation (0–60 kGy) Increased colloid surface potential due to radiolysis, increased colloid stability. Holmboe et montmorillonite, 137Cs source Co2+ sorption decreased, Cs+ sorption unchanged. al., 2009; Bentonite (MX-80) Holmboe et al., 2011 Kaolinite, γ-radiation (various γ-radiation causes amorphisation. RIDs in kaolinite, dickite, montmorillonite, Allard and Montmorillonite doses) 6oCo/ 137Cs sources and illite. Axial distortion due to hole-centre located on Si–O results in Calas, 2009; dickite, illite paramagnetic N1 RID. Isotropic N2 RID in tetrahedral sheet (EPR). Defects Allard et al., stabilised through local charge imbalance. Interlayer water reduces defect 2012 intensity. Solubility, specific surface area, and exchange capacity altered by local damage; damage/defect density increases with dose. Bentonite γ-radiation (various γ-radiation caused radiolysis and structural destabilization. Reduction in Wilson et al., doses) 60Co source particle size, increase in specific surface area. No pressure increase observed. 2010 140 Smectite γ-radiation (up to 1.5 A small increase of negative intensity of absorption peak of monomers in infra- NEA, 2013 MGy) 60Co source red spectroscopy (~ 500 nm) suggests decrease in charge 141 In the current study, we explore the combined effects of heat and subsequent γ-irradiation on montmorillonite under conditions relevant to geological disposal of heat generating radioactive waste through a combination of molecular-scale (XAS, EPR, and AFM) to macro-scale (XRF, XRD, IR, CEC, zeta potential, and extractable Fe2+) analyses. Molecular-scale information is essential to develop a mechanistic understanding of processes that would affect the performance of an engineered clay barrier in a GDF, allowing refined long-term predictions of radionuclide fate and transport to be made for any safety case. 4.3. Methods and Materials 4.3.1. Clay preparation Seven smectites were purchased from the Source Clays Repository of the Clay Minerals Society (West Lafayette, IN); four montmorillonites (SWy-2, STx-1b, SCa-3, and SAz-2) of the general formula (Na,Ca)0.3Al1.67Mg0.33Si4O10(OH)2.n(H2O) (Stanković, 2011), with the layer charge primarily localised in the octahedral sheet, a beidellite (SBId-1, Na0.5Al2(Si3.5Al0.5)O10(OH)2.n(H2O)) and two nontronites (NAu-1 and NAu-2, 3+/2+ Na0.3Fe (Si,Al)4O10(OH)2.n(H2O)), where the layer charge predominantly localised in the tetrahedral sheet. The clays were milled in a TEMA mill, sieved to obtain a size fraction of <53 μm (clay and silt-sized particles), and homo-ionised (Moore and Reynolds, 1997; Baeyens and Bradbury, 1995) to produce subsamples with Na+, Mg2+, K+ or Ca2+ as the dominant charge-balancing interlayer cation (SI). Samples were compacted using a hydraulic press (Specac) with stainless steel die (Lightpath Optical (UK) Ltd) to produce 13 mm pellets (9 tons, 300 mg, 30 secs) and 8 mm pellets (5 tons, 120 mg, 30 secs) with a compacted density of 2 g cm-3. 4.3.2. Heat treatment and irradiation experiments Homo-ionised (starting material, Na+, Mg2+, K+ and Ca2+) pellets of STx-1b (low Fe) and NAu-1 (high Fe) were used at room temperature (25 °C) and heated for two days at a range of temperatures (90, 160 and 500 °C). Following heat treatment, γ-irradiation was performed using a Foss Therapy Services Model 812 60Co γ-irradiator (Leay at al., 2014) at The University of Manchester’s Dalton Cumbrian Facility (DCF). A range of 142 irradiation conditions were used to give absorbed dose rates from 4 Gy/min up to 450 Gy/min and affording total doses of 0.2-5 MGy, determined by Fricke dosimetry (Leay at al., 2015). Sample cells consisted of quartz tubes (15 mm OD x 13 mm ID) approximately 10 cm long that were flame sealed following evacuation. One sample of SWy-1 was not evacuated to explore the effect of irradiation under aerobic versus anaerobic conditions. 4.3.3. Clay characterisation before and after heat treatment and irradiation To document the physical and chemical changes in the clays, X-ray fluorescence (XRF), X-ray diffraction (XRD), infra-red spectroscopy (IR), cation exchange capacity (CEC), extractable Fe2+ content, X-ray absorption spectroscopy (XAS), and electron paramagnetic resonance spectroscopy (EPR) were performed on clay samples before and after exposure to heat and γ-radiation. Colloidal suspensions were analysed using scanning electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering, and zeta potential measurements before and after exposure to γ-irradiation. Major element analyses were obtained over 2 hour scans on pressed pellets (15g; 12 g sample and 3 g wax binder) with a wavelength dispersive PANanalytical Axios Sequential X-ray Fluorescence Spectrometer using the standard glass AUSMON (B255). Samples (1 g) were dried at 100 °C for 1 hour, before being cooled in a desiccator; weight loss was recorded to determine the content of loosely held water. Samples were then heated for 1 hour at 1100 °C in a furnace, cooled in a desiccator and reweighed to determine loss on ignition due to dehydroxylation, release of CO2 by breakdown of carbonates, and oxidation of organic matter allowing complete analysis through comparison with XRF data using iterative methods (Rowell 1994). XRD was carried out on a Bruker D8 Advance using Cu Kα radiation (0.154 nm). A step size of 0.0197° 2θ with a counting time of 4 seconds per step over a 2θ range of 4–60° was used. Powder patterns were calibrated using an external Si standard and phases identified using a search-match routine (EVA software programme). Powder IR measurements were collected on an ID5 ATR IR spectrometer (Nicolet iS5, Thermo Scientific) to quantify water and hydroxyl loss during heating and γ-irradiation. 143 CEC was measured using a method adapted from Summer and Miller (1996). Exchangeable cations were removed by exposure to a saturating ammonium acetate solution (1 M) and the theoretical CEC was calculated indirectly by measuring the cations (Na+, Mg2+, K+, Ca2+, Fe2+/3+, Al3+, Si4+) present in the extract using inductively coupled plasma atomic emission spectroscopy (Perkin-Elmer Optima 5300 dual view ICP–AES). The extractable Fe2+ content was measured by adding 0.2 g of clay to 5 mL of 0.5 M HCl (Lovley and Phillips, 1987). Samples were centrifuged (3000 rpm, 5 mins) and the -1 extract added (0.1 mL in 2.1 mL H2O) to ferrozine reagent (0.3 mL, 1 g L ferrozine in 50 mM HEPES buffer, pH 7.0) and hydroxyl-ammonium hydrochloride solution (0.6 mL). Aqueous Fe(II) concentration was determined by measuring the absorbance at 562 nm using a UV-1800 Shimadzu UV spectrophotometer (Stookey, 1970; Violler et al., 2000). Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data were collected on beamline B18 at Diamond Light Source (Dent et al., 2009). Samples were pressed into pellets (8 mm) and sealed in Kapton tape in a sample holder. K and Ca K-edge X-ray absorption spectra were measured in fluorescence mode. Fe samples were diluted with cellulose to give the necessary absorption coefficient for the Fe K-edge and measured in transmission mode with Fe foil as an energy calibration standard. This allowed detailed analysis of Fe, Ca, and K K- edges to examine oxidation state and co-ordination geometry of both structural (Fe) and interlayer cations (Ca2+, K+). All data was processed and analysed using the Demeter software package; spectra were imported to Athena for processing (background subtraction, normalisation, data reduction) and then exported to Artemus for fitting using FEFF calculations from known crystal structures to give information on possible shells (Ravel and Newville, 2005). EPR spectra were obtained using a Bruker EMX Micro X-band Spectrometer at a frequency of 9.5 GHz and a 1 Tesla electromagnet to study radiation-induced defects in STx-1b, a model low Fe clay, after heat treatment (90, 160, 500 °C) and subsequent γ-irradiation (0.2–5 MGy). Samples (≈20 mg) were added to a fused quartz tube for measurement and strong pitch was used as a standard (g = 2.0028). The g values of the 144 main spectral features were calculated for comparison with previously published papers (Allard and Calas, 2009). Colloids were produced from the bulk STx-1b sample before and after irradiation using a preparation method adapted from Bouby et al. (2011). Particles greater than 500 nm were removed using a BOECO Germany C-29A centrifuge (4000 rpm, 11 minutes). Particle size analysis (photon correlation spectroscopy) and zeta potential measurement were carried out on colloidal suspensions (1 g L-1, pH 8–9) using a Zetasizer Nano ZSP, Mastersizer 3000, a NanoSight NS300, and a Brookhaven ZetaPlus (NanoBrook Omni) Zeta Potential Analyzer using the Smoluchowski model for zeta potential calculations. Colloids were further characterized by atomic force microscopy (AFM) to characterize the morphology of the particles allowing in addition force measurements by cantilever modification (colloid-probe technique). AFM was carried out using a Dimension 3100 (equipped with Nanoscope IV controller), from Bruker GmbH, Karlsruhe, Germany. For morphology-sample preparation, a sharp tip of ≈4 nm radius (SNL-10, Bruker GmbH, Karlsruhe, Germany) was employed. 50 μL of the clay suspension was placed on a freshly cleaved biotite substrate and left to dry in a clean environment. The dried samples were gently washed with MilliQ water to remove large aggregates or unwanted dust particles and again left to dry. The dried samples were scanned using the sharp tip to estimate the particle size distribution. The particle deposition/detachment at varying pH was noted simultaneously. Force measurements were taken using cantilevers modified with a carboxylated polystyrene particle of 1 μm attached (Novascan Technologies, Ames, U.S.A.). The dried samples were scanned using a colloid probe to trace the clay particles. An electrolyte (1 mM, NaNO3) was introduced and the samples were scanned again to image the particles. The force- volume measurements were obtained over an area of 16x16 points. The forces at every point were correlated with the image to locate the forces on the particle. At every point the force curves were averaged three times. The force measurements were performed between particle and biotite substrates under varying pH (5 & 9). The scanning probe imaging processing software (Image metrology, Denmark) was implemented to convert the obtained deflection(V)-position(nm) curves to force(nN)- 145 separation distance(nm) curves from the deflection sensitivity and spring constant (0.12 N/m) of the cantilever. The colloids were imaged before and after irradiation on a FEI QUANTA 650 FEG environmental scanning electron microscope (nm resolution at 30 kV) at the Institute for Nuclear Waste Disposal (INE, Karlsruhe) giving secondary morphology and size data. 4.4. Results 4.4.1. Clay chemical composition and exchangeable cations The elemental composition of the natural beidellite, nontronites, and montmorillonites was determined by XRF (Table 4.2). Table 4.2. Smectite chemical composition (major elements) measured using XRF. Errors (+/-): Na2O 0.006%, MgO 0.009%, Al2O3 0.11%, SiO2 0.087%, Cl 0.0007%, K2O 0.038%, CaO 0.005%, Fe2O3 0.053%, H2O and CO2 0.034%. Trace elements and metals also present (in some) but not included in Table 4.2 (F, P2O5, SiO3, TiO2, MnO, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Pb, Th, Ba, Ce, Cr, Hf, La, Nd, Ni, Ta, Tl, V, W) . Sample name Na2O MgO Al2O3 SiO2 Cl (%) K2O CaO Fe2O H2O CO2 Total (%) (%) (%) (%) (%) (%) 3 (%) (%) (%) (%) STx-1b 0.24 2.65 13.3 64.7 0.01 0.15 1.51 0.96 9.63 6.45 99.60 NAu-2 0.21 0.92 2.61 42.8 0.03 0.17 1.32 31.9 14.1 5.57 99.63 NAu-1 0.11 0.73 9.14 36.1 0.04 0.15 1.53 29.8 13.7 8.47 99.77 SCa-3 0.81 5.56 15.0 49.9 0.09 1.32 0.66 2.73 14.5 8.74 99.31 SBid-1 0.05 0.88 25.2 54.6 <0.01 0.85 0.66 1.40 8.15 7.27 99.06 SAz-2 0.04 5.17 15.7 50.2 <0.01 0.17 2.26 1.35 15.4 9.09 99.38 SWy-1 1.21 2.44 17.8 56.6 0.01 0.54 1.31 3.74 9.7 6.02 99.37 Overall, XRF analysis showed that montmorillonites had a high amount of Al3+ and Si4+ + 2+ 2+ + with varying amounts of interlayer cationic species (Na , Mg , Ca , K and FeTot) and water contents of 8–16%, all with low total carbon contents (< 3%). The nontronites 3+ had similar compositions, but with lower Al and higher FeTot contents giving a greater negative charge on each of the layers, and with a higher water content (≈14%) than most montmorillonites. Beidellite (SBld-1) had more structural Al3+ and Si4+ content than any of the other clays, high K+ content and the lowest water content. Mg2+ was 146 higher in some samples (STx-1b, SCa-3, SAz-2 and SWy-1) as it can be incorporated in the clay sheet structure as well as being present as an interlayer cation. The d001 spacing, which directly correlates with the interlayer spacing, was measured before and after cation exchange, using XRD (Figure 4.2). A similar interlayer spacing (≈15.2 Å) was observed before and after Ca2+ substitution for most of the smectites, suggesting that Ca2+ was the dominant native cation, which was also indicated by XRF analyses on the unaltered samples (Table 4.2). For SWy-1, Na+ was the main mono- valent interlayer cation, which hydrates to a lesser extent than Ca2+, resulting in a lower d001 spacing compared to the other cations (≈14.2 Å). A noticeably larger d001 was observed within the Mg-substituted montmorillonites (≈16–16.5 Å) and nontronites (≈15–15.5 Å) due to the increased hydration caused by Mg2+ due to the higher hydration energy and therefore polarisability of the cation. The interlayer spacing of the nontronites decreased significantly with Na+ substitution (≈12 Å). This decrease in interlayer spacing occurs because the larger layer charge on the nontronites, present as a result of extensive Fe-substitution, and resulting in complexation between surface oxygens and the less hydrated Na+ interlayer cations for charge balance (Savage and Authur, 2012). All samples showed a similar, smaller basal spacing with K+ (≈12 Å) and similar to past work (Sawhney 1972). This overall decrease in basal spacing was due to the larger cation size and lower hydration energy of K+ compared with the other cations, which leads to surface complexation interaction and collapse of some layers within the structure (Savage and Authur, 2012; Pusch, 2002). 4.4.2. Combined effects of heat and γ-radiation on clay structure The low Fe montmorillonite, STx-1b, and high Fe nontronite, NAu-1, were selected for extensive characterisation to contrast the behaviour of an initial buffer material with minor impurities (STx-1b) compared to a model alteration product that may surround the waste canister (NAu-1). Samples were heated to a range of temperatures relevant to heat generating waste disposal: (i) 90 °C representative of the target waste package temperature (Curtis and Morris, 2012); (ii) 160 °C to investigate effects of waste package disposal at elevated temperatures; (iii) 500 °C to investigate dehydroxylation on the clay (Panna et al., 2016); and (iv) 1000 °C for complete decomposition of the 147 clay structure. After heating, the clays were exposed to γ-irradiation and changes to the clay structure were analysed using XRD (Figure 4.3). For the cation exchanged STx-1b and NAu-1 clays, the d001 decreased by ≈0.5–1 Å upon heating to 90 °C and by ≈1–3 Å upon heating to 160 °C, due to loss of interlayer water (Table 4.3), confirmed through TGA analysis. Heating to greater than 500 °C resulted in a further collapse of the d001 basal spacing to 9.5–10 Å due to dehydroxylation, release of CO2 by breakdown of carbonates, and oxidation of organic matter, as indicated by the weight loss on ignition (Table 4.1SI). This smaller interlayer spacing was consistent with that of a highly charged illite-like phase where interlayer cations are anhydrously associated to the interlayer surfaces, but may also result from the migration of interlayer cations into vacancies in the octahedral sheet or holes within the tetrahedral hexagonal layer (Zivica et al, 2015). Heating to 1000 °C resulted in decomposition of the clay into the high energy phases of its component oxides, e.g. SiO2, Al2O3 and Fe2+/3+ oxides. As can be seen in Table 4.3, clays with divalent interlayer cations show greater resistance to heating than monovalent cations due to their higher hydration energies and larger hydration shells (Savage and Authur, 2012; Svensson and Hanson, 2013). Performing solely γ-irradiation resulted in a small decrease in the interlayer space under atmospheric conditions (25 °C) for the cation exchanged STx-1b (Figure 4.1SI), with the exception of interlayer Na+-substitution in STx-1b which resulted in significant interlayer collapse from 15.3 Å to 12.5 Å, upon γ-irradiation. Table 4.3. d001 spacing (Å) for heat-treated and γ-irradiated montmorillonite (STx-1b) and nontronite (NAu-1). Instrument error was 0.1 Å (3σ) and variability between repeat samples was 0.4 Å (3σ). Cation Temp (°C) STx-1b STx-1b γ NAu-1 NAu-1 γ Un-substituted 25 15.5 15.2 15.3 - 90 11.7 15.3 14.9 14.5 160 12.3 14.7 14.4 14.9 500 9.7 9.5 10.0 9.9 Na+ 25 15.3 12.5 12.3 - 90 14.1 12.1 12.1 12.4 160 14.7 14.4 12.0 12.4 148 500 9.4 9.6 9.9 9.9 Mg2+ 25 16.1 15.4 15.0 - 90 15.0 14.9 15.1 14.5 160 13.5 12.3 14.8 14.6 500 9.5 9.5 9.5 10.0 K+ 25 12.4 12.4 12.01 - 90 12.0 12.5 10.0 11.1 160 11.6 10.9 10.1 10.0 500 10.3 10.3 9.9 10.00 Ca2+ 25 15.6 15.4 15.1 - 90 15.1 14.6 14.8 14.5 160 12.6 14.5 13.8 15.2 500 9.5 9.4 10.0 9.5 The γ-irradiation of STx-1b and NAu-1 after heating up to 500 °C did not significantly alter the interlayer spacing beyond the effects of heating alone, with a decrease in d001 spacing clearly observable compared to the 25 °C and 90 °C samples (Table 4.3, Figure 4.1SI). However, subsequent γ-irradiation after heating to 160 °C resulted in larger d001 spacings in Ca2+- and Na+-substituted STx-1b, as well as in Ca2+-substituted NAu-1, suggesting the clays were more prone to rehydration (uptake of hydrated cations) after γ-irradiation possibly from the postulated increase in surface charge of the clays. The combined effects of heating and γ-irradiation were also studied using IR. Peaks for Si4+–O bond stretching at ≈1000–1040 cm-1 and Al3+–OH at ≈600 cm-1 were observed for all the clays, with shifts in peaks dependent on the octahedral Fe3+ and Mg2+ content and the tetrahedral Al3+ content. A Fe3+Al3+–OH peak in the nontronites and an Al3+Mg2+–OH peak in the montmorillonites were observed, along with other Si–O peaks in the region of 1000–600 cm-1 (Gates, 2005). The IR spectra for all clays showed structural OH bending groups (≈1600 cm-1) and OH stretching (≈3750–3600 cm-1), with nontronites showing OH–stretches at a higher wavenumber (Madejová and Komadel, 2001). Loss of free water (peak at 3600 cm-1) occurred with heating to 90 and 160 °C. Subsequent loss of structural OH was observed by a disappearance of these peaks at the higher temperatures of 500 and 1000 °C. After γ-irradiation, samples were maintained at constant humidity (80% MgCl2 solution) and no significant reduction in the IR peaks corresponding to water was observed (STx-1b and NAu-1, Figure 4.3SI). 149 4.4.3. Effect of γ-irradiation on CEC and extractable Fe2+ The CEC of the unaltered montmorillonites and nontronites was 65–90 meq / 100 g, in agreement with previous studies (Hicks et al., 2009); beidellite showed a low value of 50 meq / 100 g with comparison across the sample set (Table 4.2SI). The ferrozine assay (0.5 M HCl) was used to determine the effect of γ-irradiation on the extractable Fe2+ content (Table 4.2 and Table 4.2SI). The native clays all had low extractable Fe2+ (< 0.2% of the total Fe content as measured by XRF) except STx-1b and SWy-1 had 0.3–0.7% extractable Fe2+. The amount of extractable Fe2+ for the nontronites was an order of magnitude higher than the other clays but, due to the higher Fe-content, the percentage of the total Fe extracted was similar. Substituted STx-1b (Na, Mg, K, and Ca) showed higher extractable Fe2+ (up to 0.95%). SWy-1 was γ-irradiated under both aerobic and anaerobic conditions to study the effect of irradiation environment. Irradiation resulted in an increase in 0.5 N HCl extractable Fe2+ with a 2-fold increase in the aerobically irradiated sample and 3-fold increase in the anaerobically irradiated sample (Table 4.4). The effect of γ-irradiation on the CEC and extractable Fe2+ content, as a function of interlayer cation, was studied using STx-1b (Table 4.4). SWy-1 samples were used to examine the effects of γ-irradiation the extractable Fe2+ content under aerobic and anaerobic conditions. Table 4.4. CEC and ferrozine assay results for natural and cation-exchanged STx-1b and SWy-1 (anaerobic and aerobic) before and after irradiation. Error CEC; 0–8 meq / 100 g and ferrozine; 0.5%. Sample Total CEC meq/ 100 g Fe2+ extractable from total Fe present % STx-1b 78.5 0.62 STx-1b (γ) 87.9 3.75 STx-1b Na+ 83.3 0.44 STx-1b Na+ (γ) 86.9 7.95 STx-1b Mg2+ 107.7 0.95 STx-1b Mg2+ (γ) 104.3 10.4 STx-1b K+ 85.5 0.61 STx-1b K+ (γ) 84.2 7.03 STx-1b Ca2+ 110.6 0.77 150 STx-1b Ca2+ (γ) 113.6 8.52 SWy-1 91.2 0.48 SWy-1 Aer (γ) - 0.73 SWy-1 Ana (γ) - 1.21 The CEC for STx-1b with monovalent cations in the interlayer was significantly smaller than that for STx-1b with divalent cations present in the interlayer. The CEC values did not change significantly after γ-irradiation. The amount of extractable Fe2+ increased in cation exchanged STx-1b, with the smaller, higher charged cations showing the largest increase (Mg> Ca> Na> K). γ-irradiation (5 MGy) vastly increased the extractable Fe2+ in STx-1b from 0.62% to 3.75% in the starting STx-1b, from 0.7 to 8.52% in the Ca-substituted STx-1b and from 0.95% to 10.4% in Mg-substituted STx-1b. The effect of γ-irradiation on amount of extractable Fe2+ was somewhat greater with divalent interlayer cations than with monovalent interlayer cations (Table 4.4). 4.4.4. EXAFS analysis of STx-1b and NAu-1 before and after γ-irradiation XANES and EXAFS at the Ca, K and Fe K-edges were used to examine the oxidation state and co-ordination geometry of both structural Fe and interlayer cations (Ca2+, K+) in the samples (Stern, 1974; Harries and Huskins, 1986). The features in the Ca K-edge XANES spectrum correspond to the 1s to 3d transition (≈4.035 keV pre-edge feature), the 1s to 3p transition (≈4.045 keV main feature ), and also relate to the Ca interaction within the structure as a charge compensator or network modifier (≈4.040 keV shoulder) (Neuville et al., 2004 and Combes et al., 1991). γ-irradiation of low-Fe STx-1b (Figure 4.4a) and high-Fe NAu-1 (Figure 4.4d) resulted in a slight reduction in the white line intensity in the Ca K-edge XANES, with the changes more obvious for NAu-1. The Ca K-edge EXAFS and Fourier transform magnitude (Figure 4.4b and 4.4e) for both STx-1b and NAu-1 showed a decrease in intensity after γ-irradiation, presumably reflecting a more disordered system within the interlayer. The Ca K-edge EXAFS best fit for STx-1b showed that Ca2+ in the interlayer was hydrated and coordinated with six waters in octahedral configuration (Figure 4.4g and 4.4h). The Ca–O bond length (2.39±0.01 Å) remained the same after γ-irradiation but the Debye-Waller factor (σ2) showed a small increase in some cases suggesting a 151 potential increase in the disorder of the system. This effect was very small and quite possibly negligible within error for single samples but results showed the same bias across a number of samples measured suggesting a radiation induced effect, albeit a very small. NAu-1 followed similar trends with only very small changes seen. When fitting Ca2+ EXAFS a simple shell fit was used due to the interlayer environment the cation was in (surrounded by water). STx-1b also contains low but detectable concentrations of K+ (Table 4.2) and the K- edge XANES (Figure 4.6SIa) showed essentially the same features before and after γ-irradiation with a shoulder on the edge (≈3.610 keV) which increased with Si4+ content (Brigatti et al., 2008) and the white line at ≈3.612 keV arising from the excitation of the 1s electron to an excited 4p state (Sugiura and Muramatsu, 1985). The Fourier transform and an Artemis fit using an illite base file of the K K-edge EXAFS data showed contributions from the axial and equatorial O atoms around the K+ ion, as well as the Si in the tetrahedral sheet. The Debye Waller factor (σ2) did not increase after γ-irradiation, suggesting a limited disordering effect presumably due to the stronger association of K+ with the interlayer initially (Figure 4.6SIc). The Fe K-edge XANES spectra for STx-1b and NAu-1 (Figure 4.5) were all run on pellets (0.3 g and 2 g density thinned to 100 μm) and confirm that Fe(III) was the main oxidation state of iron present (Brigatti et al., 2000; Manceau and Gates, 1997). There was no change in Fe oxidation state seen as a result of γ-irradiation shown in the XANES, with very similar shape and edge positions shared, although a mixed oxidation state was possible. The low intensity of the pre-edge feature at ≈7.113 keV (1s-3d) suggests an octahedral Fe environment (Decarreau et al., 2008, Sowrey et al., 2004). The other peaks at 7.128 KeV and 7.133 KeV relate to a 1s-4s transition (main edge) and 1s-4p transition (later broad peak) respectively. No self-adsorbance was obserbed in the lower Fe STx-1b. The high Fe content in NAu-1 resulted in self-absorption effects (raw edge jump >1); comparisons were possible within sample sets as measurements were undertaken on pellets of the same sample composition (and density) allowing for analogous self-adsorption across these higher Fe sets. An increase in the Fe K-edge XANES intensity after γ-irradiation, more pronounced for NAu-1 than STx-1b (30% vs ≈1%), suggests an increase in ordering of the Fe (Figure 4.5).The STx-1b Fe K-edge 152 EXAFS and associated Fourier transform showed a slight increase in intensity (linear absorption coefficient) after γ-irradiation, implying an increase in structural order and stronger bonding between Fe and O atoms in the first co-ordination sphere. The best fit of the EXAFS data for STx-1b (Figure 4.5) gave an Fe–O bond length of 2.02±0.01 Å, which agrees with the literature for structural Fe(III) present in montmorillonite in octahedral coordination surrounded by six O atoms (Wilke et al., 2001; Vantelon et al., 2000; Manceau et al., 2000; Finck et al., 2015). Additional shells were present at 3.07±0.02 Å presumably due to Al, Fe or Mg cations in the octahedral sheet (Al being the most predominant and therefore used for the fit as was also used in previous FEFF data), and at 3.25±0.01 Å due to Si or Al in the tetrahedral sheet (Soltermann et al., 2013). After irradiation, there were minimal changes in the EXAFS data for the low-Fe STx-1b, with both the bond length and Debye-Waller factors (σ2) within error of those for STx-1b before irradiation. The Fourier transform of the Fe K-edge EXAFS for NAu-1 differs from STx-1b due to the higher Fe content resulting in contributions from structural Fe–Fe interactions (Figure 4.5f). The Fourier transform magnitude showed a pronounced increase after irradiation under both aerobic and anaerobic conditions (Figure 4.5f), indicating an increased order in the Fe–O bonding within the structure or as an oxide layer on the clay (See EPR results). The Fe–O shell was set to a single six co-ordinated species (i.e. an octahedral layer) and showed similarities before and after irradiation, with the first Fe shell remaining at 2.01±0.01 (Table 4.5). NAu-2 and SCa-3 showed similar results to NAu-1 and STx-1b, with a reduction in the Ca K-edge transform magnitude after irradiation for Ca in the interlayer, and an increase in the Fe K-edge transform magnitude after irradiation presumably due to an increase in the order of Fe present in the structure or as a hard to characterise amorphous Fe oxide on the surface (Table 4.3SI). Table 4.5. Ca and Fe K-edge EXAFS fit results for STx-1b and NAu-1 before and after irradiation 2 2 2 Sample Spectrum Path CN R (Å) σ (Å ) SO r a + + STx-1b Ca 6a Ca–O 6 2.39 0.008 0.86 0.014 − − 0.017 0.001 153 a + + STx-1b Ca γ 6b Ca–O 6 2.39 0.011 0.85 0.016 − − 0.014 0.001 a + + a STx-1b Fe 9a Fe–O 6 2.02 0.005 0.80 0.012 − − 0.006 0.001 a + + Fe–Al 3 3.07 0.009 − − 0.024 0.002 a + + Fe–Si 4 3.25 0.009 − − 0.012 0.002 a + + a STx-1b Fe γ 9b Fe–O 6 2.00 0.003 0.80 0.012 − − 0.008 0.001 a + + Fe–Al 3 3.06 0.009 − − 0.033 0.003 a + + Fe–Si 4 3.25 0.009 − − 0.015 0.003 a + + a NAu-1 Ca 10SIa Ca–O 6 2.40 0.010 0.84 0.010 − − 0.011 0.001 a + + a NAu–1 Ca γ 10SIb Ca-O 6 2.39 0.010 0.84 0.017 − − 0.016 0.003 a + + NAu-1 Fe 12SIa Fe–O 6 2.01 0.003 0.52 0.008 − − 0.005 0.001 a + + Fe–Fe/Al 3 3.06 0.005 − − 0.008 0.001 a + + Fe–Si 4 3.27 0.005 − − 0.018 0.001 a + + NAu-1 Fe γ 12SIb Fe–O 6 2.01 0.003 0.50 0.014 − − ANA 0.010 0.001 a + + Fe–Fe/Al 3 3.06 0.004 − − 0.015 0.002 a + + Fe–Si 4 3.28 0.004 − − 0.039 0.002 a + + NAu-1 Fe γ 12SIc Fe–O 6 2.01 0.003 0.63 0.016 − − AER 0.009 0.001 a + + Fe–Fe/Al 3 3.06 0.004 − − 0.012 0.002 a + + Fe–Si 4 3.28 0.004 − − 0.032 0.002 154 Notes: Path represents the scattering path, all primary scatters; CN denotes occupancy; R gives the interatomic distance; σ2 is the Debye-Waller factor (a measure 2 of disorder in the structure); SO the amplitude (from 1); and r the fit factor a measure of fit quality (0 being perfect). a denotes a fixed parameter. All reduced chi squares > 50. The XAS analyses were clear indicators for the oxidation state and local structure of the clays (nearest neighbours) but were less clear with respect to the exact location of Fe within the clay structure. The XANES and EXAFS showed that there was neither any significant change in valence of the elements studied, nor was there observable significant change in coordination structure. Potentially, an increase in the ordering of the Fe(III) coordination sphere post-irradiation occurred (such as that could be brought about through an increase in grain size or annealing of original defects, as irradiation causes sample heating); however, self-absorption effects during the measurements prohibit conclusive interpretation. Further EPR analysis allowed clear analysis of both the amorphous Fe associated with the clay and the structural Fe. 4.4.5. EPR analysis of combined heating and γ-irradiation effects on STx-1b EPR was used to study the electronic and structural effects of γ-irradiation on STx-1b before and after cation exchange, and after heating to 25, 90, 160, and 500 °C. The lower Fe-content of STx-1b allowed for changes in radiation-induced defects (RIDs) to be observed without being overwhelmed by the ferromagnetic response, a signal present as a result of Fe–Fe interactions in the high Fe-content clays. Radiation induced defects are mostly comprised of unpaired electrons (electron holes) on the oxygen associated with Si or Al in the dominantly tetrahedral and octahedral sheets in the montmorillonite crystal structure (Allard and Calas, 2009; Allard et al., 2012). The STx-1b starting material (Figure 4.6) showed a number of characteristic resonances attributable to paramagnetic species within the structure of the clay, either from substitution of lower charge cations into layers or from electron holes due to natural background γ-irradiation. A broad rhombic band at g~2, which was typical of nanostructured Fe(III)-oxides and –oxyhydroxides, either as amorphous Fe material or Fe coatings and/or a discreet phase that was less than 5% which gives rise to the background observed in the spectrum with large zero field splitting (c) (Brigatti et al., 155 2000; Sorieul et al., 2005). Fe2+ was usually not observed using 9 GHz EPR; a low-spin d6 complex with no unpaired electrons is diamagnetic and high-spin d6 has large zero- field splitting and was therefore EPR silent. Structural substitution of Fe3+/Al3+ for Si4+ in the tetrahedral and octahedral) sheets (a) and substitution of lower valent cations (e.g.Mg2+) for Fe3+/Al3+ in the octahedral sheet (b) lead to a charge imbalance within the structure giving rise to peaks observed in Figure 4.6 (Gournis et al., 2001; McBride et al., 1975a and 1975b; Berkheiser and Mortland, 1975). Fe(III) peaks are shown initially in the spectrum from charge imbalances predominantly in the octahedral sheet. Manganese, present in small quantities in STx-1b (0.01%), gives rise to a weak 55Mn2+ (I = 5/2) sextet centred at ≈ g=2 (d) due to hyperfine splitting (Jones et al., 1974). The natural radiation induced defect present (e) in STx-1b was also centred at ≈ g=2 (Allard and Calas, 2009; Sorieul et al., 2005; Hall. 1980) and when irradiated the Mn2+ sextet was masked due to the similarity in positioning. The EPR spectra for a series of un-substituted STx-1b heated between 25–500 °C was exemplified in Figure 4.7, using the two end members of the heated series (25 and 500 °C). A small decrease in amorphous Fe3+ oxides (coatings and/or discreet phase) was observed (Figure 4.6c) with a concomitant increase in more crystalline structural Fe3+ (Figure 4.6a). This suggests that amorphous Fe3+ was transformed to more crystalline phases with Fe3+ in octahedral coordination. The RID and the weak Mn2+ sextet at g≈2 (≈3500 G or 350 mT) did not change with heating to 500 °C. However, the signal at g≈3.5 (≈2000 G or 200 mT, b Figure 4.6), from the substitution of lower valent cations (e.g. Mg2+) into the octahedral sheets of montmorillonite, was lost when the structure was heated and annealing of the electronic defect occurred with heating to 500 °C presumably as the charge defects in the clay structure were balanced through migration of cations into octahedral vacancies (McBride et al., 1975a and 1975b). The predominant RID produced in montmorillonite as a result of γ-irradiation (Figure 4.8a inset) was a doublet at g≈2 (N-centre, Allard et al., 2012) corresponding to isotropic axial (N) and equatorial (N’) defects on the π Si–O orbitals in the tetrahedral sheet (Sorieul et al., 2005). The loss of an axial Si-O π electron was favoured in montmorillonite as charge loss was balanced by substitution of Mg2+ into the octahedral sheet (Figure 4.8c), not possible from the equatorial position, hence the 156 predominance of the axial associated defect in which the magnetic axis was also maximized with the clay sheets parallel to the magnetic field (Berkheiser and Mortland, 1975). A second defect (B-centre) induced by irradiation occurred in the octahedral sheet (Figure 4.8b) and was assigned to the loss of an electron from an Al– bound oxygen (equatorial) (Clozel et al., 1995). Similar defects have been previously observed by Allard et al. (2009, 2012), Sorieul et al. (2005), Clozel et al., (1995), Hall (1980), and Jones et al. (1974) on smectites, dickite, illite, and kaolinite assignable through EPR analysis of their thermal stability. The majority of the research to date has been done on kaolinite, which closely resembles the defect centres in montmorillonite, with the absence of water (Allard et al., 2012). In the current study on STx-1b, the nature of the defects remained the same but the number of defects (i.e. the number of electrons displaced) increased proportionally with dose (Figure 4.9). The effect of heating STx-1b to 25, 90, 160, 500 °C and subsequent γ-irradiation (5 MGy) was studied (shown in Figure 4.10). At 25 °C, the predominant RID was present as an axial doublet at ≈ g=2, corresponding to the displacement of an electron within the tetrahedral layer. At 90 °C, the axial doublet corresponding to the RID increases in intensity as a greater number of defects are stabilised due to thermally-induced disorder in the interlayer. At 160 °C, a reduction in the number of RIDs was observed due to an increase in ordering within the clay structure, through the loss of interlayer water increasing interlayer interaction with the clay structure and alleviating some of the charge inbalance, and further to this the migration of small interlayer cations into tetrahedral spaces and octahedral vacancies (Gournis et al., 2001). At 500 °C, the axial doublet evolved into a broader, more isotropic signal related to production of new defect centres, as a result of structural alteration and the formation of new minerals, observed by XRD (Figure 4.3). 4.4.6. Effect of γ-irradiation on STx-1b colloid formation Zeta potential measurements, providing information on the magnitude of electrostatic repulsion or attraction between particles, were used to study the effect of γ-irradiation on colloid stability calculated using colloid mobility measurements following a basic Smoluchowski model. Zeta potential results (Table 4.6) show that the γ-irradiated colloids had a slightly higher negative surface charge, resulting in greater charge 157 repulsion and therefore increased colloid stability. Irradiation generates defect sites in the clay; these are oxygen radicals that have been deprotonated. This imparts an overall negative charge to the clay structure. For colloids, this manifest itself as a negative surface charge because the particles are so small that they are basically all surface. γ-irradiation had little effect on the scattering intensity weighted colloidal particle size distribution as measured by PCS with 100–500 nm aggregates present before and after irradiation although average variability within the sample did decrease suggesting less agglomeration (Table 4.4SI). Table 4.6. Zeta potential measurements on STx-1b before and after irradiation (5 MGy). Error was 3% on measurements. Sample Zeta Potential (mV) STx-1b -26.6 ± 0.72 STx-1b γ-irradiated -28.8 ± 0.52 Atomic Force Microscopy (AFM) was used to study changes in morphology and electrostatic repulsion as a function of γ-irradiation. The size (≈400 nm) and morphology of the colloids remained predominantly unchanged; although some more rounded structural aggregates appeared to be present in the γ-irradiated samples (Figure 4.11). SEM analysis also showed limited changes to the morphology of the colloids before and after γ-irradiation (Figure 4.15SI). The effect of pH on colloid detachment from a freshly cleaved biotite surface, before and after γ-radiation, was measured. The colloids show minimal detachment at pH 5, irreversibly binding to the biotite surface after drying. At pH 9 the colloids show detachment (Figure 4.14SI) as expected from the increased repulsive forces with past work showing maximum stability of clay colloids at pH 8 (Laakosoharju and Wold, 2005). The time resolved image measurements showed a gradual detachment of particles and, after 120 minutes, no particles were found on the biotite surface. In order to confirm small differences seen in zeta potential measurements, the effect of pH and γ-irradiation on forces between polystyrene particles and clay particles was studied (Figure 4.12 and 4.13SI). A slightly larger negative potential (repulsive) force 158 was present in the γ-irradiated sample (highlighted by arrow), at both pH 5 (Figure 4.13SI) and pH 9 (Figure 4.12), compared with the non-irradiated sample, further assuring the zeta potential results; the small increase in zeta potential (negative) can be assumed to be a real effect. 4.5. Discussion 4.5.1. Effect of chemical composition on clay barrier performance The nature of the interlayer cation will play a significant role in the long-term performance of the clay barrier. Specifically the swelling capacity of the clay will impact on limiting groundwater flow and retarding the release of radionuclide after canister failure. Divalent cation (Mg2+ and Ca2+)-substituted montmorillonites had greater resistance to heating and γ-irradiation, showing larger d-spacings (≈15–16 Å), higher CEC, and reduced distortion of the structure as a function of γ-irradiation. This variation in interlayer spacing for clays with different cations was due to changes in hydration shell co-ordination by Hendricks et al. (1940). The size of the interlayer spacing in mono-valent cation (Na+- and K+-) substituted montmorillonite was reduced compared with the native clay, with Na+-substituted clay showing a further reduction after γ-irradiation (especially in high Fe-bearing clays) and K+-substituted clays showing a d001 basal plane reduction from ≈16 Å in natural clay samples to 12 Å, with no further decrease upon γ-irradiation. Previous studies have also shown that the infiltration of K+-containing groundwater may cause a reduction in radionuclide sorption capacity of the clay (Comans and Hockley, 1992). If K+ was incorporated into the interlayer, a reduction in d001 occurs due to the lower hydration energy and increased cationic radius (Sawhney, 1972), which could lead to: (i) a reduction in pressure within the GDF; (ii) fracturing of the clay; and (iii) possible alteration to illite with heating over geological time scales (Keller, 1964). However, theoretical studies (Johnson et al., 1994a; SKB; 2006b), natural analogue studies over geological timescales (Sellin and Leupin, 2013) and laboratory-based experiments (Wersin et al., 2006; Posiva, 2009) have shown negligible transformation with expected K+ concentration. Studying Fe-rich nontronite provides information on the potential effects of Fe incorporation into clay as a result of canister corrosion. Svensson (2015) estimated 159 that transformation through interaction with Fe-based canister corrosion products would affect the clay up to a distance of 10 mm from the canister in MX-80 (bentonite) under representative SKB repository conditions (≈50 °C, 2.5 years). Fe-rich clays exhibited a decrease in thermal and radiation stability, as demonstrated by a lower initial d001 of 15 Å, likely due to the increase in layer charge, particularly when the interlayer contained monovalent cations where interlayer space reduces from 15 to 12 Å without heating. Other factors affecting charge are impurities present within the structure and thermal conductivity (Karnland, 2010). The higher Fe content (Table 4.2) in the nontronite provides electron transport pathways not possible in lower Fe clays (Latta et al., 2017). This could presumably influence radiolysis pathways through charge transfer to the interlayer space and charge trapping within the structure (Fourdrin et al., 2013; Lainé et al., 2017). The higher negative surface charge, occurring 2+ 3+ 3+ i4+ because of Fe /Fe substitution for Al /S in these higher Fe systems, would not be balanced as effectively by the cations in the interlayer leading to a higher net negative charge. This could lead to a higher affinity of the clay for cationic radionuclides. In this study, the amount of extractable Fe2+ increased (between 1 – 10%) after irradiation (5 MGy), depending on the initial concentration of Fe present in the structure. This increase in extractable Fe2+ could be due to radiolysis products from γ-irradiation, such as e− or H•, partially reducing Fe3+ to Fe2+ within the clay octahedral layers. The effect was enhanced when irradiation was carried out under anaerobic conditions reflecting the fact that molecular oxygen was a good scavenger of e− and H•. An increase in extractable Fe2+ has been previously observed at γ-irradiation doses on the order of 100 kGy (Gournis et al., 2000; Bank et al., 2008). Fe K-edge XANES showed no change (within error) in the bulk Fe oxidation state during the experiment as the amount of γ-radiation induced reduction seen using chemical extractions was within the detection limit for the technique (≈5–10%, see Table 4.4) (Calvin, 2013; Baker and Strawn, 2012). 4.5.2. Effect of heat and γ-irradiation on clay barrier performance Results from XRD, IR, and CEC measurements show that γ-radiation had no significant effect on bulk clay structure. However, atomic scale information provided by EXAFS demonstrated that γ-radiation may have caused distortion of oxygens surrounding divalent interlayer cations (Ca2+), possibly increasing the disorder in the system 160 minimally, evidenced by a slight increase in the Debye Waller factor and a reduction of the amplitude in the Fourier transform post irradiation over multiple samples (Table 4.5). Additionally, after γ-irradiation, an increase in the amplitude of the Fourier transform corresponding to the Fe K-edge EXAFS oscillations suggests an increase in structural order of the local bonding environment for Fe, present either within the clay structure or as an amorphous Fe-oxyhydroxide coating on the clay particles, although this effect was small and could be said to be negligible or within error of measurements due to the self-absorbance effects seen. This trend of Fe changing from an amorphous to a more crystalline environment with γ-irradiation, was observed in the EPR data (Figure 4.6) in which the broad background decreased slightly in intensity, and was associated with lower γ-radiation doses (<1000 MGy); at higher doses, the opposite trend, i.e. amorphisation as a function of dose, was observed (Allard and Calas, 2009). Most previous work on γ-irradiation of Fe-bearing clay has focused on the reduction of Fe within the clay structure; however, the results presented here show small structural responses to irradiation that are not associated with a change in oxidation state. In addition, EPR analysis highlighted that γ-radiation resulted in an increase in the electronic defects in the samples. This had a clear impact on the overall surface charge of the clay, increasing the magnitude of the zeta potential in colloidal samples through electrons trapping at surface sites (as colloid mostly surface) (Table 4.6). In the bulk clay structure electrons could become mobile through redox active transport pathways in the clay such as Fe (Neumann et al., 2015). The creation of radiation induced defects (RIDs) from γ-irradiation caused local charge imbalance within the clay mineral lattice as seen in past work across a range of minerals (Allard and Calas, 2009; Hall, 1980; Jones et al., 1974), which was maximized when the clay sheets were parallel to the magnetic field in the EPR measurments (Berkheiser and Mortland, 1975). Minimal changes in the interlayer space with heating up to 160 °C were shown for both STx-1b and NAu-1 substituted with divalent interlayer cations, as well as an increased susceptibility for the interlayer to rehydrate after γ-irradiation (5 MGy). Clays with monovalent Na+ or K+ in the interlayer showed interlayer collapse at a lower temperature and a reduced affinity for rehydration after γ-irradiation. In a study by Karnland and Birgersson (2006), new phases were formed at temperatures above 161 160 °C, which had a negative effect on the ability of the clay to swell, causing crack formation and accelerated transport via advection flow. The high thermal conductivity of clay would help to mitigate these effects, even at elevated temperatures (<160 °C) (Hicks et al., 2009). In this study, new phases were formed at higher temperatures (500 and 1000 °C), with a decrease in the d001 interlayer space (10 Å), resembling that of an illite-like species at 500 °C (both before and after γ-irradiation). Heating STx-1b to 500 °C prior to irradiation had little effect on naturally occurring RIDs, but did result in migration of interlayer cations into gaps within clay tetrahedral sheets and vacancies in the octahedral sheets, which served to mitigate the charge imbalance caused by substitution of lower valent cations (i.e. Mg2+) for Fe3+/Al3+ in the octahedral sheet, observable by a change in intensity in the EPR spectrum at g≈3.5 (McBride et al., 1975a and 1975b). A decrease in Fe(III)-oxides and –oxyhydroxides, either as amorphous Fe material or Fe coatings and/or a discreet phase and a corresponding increase in structural Fe3+ (g~4.4) were also observed with EPR (McBride et al., 1975b). Thus, heating at temperatures up to 160 °C was shown to have a minimal effect on the clay structure and, even at the upper end of GDF lifetime expected γ doses (5 MGy), the clay barrier would maintain its swelling properties and thus limit groundwater flow around the waste canister. The effect of heating on the RIDs, corresponding to the displacement of electrons from within the structure, varies depending on the charge stabilisation mechanism. Previous studies have shown that heating after γ-radiation annealed the defects within the structure (Pusch, 2002; Sorieul et al., 2005; Sorieul et al., 2008; Holmboe et al., 2009; Tabacaru et al., 2010; Wilson et al., 2010; Allard et al., 2012). Heating post irradiation mobilises trapped electrons (e−), which can then recombine with trapped (hole) defects. In the current study, the clays were heated prior to γ-irradiation, and the observed results were different to annealing studies previously conducted as the e− remain trapped. At room temperature, the presence of interlayer water reduced the intensity of the main RID in the EPR spectrum (g≈2). At 90 °C, the amount of RIDs increased and charge stabilisation was achieved through thermally-induced disorder in the hydrated interlayer. A slight increase in structural order in the clay, through loss of free interlayer water and the interaction of interlayer cations with surface sites, occurred with heating up to 160 °C and this stabilised some of the charge defects 162 present within the clays reducing the number of RIDs seen. Generation of new phases at 500 °C (montmorillonite still present), confirmed by XRD, caused the axial doublet to broaden into more isotropic signal due to the production of multiple defect sites in newly-formed mineral phases. The presence of the increased RIDs after γ-radiation could have an effect on the sorption capacity of the clay, and the potential for colloid formation. Colloids have a high affinity for radionuclides due to their high surface area and have the potential to actively transport radionuclides out of the GDF and into the surrounding environment (Buddemeier and Hunt., 1988; Kersting et al., 1999; Schäfer et al., 2012). The STx-1b montmorillonite colloids showed increased stability after γ-irradiation through a small increase in net surface charge (and repulsion) observed in both zeta potential and AFM measurements, with colloidal size and morphology remaining largely unchanged. The STx-1b clay colloids were most stable at pH 9; the pH within the engineered clay barrier of a geological disposal facility for heat generating wastes is predicted to be ≈7–9 (Kaufhold et al., 2008). Here, the modestly enhanced colloid stability as a function of γ-irradiation may impact on the retardation function of the barrier. The increase in negative surface charge as a result of γ-irradiation (trapped electrons) could lead to an increased affinity for cationic radionuclides and, if irreversibly bound, could influence the colloid associated radionuclide transport in the far-field environment. The aforementioned charge increase was induced in the clay structure through the knocking off of an electron from oxygen within the structure (3 defect types, EPR) via the interaction of a γ-ray (high energy electron) with a pi electron of a bonding oxygen, forming oxygen radicals and amassing electrons from this interaction. These electrons are stored at the surface of the structure and induce reactions with redox active species in the clay (i.e. Fe). This imparts an overall negative charge to the clay structure. For clay colloids, this manifest itself as a negative surface charge because the particles are so small that they are basically all surface but position of the charge in the bulk clay itself was still hard to pinpoint exactly but it should be assumed this would be associated with surface groups or redox activite species within the clays. 163 4.6. Conclusions Overall, we have highlighted that minimal changes to the clay structure occur on a macro-scale as a function of the coupled effects of temperature (up to 160 °C) and γ-irradiation (up to 5 MGy). The robustness of the clay to these conditions provides a positive outlook and was encouraging for the safety function of clay proposed for use in a GDF as heat and γ-irradiation would be two of the major challenges faced by the engineered clay barrier after the heat generating wastes are deposited in the GDF. We also highlight the importance of molecular-scale effects, with the formation of radiation induced defects in the electronic structure of the clay having small, but measureable, effects on the clay properties. Further research should be undertaken on the sorption properties of montmorillonite after being subjected to the coupled effects of heating and γ-irradiation because the changes on a molecular scale shown here could have implications for both the bulk and the colloidal fractions of clay over geological timescales in relation to contaminant interaction. Further molecular-scale research could be targeted to optimise the clay structure and further maximise resistance to challenges from heat and γ-irradiation. Extended studies into concurrent heating and γ-irradiation of clay buffer material would give an even clear picture of the behaviour to expect within a GDF at the initial stages of storage but this would pose a the challenge of using a heating source inside and irradiator. Ultimately the work carried out here shows challenges form γ-irradiation at lifetime doses expected within a GDF, at temperatures above that expected of canisters at disposal, can be withstood from the buffer material with only molecular scale changes seen in the form of charge defect accumulation. This may allow HLW canisters to be disposed of closer together, thus reducing the costs associated with GDF construction allowing for a smaller GDF footprint. 4.7. Acknowledgements We acknowledge the Dalton Nuclear Institute, University of Manchester for funding this project and the support of NERC via the Lo-RISE Consortium (NE/L000547/1). T.S. and G.K.D. received partial funding by the Federal Ministry of Economics and Technology (BMWi) under the joint KIT-INE, GRS research project ‘‘KOLLORADO-e” 164 (02E11203B) and the European 7th Framework Programme (FP7/2007-2011) under grant agreement no. 295487 (BELBaR Project). We thank Prof Michael Henderson for his recommendations on draft scripts. We recognize the EPSRC National EPR facility at the University of Manchester for the use of the Bruker EMX Micro EPR spectrometer and thank Adam Brookfield, Daniel Sells, and Prof. David Collison for help with EPR spectroscopy. This work also made use of the 60Co γ irradiator at the Dalton Cumbrian facility (The University of Manchester), which was funded as part of the UK National Nuclear User Facility. We thank Diamond Light Source for access to beamline B18 (SP9647) that contributed to the results presented here; and Dr. Stephen Parry, Dr. Giannantonio Cibin and Prof. Andy Dent are thanked for their assistance in the collection of the XAS data. 4.8. References Ahn, J. and Apted, M.J. (2010) Geological Repository Systems for Safe Disposal of Spent Nuclear Fuels and Radioactive Waste, Woodhead Publishing Limited 1, 3-28. ISBN: 978-1-84569-542-2. Allard, T. Balan, E. Calas, G. Fourdrin, C. 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Similar values for beidellite and nontronite with variable parameters (α, β, γ (all ≈1.4 – 1.6), and colour) depending on compositions (Wilson, 2013). 177 Figure 4.2. d001 X-ray diffraction data for cation exchanged smectites; () starting material, () Na exchanged, (▲) Mg exchanged, (▼) K exchanged, (♦) Ca exchanged. 178 90z °C 25 °C 500 °C 160 °C 1000 °C 500 °C 160 °C 90 °C 25 °C Figure 4.3. a) X-ray diffraction measurements for Ca-montmorillonite (STx-1b) at 25 °C (black - bottom), 90 °C (red), 160 °C (green), 500 °C (blue) and 1000 °C (Dark yellow - top) from 4 to 60 2θ. b) Zoom in of highlighted area showing the shift in 2θ with heating relating to interlayer spacing changes. Index: (1) montmorillonite d001 (2) illite d001 (3) montmorillonite d003 (4) nordstrandite d002 (5) montmorillonite d111 /Quartz d100 (6) Cristobalite d101/ d111/ d102/ d200 (7) montmorillonite d005 (8) Quartz d101 (9) montmorillonite d110 (10) montmorillonite d200. (Sorieul et al., 2008; Chung-lun et al., 2002; Morris et al., 1981) 179 d e f g h Figure 4.4. XAS spectrum of STx-1b Ca K-edge before (Black) and after (Red) 2.7 MGy γ irradiation; a) XANES region b) EXAFS (k-space) c) Fourier transform in R-space. XAS spectrum of NAu-1 Ca K-edge before (Black) and after (Red) 2.7 MGy γ irradiation; d) XANES region e) EXAFS (k-space) f) Fourier transform in R-space. Ca K-edge EXAFS fit data from Artemis in both k-space and R-space (Fourier transform) for STx-1b before g) raw data (black) and fit (green) and after h) γ irradiation (red), fit (blue). 180 d e f g h Figure 4.5. XAS spectrum of STx-1b Fe K-edge before (Black) and after (Red) 2.7 MGy γ-irradiation; a) XANES region b) EXAFS (k-space) c) Fourier transform. XAS spectrum of NAu-1 Fe K-edge before (black) and after anaerobic (red)/ aerobic (blue) 2.7 MGy γ-irradiation; d) XANES region e) EXAFS (k-space) f) Fourier transform. Fe K-edge EXAFS and Fourier transform for STx-1b before g) raw data (black) and fit (green) and after h) γ-irradiation (red), fit (blue). 181 Figure 4.6. EPR spectrum of STx-1b at 25 °C with the peaks ascribed to (a) Structural Fe3+/Al3+ - tetrahedral and octahedral, (b) Structural Fe3+/Mg2+/Metal2+ - octahedral, (c) Fe3+ Oxide, (d) Mn2+ Sextet (first peaks), and defects: (e) Natural defect (radiation induced). 182 Field (G) Figure 4.7. EPR spectra of STx-1b (black) compared to STx-1b heated to 500 °C (red). 183 Figure 4.8. (a) Section of EPR spectrum showing the predominant defect centres of STx-1b γ-irradiated to 5 MGy without heat treatment (25 °C). (b) Structural representation of montmorillonite layers with corresponding defects centres (Adapt. Bergaya and Lagaly, 2013; Allard and Calas, 2009). (c). Molecular representation of main axial defect present in montmorillonite (adapted from Allard et al., 2012). 184 Figure 4.9. EPR spectra of un-irradiated STx-1b (black), γ-irradiated STx-1b at 0.2 MGy (red), 2.7 MGy (green) and 5 MGy (blue) all at 25 °C. 185 Figure 4.10. EPR spectra of γ-irradiated STx-1b at 5 MGy at 25 °C (blue), 90 °C (orange), 160 °C (red) and 500 °C (burgundy). 186 Figure 4.11. AFM morphology pictures of dried colloid suspensions at a 0.5 μm and 1 μm scale on the starting STx-1b and the γ-irradiated (5 MGy) STx-1b. 187 Figure 4.12. Force vs. separation graph between polystyrene particles and clay particles showing the effect of γ-irradiation at pH 9 (colloid stability). 188 Molecular- to meso-scale effects of heat and subsequent gamma radiation on engineered clay barrier performance for radioactive waste disposal A. P. Simsa,b, J. M. Devinea, W. R. Bowera, K. Morrisb, R. A. D. Pattrickb, R. Edgec, S. M. Pimblotta,c,d, A. J. Fieldinge, T. Schäferf,g, G. K. Darbhag,h, J. F. W. Mosselmansi, F. R. Livensa, C. I. Pearce*a,j a Centre for Radiochemistry Research, School of Chemistry, University of Manchester, M13 9PL UK ; b Research Centre for Radwaste Disposal and Williamson Research Centre, School of Earth and Environmental Sciences, University of Manchester, M13 9PL UK; c Dalton Cumbrian Facility, University of Manchester, M13 9PL UK; d Idaho National Laboratory, Idaho Falls, 83402, Idaho, USA; e School of Pharmacy and Biomolecular Sciences, Liverpool John Moores University, L3 3AF UK; f Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Germany; g Friedrich- Schiller-University Jena, Institute of Geosciences, Applied Geology, Burgweg 11, D-07749 Jena, Germany; h Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, West Bengal, India; i Diamond Light Source, Harwell, Oxford, UK; j Pacific Northwest National Laboratory, Richland, Washington, 99352, USA. 4.10. Supplementary Information: Methods Homo-ionisation: the same method was used for each homo-ionisation completed with the only substitution dependent on the ion required for homo-ionisation (i.e. Na, Mg, K, and Ca) and clay preparing; the following example uses sodium and STx-1b as an example. A 2 M solution of sodium chloride (2.34 g) in deionised water (20 mL) was prepared and left to equilibrate (2 hours). STx-1b (0.5 g) was added to the solution and rocked (48 hrs) to allow exchange. Solutions were centrifuged (4500 rpm, 10 minutes) to separate out Stx-1b from solution and supernatant decanted. The resulting clay was washed twice with 1 M sodium chloride (2x 20 mL, 2.34 g), once with sodium acetate 189 (40 mL, 3.29 g), and again with sodium chloride (20 mL, 1.17), re-suspending prior to each centrifugation and decanting the solution after. A final wash with a water ethanol mix (20 mL, 50:50) was completed with a final centrifugation and resulting clay dried over MgCl2 in a desiccator until use and in sealed glass vials after use for storage. WLOI: carbonate results using thermogravimetric analysis on a Metter Toledo TGA/DSC 1 STAR System run from 0–1000 °C at a ramp rate of 10 °C/ min under an air purge atmosphere under atmospheric pressure. XRD: pelleted samples mounted on holder and levelled with top of holder. Powder diffraction samples were run under the same parameters but dissolved in amyl acetate and a thin layer was applied evenly to a glass slide. A leeching study of all untreated clays (2 g in 10 mL) and two sets of treated clays (Na, Mg, K, Ca), SWy-1 and NAu-2 (0.2 g in 10 mL), was undertaken to look at water chemistry associated with homo-ionisation reactions using ion chromatography. Colloid preparation was adapted from Bouby et al (2011) and removed particles of a size greater than 500 nm. STx-1b (1.0 g) in 100 mL of deionised water was stirred for 7 days; resulting solutions were pipetted into centrifuge tubes. Samples were centrifuged (4000 rpm, 11 min) and resulting supernatants decanted and clay was re- suspended. The process was repeated two more times and final supernatant keep as stock colloid solution. 190 XRD Figure 4.1SI. Comparison of irradiated (red) vs natural (black) X-ray diffraction patterns of Mg- montmorillonite (STx-1b) heated to 90 °C. Shows similar patterns with identical d001 spacing before and after γ-irradiation. Only real change seen was in amplitude which in XRD measurements due to orientation of the samples when measured. Scherrer equation 퐾휆 휏 = Relates to particle size – liking to the broadening of the peaks 훽푐표푠휃 WLOI Table 4.1SI. Weight loss of ignition data for natural smectites. Soil Amount Organic Inorganic Total C Sample (g) C (%) C (%) (%) SWy-1 2.00 1.08 4.58 5.66 STx-1b 2.00 1.86 3.76 5.61 SCa-3 2.00 3.28 4.74 8.02 SAz-2 2.01 3.34 5.08 8.41 SBld-1 2.02 2.15 5.06 7.21 NAu-1 2.00 3.95 3.43 7.37 NAu-2 2.01 1.75 4.00 5.75 All show similar results for total carbon as seen in the XRF results 191 IR Figure 4.2SI. Assigned infra-red spectrum of a nontronite and montmorillonite. Figure 4.3SI. Effects of γ-irradiation on montmorillonite (STx-1b) shown for unirradiated (black and red), STx-1b 0.2 MGy (blue), 2.7 MGy (pink), and 5 MGy (green). Little effect seen with γ-irradiation of STx-1b – normalised curves show little change, possible more hydration after irradiation – stored aerobically. Annotated spectrums show relevant peaks with structural and free water peaks at ≈1600 cm-1 and ≈3400 cm- 1 and hydroxyl peak at 3600 cm-1; heating shows reduction of water associated with structure and heating above 160 °C shows loss of some structural peaks. Table 4.2SI. CEC and ferrozine measurements on natural clays. Sample Total CEC meq/ 100 g Fe extractable from total Fe present % STx-1b 90.93 0.67 192 SWy-1 71.92 0.47 SCa-3 84.53 0.10 SAz-2 90.55 0.23 SBld-1 51.84 0.21 NAu-1 78.21 0.05 NAu-1 78.21 0.05 EXAFS a b c Figure 4.4SI XAS spectrum of SCa-3 Ca K-edge before (Black) and after (Red) 2.7 MGy γ-irradiation. a) XANES region b) EXAFS (k-space) c) Fourier transform in R-space. a b c Figure 4.5SI. XAS spectrum of NAu-2 Ca K-edge before (Black) and after (Red) 2.7 MGy γ-irradiation. a) XANES region b) EXAFS (k-space) c) Fourier transform in R-space. 193 a b c Figure 4.6SI. XAS spectrum of STx-b potassium K-edge before (black) and after (red) 2.7 MGy γ-irradiation. a) XANES region b) EXAFS (k-space) c) Fourier transform in R-space. a b c Figure 7SI. XAS spectrum of SCa-3 potassium K-edge before (black) and after (red) 2.7 MGy γ-irradiation. a) XANES region b) EXAFS (k-space) c) Fourier transform in R-space. a b c Figure 4.8SI. XAS spectrum of SCa-3 Fe K-edge before (Black) and after anaerobic (Red)/ aerobic (blue) 2.7 MGy γ-irradiation. a) XANES region b) EXAFS (k-space) c) Fourier transform. a b c 194 Figure 4.9SI. XAS spectrum of NAu-2 Fe K-edge before (Black) and after (Red) 2.7 MGy γ-irradiation. a) XANES region b) EXAFS (k-space) c) Fourier transform. a) b) Figure 4.10SI. Calcium K-edge EXAFS and Fourier transform for NAu-1 before a) raw data (black) and fit (green) and after b) γ-irradiation (red) and fit (blue). 195 a) b) Figure 4.11SI. Potassium K-edge EXAFS and Fourier transform for STx-1b before a) raw data (black) and fit (green) and after b) γ-irradiation (red) and fit (blue). 196 a) b) c) Figure 4.12SI. Iron K-edge EXAFS and Fourier transform for NAu-1 before a) raw data (black) and fit (green) and after b) anaerobic γ-irradiation (red) and fit (navy), c) aerobic γ-irradiation (blue) and fit (pink). Table 4.3SI. EXAFS fits for STx-1b, NAu-2 and SCa-3 from Artemus using a set co- ordination number (a) for interlayer Ca2+, K+ and Fe2+/3+. 2 2 2 Sample Path CN R (Å) σ (Å ) SO r + + a 2.81 0.024 a STx-1b K K-O 1 6 − − 1.00 0.018 0.080 0.005 + + a 3.06 0.019 K-O 2 4 − − 0.219 0.007 + + a 3.64 0.034 K-Si 1 10 − − 0.122 0.004 + + a 2.87 0.022 a STx-1b K γ K-O 1 6 − − 1.00 0.016 0.023 0.003 + + a 3.15 0.018 K-O 2 4 − − 0.120 0.006 + + a 3.68 0.040 K-Si 1 10 − − 0.083 0.006 197 + + a 2.41 0.012 a SCa-3 Ca Ca-O 6 − − 1.00 0.006 0.009 0.001 + + a 2.39 0.013 a SCa-3 Ca γ Ca-O 6 − − 1.00 0.013 0.016 0.001 + + a 2.00 0.005 a SCa-3 Fe Fe-O 6 − − 0.77 0.018 0.008 0.001 + + a 3.06 0.005 Fe-Al 3 − − 0.024 0.002 + + a 3.23 0.005 Fe-Si 2 − − 0.019 0.002 + + a 3.78 0.010 Fe-O 4 − − 0.060 0.008 + + a 2.00 0.005 a SCa-3 Fe γ ANA Fe-O 6 − − 0.77 0.011 0.007 0.001 + + a 3.01 0.006 Fe-Al 3 − − 0.019 0.002 + + a 3.21 0.006 Fe-Si 2 − − 0.016 0.002 + + a 3.76 0.006 Fe-O 4 − − 0.029 0.003 + + a 2.00 0.004 a SCa-3 Fe γ AER Fe-O 6 − − 0.77 0.011 0.008 0.001 + + a 3.06 0.005 Fe-Al 3 − − 0.021 0.002 + + a 3.23 0.005 Fe-Si 2 − − 0.018 0.002 + + a 3.78 0.005 Fe-O 4 − − 0.035 0.004 + + a 2.40 0.009 a NAu-2 Ca Ca-O 6 − − 0.87 0.012 0.015 0.001 + + a 2.39 0.012 a NAu-2 Ca γ Ca-O 6 − − 0.87 0.015 0.018 0.001 + + a 2.02 0.006 a NAu-2 Fe Fe-O 6 − − 0.95 0.017 0.011 0.001 + + Fe-Fe 3 a 3.07 0.006 − − 0.015 0.002 + + a 3.28 0.007 Fe-Si 4 − − 0.044 0.004 + + a 2.02 0.003 a NAu-2 Fe γ Fe-O 6 − − 0.95 0.017 0.011 0.001 + + a 3.04 0.004 Fe-Fe 3 − − 0.014 0.001 + + a 3.33 0.004 Fe-Si 4 − − 0.040 0.001 198 Colloids Table 4.4SI. Colloidal zeta potential and size measurements. Sample Zeta Potential Mobility Size rangea Meana Modea Meanb Modeb Meanc Modec (mV) (μs-1)/(V/cm) (nm) (nm) (nm) (nm) (nm) (nm) (nm) STx-1b -26.6 ± 0.72 -2.08 ± 0.05 250-900 406 ± 4.1 292 ± 4.1 525 ± 493 ± 342 ± 301 ± untreated* 177 177 181 181 STx-1b -28.8 ± 0.52 -2.25 ± 0.04 240- 780 426 ± 456 ± - - 326 ± 226 ± γ-irradiated* 9.2 9.2 197 197 STx-1b heated - - - - - 727 ± 594 ± - - (90 °C)* 347 347 STx-1b heated - - - - - 679 ± 594 ± - - (160 °C)* 432 432 *measurements on one sample (error on difference in repeats). a Brookhaven ZetaPlus Zeta potential Analyzer. b Malvern Mastersizer 3000. c NanoSight. 199 AFM Figure 4.13SI. Force vs. separation graphs showing the effect of pH and γ-irradiation between polystyrene particles and clay particles. 200 Colloid detachment at pH 5 0 min 60 min Colloid detachment at pH 9 0 min 60 min Figure 4.14SI. AFM images of colloidal STx-1b detachment at pH 5 and pH 9 201 SEM Before irradiation After irradiation Figure 4.15SI. SEM images of colloidal STx-1b before and after γ-irradiation. 202 5. The coupled physicochemical effects of alpha irradiation and heating on clay barriers for radioactive waste disposal This chapter is presented as a manuscript prepared for submission to the Journal of Nuclear Materials. Supporting information for this manuscript is included immediately following this paper. 203 The coupled physicochemical effects of alpha irradiation and heating on clay barriers for radioactive waste disposal A. P. Simsa,b, W. R. Bowera, K. Morrisb, R. A. D. Pattrickb, A.D. Smithc, J. Behnsend, M. Millere, J. F. W. Mosselmansf, F. R. Livensa, C. I. Pearce*a,e a School of Chemistry, University of Manchester, M13 9PL UK ; b School of Earth and Environmental Sciences, University of Manchester, M13 9PL UK; c Dalton Cumbrian Facility, University of Manchester, M13 9PL UK; d Henry Moseley X-ray Imaging Facility | School of Materials, University of Manchester, M13 9PL UK; e Pacific Northwest National Laboratory, Richland, WA, USA; f Diamond Light Source, Harwell Campus, Didcot, OX11 0DE UK. 5.1. Abstract The safe containment of long lived, radioactive waste is crucial for the future of nuclear power. Clay is an integral part of the engineered barrier system surrounding the waste canisters in many proposed geological disposal facilities (GDFs) for heat generating radioactive waste. The engineered clay barrier (ECB) protects the waste canister against corrosion, but it must have the necessary physicochemical properties to limit radionuclide release after canister failure, when it is exposed to alpha radiation and heat from the decay of actinides in the waste (>120 °C). Characterising the clay after exposure to alpha particles (α) and heat (25 and 120 °C), using a combination of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray computed tomography (XCT), is fundamental to understanding the limitations of the ECB. Heat showed minimal change to the clay structure on a macro-scale, but α-irradiation caused amorphisation in areas of moderate dose (> 0.15 dpa) with a collapse or loss of the interlayer (d001 basal spacing), an increased pore density, and visible damage to the clay surface. Clays with a higher Fe content showed resilience to alpha damage with only partial collapse of the interlayer observed due to the stabilising presence of Fe dampening radiolysis effects. These results suggest a self-propagating transport mechanism for radionuclides as a function α-irradiation, through a possible sorption- desorption cycle, potentially leading to radionuclide release from the GDF. This 204 physicochemical characterization of clay after exposure to the challenging GDF environment is required for a predictive understanding of local radionuclide transport. 5.2. Introduction The UK Government has set out a framework for managing higher activity radioactive waste in the long-term through geological disposal (DECC, 2014; NDA, 2010). All proposed GDFs, in the UK and worldwide, incorporate the use of a multi-barrier system designed to isolate the waste from the surrounding environment and to contain the slowly decaying radioactivity for 105-106 years (Sellin and Leupin, 2013; Ahn and Apted, 2010; Svenke and Nilsson, 1983). In the case of heat-generating wastes, including high level wastes (HLW) and spent nuclear fuel (SNF), the multi- barrier concept comprises of an engineered clay barrier (ECB) surrounding a central robust containment system. HLW is incorporated into a glass matrix and welded into a stainless steel canister (Lee and Tank, 1985) and SNF is placed in a similarly robust canister and over-packed, e.g. with copper, before emplacement underground (RWM, 2016, Baldwin et al., 2008). In both cases, the waste package container is surrounded by an ECB which is intended to swell and minimise groundwater flow around the canister for several hundred years. At longer times, when the canister can no longer be assumed to be intact due to inevitable chemical and physical degradation, the ECB ’safety function’ evolves to retard radionuclide release from the waste (Wilson et al., 2011; Bernier et al., 2007, Holton et al., 2012). Due to its low hydraulic conductivity, high swelling properties, high ion exchange and sorption capacity for radionuclides, the aluminium phyllosilicate bentonite is a key ECB component (Sellin and Leupin, 2013; Wilson et al., 2011; Hicks et al., 2009). Bentonite consists predominantly (> 75%) of the dioctahedral smectite, montmorillonite, with limited accessory minerals and organics (Posiva, 2009; Ahn and Apted, 2010; Karnland, 2010; Stanković et al., 2011). Montmorillonites have different chemical compositions (Si4+, Al3+, Fe2+/3+, K+, Na+, Ca2+, and Mg2+ content), structural site occupancies (octahedral, tetrahedral) and layer charges, with these factors significantly affecting ECB swelling and sorption properties (Savage and Authur, 2012; Brigatti et al., 2011). Even after several decades of interim storage, disposal of heat generating waste would likely lead to temperatures of 80-90°C in the ECB. As well as heat, the ECB would also 205 be exposed to ionizing radiation. Over time, the primary containment would corrode releasing corrosion products (Fe3O4, Fe(OH)2, Fe(II), Cu(I)) which would interact with the ECB and change the chemical composition of the montmorillonite (Gorski et al., 2013; Neumann et al., 2015). Without the protection of the waste container, the ECB would then be exposed to the waste and irradiation from the longer-lived radionuclides. All types of radiations interact efficiently with clay particles because their small size and high surface area (Lu et al., 2013). To predict the ability of the ECB to perform its safety function, i.e. limiting radionuclide transport out of the GDF to diffusive processes (Nagra, 2002; Missana et al., 2008; Beattie and Williams, 2012; Sellin and Leupin, 2013), the coupled effects of radiation and heat on the swelling and cation exchange capacity of montmorillonite must be investigated (Ewing et al., 2000; Pusch, 1994; Petit et al., 1987; Weber et al., 1998). 5.2.1. Effects of heat on ECB performance Heat on its own has limited effects on the ECB (Couture, 1985). Smectites are stable up to 80-100 °C over millions of years and maintain swelling properties below 200 °C (Pusch, 1983; Kamei et al., 1999; Hansen et al., 2012). Disposal of heat-generating waste in the GDF may lead to canister temperatures of ≈160 °C, resulting in an adjacent ECB temperature of <100 °C. Previous studies at 90 and 120 °C showed no significant changes in advantageous clay properties or clay mineralogy, suggesting that the ECB would remain stable at even at the higher temperatures during initial periods of storage and before ground water infiltration (Honty et al., 2012; Hicks et al., (2009); Wersin et al., 2007). Dehydration upon heating up to 400 °C had no effect on the Cs+ ion exchange capacity of smectite, but dehydroxylation, amorphisation, and phase transformation above this temperature caused a decrease in the exchangeable cations (Gu et al., 2001). 5.2.2. Effects of Radiation on ECB performance During initial periods of storage, the ECB would be exposed to ionizing gamma radiation and high energy electrons produced by interaction of γ-rays with the waste container, which cause small changes to the clay over a large scale (m), at high doses of several MGy (Huang and Chen, 2004; Negron et al., 2002; Plötze, 2003; Plötze et al., 2002; Pusch et al., 1992; Pushkareva et al., 2002). These small changes include 206 increases in: (i) charge (electronic) defects; (ii) the Fe(II)/Fe(III) ratio; and (iii) colloid stability, which could influence ECB performance with respect to radionuclide retardation (Sims et al., In Press; Allard et al., 2012; Holmboe et al., 2011; Holmboe et al., 2009; Souriel et al., 2005). Over time, the primary containment would corrode, without the protection of the waste container, the ECB would be exposed to α-emitters (e.g. U, Np, Pu, Am), β- emitters (e.g. Cs, Sr), and ionising γ-radiation (inelastic) as well as heavy ion (elastic) effects from the actinides and fission products in the waste (Holton et al., 2012; NDA, 2+ 2010; Ewing, 2001). Damage caused by emission of α-particles (4He ions) from actinide nuclei would dominate over longer (>500 years) time periods (Allard and Calas 2009, Ewing et al., 2000, Pusch, 1994; Ewing et al., 1995) and is quantified by average displacements per atom (dpa), which measures the effect of ion bombardment on each individual atom in the structure and is based on the energy of the particle, material density, and crystallite size (Bower et al., 2015; Lu et al., 2013; Lieser and Kratz, 2008; Weber, 2000). Radiation damage occurs through two distinct processes; α- particles themselves (4.5–5.5 MeV) causing displacements and ionisation, and the recoil of heavy actinide nuclei (70–100 keV) causing dislocations (Farnan et al., 2007). α-particles and recoil nuclei affect the oxidation state of redox active elements in the ECB, as well its physical integrity, by causing structural breakdown, vacancy formation (Frenkel defects), radiolysis within the material, and lattice distortions (Ewing, 2001; Ewing, et al., 1995; Pattrick et al., 2013; Allard and Calas, 2009; Suckow, 2009; Lieser and Kratz, 2008). For example, Sorieul et al. (2005) showed the formation of hole defects (electronic), with an associated reduction in Fe3+ (Allard and Calas, 2009). This was also the case for α-irradiated biotite, in which the electron-liberating radiolysis of structural OH− groups reduced Fe3+ to Fe2+, as demonstrated by a shift to lower energy in the Fe K-edge X-ray absorption spectrum (Pattrick et al., 2013; Bower et al., 2015). The structural effects of α-irradiation have been demonstrated in montmorillonite (SCa-3), where the d001 basal spacing (interlayer space) collapsed from 14.1 Å to 9.5 Å, which would significantly affect the cation exchange capacity and swelling properties (Mosselmans et al., 2016). As the α-particles (He2+ ions) move through materials such as smectite, they can pick up electrons to form He gas resulting in bubble formation, even at relatively low doses (0.2-0.5 dpa) (Wang et al., 1999; 2004). However, it is 207 likely that the increased porosity caused by concurrent heavy ion damage would allow the He to diffuse through micro-cracks avoiding bubble accumulation (Kitamura and Takase, 2016). The ECB has a high sorption capacity for actinides, with the trapped α-emitters causing localized amorphisation in the montmorillonite and potential release of the actinide, with subsequent structural disintegration along the migration path (Kitamura and Takase, 2016; Jonsson, 2012; Sorieul et al., 2008). These sorption/desorption cycles can cause propagation of the damage away from the source and potentially provide an advective flow path for radionuclides out of the GDF (Gu et al., 2001). Migration of radionuclides through these damaged areas under flow conditions would cause damage in the wider clay area (Wilson et al., 2011). Conversely, amorphisation may enhance sorption and retard release of sorbed radionuclides, as has been shown for zeolites (Wang et al., 2004). Amorphisation also increases the reactive surface area, - with α-irradiated biotite showing enhanced sorption of selenite (SeO4 ) (Bower, 2016). The α-dose required for destruction of the montmorillonite crystal structure to form an amorphous, siliceous mass is on the order of 1018 α-particles per gram of clay (Beall, 1984; Grauer, 1986; Meunier et al., 1998). However, damage due to α-emitters has been shown at doses as low as 1015 α/ g (SKB, 2006a). In addition to the α-radiation damage in the ECB, α-radiolytic decomposition of water can cause production of H2O2 and O2. An oxidising environment in the ECB would enhance the migration of redox sensitive radionuclides that are more soluble in their - oxidised form, e.g. technetium as Tc(VII)O4 (Liu et al., 2003; Jansson et al., 2006). However, H2O2 can also block surface sites and slow the dissolution of radioactive phases, restricting the transport of radionuclides (Kitamura and Takase, 2016). Minerals containing radioactive inclusions provide useful natural analogues for long term α-irradiation effects on materials (Pattrick et al., 2013; Bower et al., 2015; Ewing et al., 1995). Quartz from a natural deposit in Canada contains radiation-induced defects (RIDs) from α-decay of structurally incorporated U and Th (and daughter isotopes) (Cerin et al., 2017). These RIDs were observable by EPR as homogenously distributed silicon vacancy hole centres, like the electron hole associated with axial Si- O in the tetrahedral layer of smectites (Cerin et al., 2017; Allard et al., 2012; Allard and Calas, 2009; Sorieul et al., 2005; Sims et al., In review). Biotite, chamosite and 208 cordierite contain α-particle emitting inclusions, e.g. Th-rich monazite (CePO4) or U- rich zircon (ZrSiO4), and exhibit 30-50 μm silver, brown and yellow radio-halos respectively, caused by atomic displacements, e.g. Frenkel defects, and ionisation events, e.g. electrons holes, resulting in increased light absorption in the visible range (Nasdala et al., 2001; Pattrick et al., 2013; Bower et al., 2015; Bower et al., 2016). Radio-centres show concentric aureoles due to the numerous α-emission energies in the actinide decay series (i.e. 238U and daughters) (Pal, 2004). Linear energy transfer of α-radiation and recoil nuclei produce the most intensive damage, but only at the scale of ≈100 nm to 20 μm in minerals such as montmorillonite. Beyond this short-range disorder, long-range order is maintained with only scattered point defects, as evidenced by the electron diffraction pattern (Nasdala et al., 2001; Nasdala et al., 2006). To examine the effects of α-radiation on a laboratory timescale, materials can be doped with 238Pu or 244Cm to impart large α-doses (>1000 decay events/ g) over relatively short periods of time (Ewing, 1999, Weber et al. 1998). For examples, Pu- doped zircon has been examined with solid state nuclear magnetic resonance (NMR) to quantify long-term structural damage due to self-irradiation (Farnan et al., 2007). Accelerated Ion beam experiments are another way to simulate damage (5 MeV, He2+ ions), seeing ionisation on the range of 20 μm, and α-recoli damage causing around 1000 dpa (Ewing, 2001). The simulated 5 MeV alpha particles (ionizing particles) and 100 keV recoil nuclei (displacive atoms) can be estimated at a dose of 4 GGy and 0.2 displacements per atom (dpa), respectively (Sorieul et al., 2008). Deschanels et al. (2014) studied zirconolite and monazite matrices for the containment of actinides (Np, Cm, Am, Pu), using an external irradiation source (Au) and internal source (238Pu doping). Zirconolite became amorphous in both cases at a dose of 4 × 1018 α/ g (0.3 dpa at RT), with macroscopic swelling (≈6%). In monazite, externally irradiated samples showed the same trend, but 238Pu-doped samples showed smaller swelling (≈1%) and resistance to amorphisation up to doses of 7.5 × 1018 α/ g (0.8 dpa), suggesting α- annealing within the structure. Exchanging the interlayer cations of clay minerals with 253Es (6 x 1010 α min-1 μg-1, 6.6 MeV) also resulted in morphological changes through the loss of structure but maintained sorption of 253Es from solution (Haire and Beall, 1978). 209 5.2.3 Coupled effects of heat and radiation on ECB performance As discussed in Section 2.3.1, heating prior to irradiation has limited effects on the ECB, but coupling heat with radiation-induced structural disorder may result in processes such as thermal annealing; for example, heating of monazite and zircon suppressed amorphisation (Meldrum et al., 1998). Sorieul et al. (2008) examined the effect of temperature on radiation damage in smectites and found that annealing of the structure increased the α-irradiation required for amorphisation up to 300-400 °C, beyond which thermally induced instability of the structure due to dehydration and dehydroxylation reduced the dose required (Gu et al., 2001; Wang et al., 2004). Removal of structural water and fixation of exchangeable cations at temperatures between 25 and 400 °C also increased the amorphisation dose required (Wang et al., 2004). In this study, clay samples were subjected to challenges from alpha particles (actinide decay) and heat from the decaying waste (>120 °C) to mimic conditions in a GDF after waste canister corrosion. The effects of α-particle damage and heating (25 and 120 °C) were characterized using a combination of X-ray diffraction (XRD, mineralogical changes), X-ray absorption spectroscopy (XAS; X-ray absorption near edge structure (XANES, oxidation) and extended X-ray absorption fine structure (EXAFS, co- ordination)) and X-ray computed tomography (XCT, physical damage and pore density) to track physicochemical changes in the clay and assess their impact on the radionuclide retention properties of the ECB. 5.3. Methods and Materials 5.3.1. Clays Seven smectitic clays were purchased from the Source Clays Repository of the Clay Minerals Society (West Lafayette, IN); four montmorillonites (SWy-2, STx-1b, and SCa- 3) of the general formula (Na,Ca)0.3Al1.67Mg0.33Si4O10(OH)2.n(H2O) (Stankovic, 2011), where the layer charge was primarily in the octahedral sheet (Vantelon et al., 2003), a beidellite (SBId-1, Na0.5Al2(Si3.5Al0.5)O10(OH)2.n(H2O)) and two higher Fe (≈30%) 3+/2+ nontronites (NAu-1 and NAu-2, Na0.3Fe (Si,Al)4O10(OH)2.n(H2O)), where the layer charge was predominantly in the tetrahedral sheet. 210 The clays were milled in a TEMA mill, sieved to obtain a size fraction of < 53 μm (clay and silt-sized particles). One montmorillonite, STx-1b, was homo-ionised (method adapted from Moore and Reynolds, 1997; Baeyens and Bradbury, 1995) to produce samples with Na+ or Mg2+ as the dominant charge-balancing interlayer cation. All samples were compacted using a hydraulic press (Specac) with stainless steel die (Lightpath Optical (UK) Ltd) to produce 13 mm pellets (9 tonnes, 300 mg, 30 secs) with a compacted density of 2 g cm-3. Pellets were mounted on individual glass slides; a circle of 0.5 cm diameter was polished to a thickness of 100 μm through the addition of resin to one side allowing the removal of the glass backing. 100 μm thinned pellets of the STx-1b (starting material, Na+, and Mg2+), a model montmorillonite, both before and after being heated to 120 °C for 2 days in a Gallenkamp heatbox oven prior to thinning, were separated from the glass slides and mounted on the end of a pin on kapton tape. Samples were analysed using XCT prior to irradiation and subsequently all mounted on the same glass slide with a quartz screen (beam alignment) for α-irradiation, as described below, before re-mounting on the pins on Kapton tape for final tomography analysis (See Figure 5.1 and Figure 5.21SI). Figure 5.1. Showing the set up for tomography samples before and after α-irradiation. Samples were roughly 0.5 mm in diameter and were set up on the same holder with the beam centred in the middle to give equal dose on each sample. Single samples mounted on glass slides (not shown) were irradiated individually. 5.3.2. Alpha irradiation α-particles were simulated using an electrostatic ion beam accelerator to generate α- decay events comparable to that of the U/Th decay chain (∼4‒8 MeV) (Bower et al., 2015, Bower et al., 2016). The α-irradiations were carried out at the University of 211 Manchester’s Dalton Cumbria Facility (DCF) using a 5MV NEC 15SDH-4 tandem Pelletron ion accelerator with a high current Toroidal Volume Ion Source (TORVIS) (Leay et al., 2015; Bower et al., 2015). Beam-induced heating was measured using a thermocouple and resulted in temperatures of up to 75 °C. A quartz screen was used to align the beam and the number of collisions estimated using SRIM modelling with fluence calculated from the circular Gaussian beam profile (Figure 5.21SI, Figure 5.22SI, and Table 5.5SI). Clay samples subjected to XCT analysis received a dose of approximately 0.10-0.25 (0.15) displacements per atom (dpa). Clay thin sections for XRD and XAS received a dose of 0.01-1.50 dpa over the area of analysis (a 0.5 cm diameter circle cut in the supporting glass slide). The average beam current used was 238 nA and each sample was irradiated for 204 minutes. 5.3.3. Clay characterisation before and after heat treatment and irradiation To fully determine the physical and chemical changes in the clays XRD and μXRD, XCT and μXAS were performed on bulk clay samples before and after exposure to heat (25 and 120 °C) and α-irradiation (He2+, 5 MeV). XRD was carried out on a Bruker D8 Advance using Cu Kα radiation (8.05 keV, 0.154 nm). A step size of 0.0197° 2θ, with a counting time of 4 seconds per step, over a 2θ range of 4-60° was used. Powder patterns were collected using a Si standard and phases identified using a search-match routine (EVA software program). μXRD was collected with a using a CCD detector on beamline I18 at Diamond Light Source, using synchrotron radiation with a spot size of 3 μm at 6700 eV allowing high resolution X- ray diffraction. The beam was calibrated using a LaB6 standard sample with two distinct diffraction peaks (16 keV, 0.7749 Å; 6.7 keV, 1.8505 Å). Clay samples were mounted as previously described, as thin sections on glass slides, allowing diffraction mapping across the 100 µm thick clay sample; changes across the sample were visualised in terms of d-spacing relating in particular the d001 basal plane relating to interlayer modifications. The smaller tomography STx-1b samples (natural, Na, and Mg; heated and unheated) were analysed after α-irradiation mounted on kapton to allow comparisons to be drawn. The XRD images were analysed in the program DAWN (Basham et al., 2015). 212 μXAS analysis comprising of both XANES and EXAFS measurements were also carried out on beamline I18 (Diamond Light Source) using a beam size of 6x2 and cryo-cooled Si(111) double crystal monochromator. Iron K-edge data was collected in transmission mode with iron foil as an energy calibration standard and aluminium inserted to reach correct attenuation of the beam (≈10,000 counts). This allowed detailed analysis of Fe K-edges to examine oxidation state and co-ordination geometry of structural iron. Ca K-edge data was collected in fluorescence mode on a number of spots across some select clays (NAu-2, SCa-3, Bld-1, SWy-1 and STx-1b) to examine interlayer cation behaviour before and after α-irradiation. All XAS data were analysed in the Demeter Suite (Ravel and Newville, 2005). X-ray computed tomography was carried out on a Zeiss VersaXRM-520 system at the Henry Moseley X-ray Imaging Facility (HMXIF) at the University of Manchester. The samples were run at 60 kV (5 W) under an atmospheric environment. A 4x objective lens was used and source and detector position selected to achieve a 3 um x 3 um pixel size with a FoV of 3 mm x 3mm. An exposure time of 1.5 s per projection and full tomography scans with 1601 projection resulted in total scan times of around 2 h. Higher resolution runs used a 20x objective lens with a longer exposure time of 30 seconds per projection with a pixel size of ≈0.5 μm (≈20 hrs scan time). Datasets were reconstructed using proprietary Zeiss XRMReconstructor software. AvizoFire 7.1 software (FEI: Thermo Fisher Scientific) was used to visualize3D X-ray images and analyse pore density before and after α-irradiation on smaller sub-volumes of each corresponding montmorillonite. Data was exported as a 2D schematic and plotted as a 1D graph in MATLAB. 5.4. Results 5.4.1. Clay composition Clay composition was analysed by X-ray fluorescence as described previously (Sims et al., in review) and was provided in Table 5.1. Table 5.1: Smectite chemical composition (major elements) measured using XRF. Errors (+/-): Na2O 0.006%, MgO 0.009%, Al2O3 0.11%, SiO2 0.087%, Cl 0.0007%, K2O 0.038%, CaO 0.005%, Fe2O3 0.053%, H2O and CO2 0.034%. Trace metals also present 213 but not included in Table 5.1. Sample Na2O MgO Al2O3 SiO2 Cl (%) K2O CaO Fe2O3 H2O CO2 Total name (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) STx-1b 0.24 2.65 13.3 64.7 0.01 0.15 1.51 0.96 9.63 6.45 99.60 NAu-2 0.21 0.92 2.61 42.8 0.03 0.17 1.32 31.9 14.1 5.57 99.63 NAu-1 0.11 0.73 9.14 36.1 0.04 0.15 1.53 29.8 13.7 8.47 99.77 SCa-3 0.81 5.56 15.0 49.9 0.09 1.32 0.66 2.73 14.5 8.74 99.31 SBid-1 0.05 0.88 25.2 54.6 <0.01 0.85 0.66 1.40 8.15 7.27 99.06 SWy-1 1.21 2.44 17.8 56.6 0.01 0.54 1.31 3.74 9.7 6.02 99.37 The montmorillonites (STx-1b, SWy-1, SAz-2, SCa-3) have a high SiO2 concentration (≈50% by weight), the nontronites (NAu-1, NAu-2) have a higher Fetotal concentration (≈30% by weight), and the beidellite has a higher Al2O3 and SiO2 concentration (25.2 and 54.6% respectively by weight). After homo-ionisation, the Ca2+ in the STx-1b interlayer was expected to be replaced by Na+ and Mg2+ respectively. Mg2+ was high in some of the samples already compared to the other cations as it can be incorporated into the clay structure and was often used to stabilised radiation induced defects in clays (Allard et al., 2009; 2012). 5.4.2. Effect of interlayer cation on resistance to heat and α-irradiation damage The effect of interlayer cation was studied using a model montmorillonite, STx-1b, with three different interlayer cations; Ca2+ (starting material), Na+ and Mg2+, and the results of homoionisation with Na+, Mg2+, and Ca2+ are summarised below in Table 5.2. Table 5.2: Showing the effect of interlayer cation on the d-spacing of the clays (d001 basal spacing). Error (+/-): 0.14 Å. The SBld-1 was hard to cation exchange and only proved successful with Ca2+ exchange. Cation STx-1b NAu-1 SWy-1 SCa-3 NAu-2 SBld-1 Starting 15.45 15.27 14.31 15.73 14.88 15.71 material Na+ 15.28 12.29 15.63 14.74 11.30 - Mg2+ 16.13 15.00 16.05 16.72 15.35 - K+ 12.41 12.01 11.91 12.24 11.59 - Ca2+ 15.56 15.09 15.59 15.79 15.04 15.43 214 Clay samples were chosen to investigate the effect of different chemical compositions (Table 5.2, highlighted in orange) and different interlayer cations (Table 5.2, highlighted in green). Before α-irradiation, the samples analysed have a d001 basal spacing of 14.31 -15.71 Å in the starting materials and 15.28 and 16.13 Å after cation exchange with Na+ and Mg2+ respectively. All the clays showed an expected collapse of the interlayer to ≈12 Å with K+ substitution (Sawhney, 1972), and high Fe clays show a collapse to ≈12 Å with Na+ substitution due to the higher charge on the clay. The changes between the Na-STx-1b and Mg-STx-1b, with and without heating, are summarised in Figure 5.2. STx-1b starting material was previously visualised using standard XRD practices (Table 5.2). Heating the samples to 120 °C prior to α-irradiation showed the same trend as samples without a heat treatment step. Low energy (larger d-spacing) XRD scans (6.5 KeV) after α-particle irradiation (0.1-0.25 dpa), showed a + collapse of the interlayer peak (d001) in the Na -treated STx-1b (Figure 5.2a) whereas Mg2+-treated STx-1b showed a resistance to collapse with a minimal change in the interlayer (Figure 5.2b). Higher energy (smaller d-spacing) XRD scans (12 keV) showed that long range order was maintained in the heated and unheated samples for STx-1b exchanged with monovalent and divalent cations (Figure 5.2c and 5.2d), and the following peaks were identified: d110/d020 clay peak (4.45 Å), d100 quartz peak (4.1 Å, seen in STx-1b), a d130/d004 clay peak (≈3.20 Å), a d005 clay peak (2.56 Å) and a d110 quartz peak (2.48 Å) a d007 clay peak (1.70 Å) and a d060 quartz peak (1.50 Å)/ a d008 clay peak (1.50 Å). The STx-1b starting material looked much the same as the resilient divalent treated STx-1b samples (Mg and Ca). 215 a b Transect Transect d-spacing d-spacing c d Transect Transect d-spacing d-spacing d-spacing d-spacing Figure 5.2. Showing the effect of α-irradiation on the low and high energy XRD measurements (6.7 keV top, a/b and 12 keV bottom c/d) on unheated monovalent Na+-substituted STx-1b (left, a and c) and divalent Mg2+-substituted STx-1b (right, b and d). Each line is a different point and values are d-spacings (Å). The same effects were seen in heated Na+/ Mg2+ exchanged STx-1b samples and the STx-1b starting material with Ca2+ in the interlayer (Figure 5.1SI and Figure 5.2SI). Ca and Fe K-edge XANES and EXAFS of the STx-1b samples were used to examine oxidation state and co-ordination geometry of both structural Fe and interlayer cations (Ca2+) in the samples (Stern, 1974; Harries and Huskins, 1986). Ca K-edge XANES and EXAFS spectra were obtained for the STx-1b starting material (Un), prior to and after α-irradiation. Features in the Ca K-edge XANES have been previously described in a paper by Sims et al. (in review) that showed the 1s to 3d transition (≈4.035 keV) give a pre-edge feature; the 1s to 3p transition (≈4.045 keV) manifests itself as the main 216 feature; and the Ca interaction within structure (≈4.040 keV) as a charge compensator or network modifier gives rise to a shoulder between these (Neuville et al., 2004; Combes et al., 1991). Figure 5.3. Showing the Ca K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for the STx- -1b (Un) before (black) and after (red) α-irradiation; and effect of prior heat treatment after α-irradiation (blue). The features in the Ca K-edge XANES spectrum correspond to changes from the α-irradiation of STx-1b (Figure 5.3) with and without heating and resulted in a slight reduction in white line intensity in the Ca K-edge XANES and no visble change of oxidation state. A decrease in magnitude in both the k-space EXAFS and Fourier transform after α-irradiation suggest a more disordered system and a weakening of the Ca-O bond possibly due to radiolysis in the interlayer. The Ca K-edge EXAFS data for STx-1b (Table 5.3) were fitted using a fixed 6-fold co-ordination number to represent hydrated Ca2+ in the interlayer (octahedral configuration), and a fixed amplitude for comparison. The Ca-O bond length did not change significantly as a function of α-irradiation. The Debye-Waller factor (σ2) showed a small increase after α-irradiation suggesting an increase in the disorder of the system but this change could be seen as trivial as it was only a very small variation. The heat treated sample showed much the same trend but to a lesser extent, possibly due to an annealing affect. Table 5.3: Ca K-edge EXAFS fit results for STx-1b before and after α-irradiation (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r STx-1b Un Ca–O 6* 2.45 ± 0.007 ± 0.80* 0.004 0.01 0.0006 STx-1b Un α Ca–O 6* 2.44 ± 0.009 ± 0.80* 0.002 0.01 0.0004 STx-1b Un Ca–O 6* 2.44 ± 0.009 ± 0.80* 0.006 217 heated α 0.01 0.0007 Notes: Path represents the scattering path, single scattering paths only; CN denotes degeneracy; R gives the interatomic distance; σ2 is the Debye-Waller factor (a measure of disorder in the structure); S02 the amplitude reduction factor; and r the fit factor a measure of fit quality. * denotes a fixed parameter. Fe K-edge XANES and EXAFS were used to examine structural changes in Fe. The K- edge XANES spectra for STx-1b (Figure 5.4) was indicative of ferric Fe with an edge position of ≈7.126 keV (Manceau and Gates, 1997) and there was no change in the Fe3+ oxidation state after α-irradiation. The low intensity of the pre-edge feature at ≈7.114 keV suggests an octahedral Fe environment (Decarreau et al., 2008, Sowrey et al., 2004). A decrease in the Fe K-edge XANES white line intensity after α-irradiation suggests an increase in disorder. Figure 5.4. Showing the Fe K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for the STx-1b (Un) before (black) and after (red) α-irradiation; and effect of prior heat treatment after α-irradiation (blue). The STx-1b Fe K-edge EXAFS and associated Fourier transform showed a large decrease in intensity after α-irradiation, also implying a decrease in the structural order of the Fe in the STx-1b. Both the 6-fold co-ordination number and amplitude were fixed for the fit, with Fe predominantly substituting for Al3+ in the octahedral sheet. The fit of the EXAFS data for natural STx-1b (Table 5.4) gave an Fe-O bond length of 2.01±0.02 Å, which was in agreement with structural Fe(III) present in montmorillonite, in octahedral coordination surrounded by six oxygen atoms (Wilke et al., 2001; Vantelon et al., 2003; Finck et al., 2015). Additional shells were present at 2.89±0.03 Å presumably due to Al, Fe or Mg cations in the octahedral sheet, and at 3.15±0.01 Å due to Si or Al in the tetrahedral sheet. After irradiation, the first shell Fe-O bond length showed no significant change within error from the untreated STx-1b values. The 218 Debye-Waller Factor increased slightly in all the samples after irradiation suggesting a decrease in structural order and a weakening of bonding interaction in the initial Fe-O shell. The longer-range order was not affected, with similar bond lengths and Debye- Waller Factors in comparison to the STx-1b starting material. The Na-STx-1b showed a lengthening of the bonds in the outer two shells to 2.98±0.04 Å and 3.22±0.03 Å for both the heated and unheated samples, again implying a weakening of these bonds and increased disorder of the system after α-irradiation. Table 5.4: Fe K-edge EXAFS fit results for treated series of STx-1b before and after α-irradiation (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r STx-1b Un Fe-O 6 * 2.00 ± 0.006 ± 0.80* 0.012 0.02 0.001 Fe-Al 3 * 2.89 ± 0.013 ± 0.03 0.007 Fe-Si 4 * 3.15 ± 0.012 ± 0.01 0.005 STx-1b Un α Fe-O 6 * 1.98 ± 0.009 ± 0.80* 0.003 0.01 0.001 Fe-Al 3 * 2.89 ± 0.014 ± 0.03 0.003 Fe-Si 4 * 3.14 ± 0.019 ± 0.04 0.004 STx-1b Un Fe-O 6 * 1.99 ± 0.009 ± 0.80* 0.006 heated α 0.01 0.001 Fe-Al 3 * 2.90 ± 0.013 ± 0.03 0.004 Fe-Si 4 * 3.14 ± 0.014 ± 0.03 0.003 STx-1b Mg α Fe-O 6 * 1.97 ± 0.008 ± 0.80* 0.006 0.01 0.001 Fe-Al 3 * 2.89 ± 0.010 ± 0.03 0.003 Fe-Si 4 * 3.10 ± 0.013 ± 0.04 0.004 STx-1b Mg Fe-O 6 * 1.99 ± 0.007 ± 0.80* 0.006 heated α 0.01 0.001 Fe-Al 3 * 2.92 ± 0.016 ± 0.05 0.007 Fe-Si 4 * 3.14 ± 0.019 ± 0.06 0.008 STx-1b Na α Fe-O 6 * 2.00 ± 0.009 ± 0.80* 0.009 0.01 0.001 Fe-Al 3 * 2.98 ± 0.011 ± 0.04 0.004 Fe-Si 4 * 3.22 ± 0.009 ± 0.03 0.003 219 STx-1b Na Fe-O 6 * 2.00 ± 0.010 ± 0.80* 0.006 heated α 0.01 0.001 Fe-Al 3 * 2.98 ± 0.014 ± 0.04 0.005 Fe-Si 4 * 3.22 ± 0.012 ± 0.04 0.003 Notes: Path represents the scattering path, all single scattering paths; CN denotes degeneracy; R gives the interatomic distance; σ2 is the Debye-Waller factor; S02 the amplitude reduction factor; and r the fit factor a measure of fit quality. * denotes a fixed parameter. X-ray computed tomography (XCT) was used to look at damaged regions in STx-1b and map pore density allowing the effect of α-particle damage to be visualised at a μm- scale. This visualisation allowed a 3-D evaluation of crack distribution within the clay (Kaufhold et al., 2016; Ma et al., 2017). Individual clay grains are below the resolution of XCT, with grain densities causing absorption, making segmentation and processing challenging. Use of µ-XCT to map cracks on a large scale (cm) tends to over-estimate cracks, due to particle volume effects (Kaufhold et al., 2016). However, changes in the average micro-crack and meso-pore density across the same small sections of STx-1b before and after α-irradiation could be mapped. Figure 5.5. Tomography images (XY plane) for Un STx-1b unheated (top) and heated (bottom) before (green) and after (red) α-irradiation with corresponding pore density plots from small sections within the clays (yellow boxes). Samples were roughly 0.5 mm in diameter and 100 μm thick. Figure 5.5 shows all the variations of the untreated STx-1b samples (α and heated); it was possible to see the damage in the cross section images (Figure 5.6), with a small 220 increase in pore density across all samples after α-irradiation (Figure 5.5SI and Figure 5.6SI). Figure 5.6. Showing a selection of images of Mg treated STx-1b from before (left, Green) and after α-irradiation (right, Orange). Top images are 3D images from front and side (XY plane) and the bottom images are cross-section slices (2D) through the small tomography samples (0.5 mm in diameter and 100 μm thick). Figure 5.6 above shows the visual damage observable after α-irradiation. Comparing the unirradiated Mg treated STx-1b (green, and cross-sections images below) to the α-irradiated Mg treated STx-1b (Orange, and cross-sections images below); a visible increase in the cracks present on the surface and loss of some of the sample thickness was shown. The cross-section images below show a similar story with the number of cracks at the damaged side surface of the clay increasing visibly only a short way into the clay after α-irradiation. Throughout, the STx-1b sample sets (Un, Na, and Mg), the effect of α-irradiation was shown to induce further crack propagation and increase the number of larger cracks within the samples (Figure 5.6 and Figures 5.7SI, 5.8SI, 5.9SI, 5.10SI, 5.11SI). An increase in the cracks and the porosity in the near-surface region (≈26 μm) also was observed, corresponding to the depth of α-particle penetration. In Figure 5.7 examples of SAz-2 and STx-1b samples before and after α-irradiation are used to show this surface damage of the clays. 221 Increased cracks (lighter regions) a b c d Figure 5.7. Showing 2D slices of SAz-2 before (a) and after (b) α-irradiation with the effect at the edge of the damage region visualised (arrowed). The same effect was visualised on a 100 μm sample of STx-1b before (c) and after (d) α-irradiation. 5.4.3. Effect of chemical composition on alpha irradiation damage in clays The effect of α-irradiation on clays with different chemical composition was examined with XRD and XAS, with a focus on Fe content. An α-damage track from an area of no damage into an area of high damage was shown for: (i) the low Fe clay STx-1b (Fe ≈1%) in Figure 5.8; and (ii) the high Fe clay NAu-1 (Fe ≈30%) in Figure 5.9. The high Fe clay represents a pseudo alteration product of clay interaction with Fe2+, formed by corrosion of the steel canister with a GDF setting. The other clays studied, SCa-3, SBld- 1, NAu-2, and SWy-1 (Figure 5.10SI, 5.13SI, 5.14SI, 5.15SI) fall between these two ‘end- members’. 222 The XRD of the α-irradiation track in STx-1b (Figure 5.8) showed a loss of basal d001 spacing with progressive movement into the damage zone. The discoloured damaged area at the centre of the α-irradiation received a dose of ≈1.5 dpa. The extent of damage decreased with increasing distance from the centre, as the dose decrease to below that expected for amorphisation (<0.1 dpa). The XRD patterns at higher energy (12 keV) showed the STx-1b to maintain long range order even into the area of high damage showing the damage observed to be localised effect (at the surface/ interlayer) within the clay structure. d-spacing d-spacing Transect Transect d-spacing d-spacing Higher energy peaks still present throughout – long range order (12 keV scans) Figure 5.8. Showing the low and high energy (6.7 and 12 keV) XRD d-spacing (Å) scans of STx-1b, mapping from an un-irradiated area to an area of high α-damage (path shown on top image) on a thinned section of a 13 mm pellet (0.5 mm diameter, 100 μm). Collapse of d001 spacing shown in left images (6 keV scans) and longer range order maintained (12 keV scans). The Ca K-edge XANES for STx-1b (Table 5.5) showed no change between sample points, but EXAFS showed an increase in the Debye Waller Factor (σ2) and a decrease in 223 intensity of the Fourier transform with movement further into the α-irradiated regions suggesting an increase in the disorder caused by α-particle damage. The Fe K-edge XANES showed minimal changes with a slight decrease in the edge position at the most damaged point suggesting some reduction of Fe3+ to Fe2+ (Figure 5.18SI). The Fe K-edge EXAFS showed reduction in intensity of the Fourier transform and an increase in the σ2 with movement into the damaged region, again suggesting disorder and damage at doses >0.3 dpa. There was little damage in the low dose area (≈0.1-0.25 dpa), with the EXAFS fits matching that for the un-irradiated region or being within error of one another in both cases. Table 5.5: Ca and Fe K-edge EXAFS fits for STx-1b along a α-irradiation track from an area of low damage (dpa) to an area of higher damage (dpa) (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r STx-1b Ca–O 6* 2.41 ± 0.008 ± 0.80* 0.002 damage 0.01 0.0006 Track - edge STx-1b Ca–O 6* 2.41± 0.009 ± 0.80* 0.007 damage 0.01 0.0007 Track - low STx-1b Ca–O 6* 2.42 ± 0.010 ± 0.80* 0.006 damage 0.01 0.0009 Track - high STx-1b Fe-O 6 * 2.01 ± 0.005 ± 0.80* 0.011 0.01 0.001 damage * Track - edge Fe-Al 3 3.04 ± 0.010 ± 0.03 0.003 Fe-Si 4 * 3.24 ± 0.004 ± 0.01 0.001 STx-1b Fe-O 6 * 1.99 ± 0.006 ± 0.80* 0.006 0.01 0.001 damage * Track - low Fe-Al 3 2.96 ± 0.008 ± 0.02 0.002 Fe-Si 4 * 3.17 ± 0.010 ± 0.03 0.002 STx-1b Fe-O 6 * 1.98 ± 0.011 ± 0.80* 0.022 0.02 0.001 damage * Track - high Fe-Al 3 3.01 ± 0.011 ± 0.05 0.005 Fe-Si 4 * 3.24 ± 0.009 ± 0.04 0.003 Notes: Path represents the scattering path, all single scattering paths; CN denotes degeneracy; R gives the interatomic distance; σ2 is the Debye-Waller factor; S02 the 224 amplitude reduction factor; and r the fit factor a measure of fit quality. * denotes a fixed parameter. For NAu-1, XRD of the higher α-damaged area showed a colour change to black and a partial collapse of the interlayer from 14.9 Å to 9.6 Å. The extent of collapse of the d001 basal spacing here resembles dehydration in the interlayer to and illite/biotite-like species (Sims et al., in review; Moore and Reynolds, 1997; Stanković et al., 2011; Villar, 2002). 14.9 Å 9.6 Å Transect d-spacing d-spacing Figure 5.9. Showing the XRD map of NAu-1 from an area of high damage to an area of lower damage on a thinned section of a 13 mm pellet (0.5 mm diameter, 100 μm). d001 basal spacing collapse shown by red arrow and image of the sample (front and back after damage (13 mm pellet) and damage track show with red line). The Fe K-edge EXAFS for structural Fe in NAu-1 shows little change with α-irradiation (Table 5.6), other than an general increase in the σ2, but this was within error of the starting material. Table 5.6: Fe K-edge EXAFS fit results for NAu-1 from a point off the α-damaged area on the pellet and at the highest point of damage (dpa). (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r NAu-1 Fe-O 6 * 2.00 ± 0.004 ± 0.80* 0.009 0.01 0.001 225 Fe-Fe/Al 3 * 3.06 ± 0.003 ± 0.01 0.001 Fe-Si 4 * 3.16 ± 0.014 ± 0.04 0.009 NAu-1 α Fe-O 6 * 1.99 ± 0.006 ± 0.80* 0.008 0.01 0.001 Fe-Fe/Al 3 * 3.04 ± 0.005 ± 0.01 0.001 Fe-Si 4 * 3.23 ± 0.018 ± 0.02 0.009 Notes: Path represents the scattering path, all single scattering paths; CN denotes occupancy; R gives the interatomic distance; σ2 is the Debye-Waller factor; S02 the amplitude reduction factor; and r the fit factor a measure of fit quality. * denotes a fixed parameter. NAu-2 showed no change in the interlayer spacing (14.3 Å) all the way into the damage region although the maximum α-damage dose was less here (0.15-0.1 dpa). The same was seen for SBld-1 (0.15-0.1dpa) with a slight reduction in the d001 basal spacing from 12.9 to 12.7 Å after α-irradiation; although the predominant interlayer cation in beidellite was K+ which could have impacted the resilience of the structure due to the lower initial hydration and tetrahedral character of the charge in the interlayer. SWy-1, with Na+ predominantly in the interlayer (monovalent), showed a lower initial value for the d001 basal spacing of 12.3 Å and a collapse of the interlayer with movement into the area of higher α-irradiation; possibly due the higher radiation damage across the sample (0.47 dpa) and monovalent interlayer cation offering less resistance to radiation (Savage and Authur, 2012). As shown in Table 5.7 and Figure 5.10, some of the clays show lower XRD d001 basal spacings than in the natural the starting materials (Table 5.2), and this may be due to the entirety of the sample region (5 mm diameter circle) receiving some dose during the experiment (as all from same sample). The Ca K-edge EXAFS for the other samples (Figure 5.19SI, Table 5.2SI) showed an increased σ2 (Debye Waller factor) for SWy-1 and SCa-3, and no change in NAu-2 and SBld-1 respectively. The increase in the σ2, coupled with a decrease in the intensity of the Fourier transform, suggest an increase in disorder in the two higher damaged montmorillonites (SWy-1 and SCa-3). The other clays showed resistance at a lower dose (0.15 dpa), the high Fe content in nontronite (NAu-2) and the lower interlayer hydration in beidellite (SBld-1) imparted some degree of resistance to structural 226 disorder caused by α-irradiation on interlayer species. The Fe XAS measurements taken showed a similar trend in all the clays (Figure 5.20SI, Table 5.2SI); a large decrease in the intensity of the Fourier transform was observed in all the samples and an increase in the σ2 for all the bonds fit suggested an increase in the disorder of the system around the Fe in each case, this was to be expected with α-irradiation causing local structural damage. The effect of dose (dpa) on the interlayer spacing as measured by XRD (d001 basal spacing) and was compared across the six clays; STx-1b, SBld-1, SCa-3, SWy-1, NAu-1, and NAu-2 as a function of Fe content (low to high), and interlayer cation. Table 5.7: Showing the effect of dpa caused by α-irradiation on d-spacing of the clays (d001 basal spacing). (Error (+/-): 0.14 Å before/ CCD detector after). Clay Undamaged α-irradiated displacements Fe % Predominant d001 spacing d001 spacing per atom interlayer cation STx-1b Un 15.45 14.8 0.16 0.67 Ca2+ STx-1b Mg 16.13 15.0 0.16 0.67 Mg2+ STx-1b Na 15.28 0.0 0.16 0.65 Na+ STx-1b 15.45 0.0 1.5 0.67 Ca2+ SBld-1 15.71 12.7 0.15 0.98 K+ SCa-3 15.73 9.5 1.5 1.91 Ca2+ SWy-1 14.31 0.0 0.47 2.62 Na+ NAu-1 15.27 9.6 0.47 20.9 Ca2+ NAu-2 14.88 14.4 0.15 22.3 Ca2+ Note: for the dpa calculation the highest point of α-damage estimation in the 5 mm diameter circle was taken, dose delivered in each sample varied with beam positioning. 227 Max 0.16 0.16 0.16 1.5 0.15 1.5 0.47 0.47 0.15 dpa d-spacing (Å) Clay Figure 5.10. Showing d001 spacing in the natural and treated clays as a function of displacements per atom caused through differing α-irradiation doses. Figure 5.10 and Table 5.7 show a clear trend with any α-irradiation dose, with the reduction in the d001 basal spacing linked to Fe content and interlayer cation. STx-1b showed minimal reduction in d-spacing and a resilience to damage with both divalent ions, Ca2+(STxUn) and Mg2+(STx-Mg), present in the interlayer after a low α-dose (0.16 dpa). Na+-STx-1b (STx-Na) showed a complete collapse of the interlayer space after the same α-dose (0.16 dpa) suggesting the weaker hydration of the Na+ affected the clays radiation resilience. SWy-1, with Na+ as the predominant cation, was also shown to completely collapse after α-irradiation, strengthening the argument that mono-valent, hydrated cations become dehydrated during α-irradiation. STx-1b (Ca2+ predominant) showed a complete collapse after a higher α-dose (1.5 dpa), showing that there was critical dose for interlayer collapse in montmorillonite, irrespective of the interlayer cation. SBld-1, with K+ as the predominant cation, showed a reduction in the interlayer from 15.71 to 12.7 Å at a lower α-dose (0.15 dpa). This was associated with a collapse, normally from dehydration, but mostly likely due to radiolysis of the interlayer water; the collapse to 12.7 Å was due to the tetrahedral character of the charge in interlayer in beidellite (Lantenois et al., 2008). The same trend was seen for SCa-3 and NAu-1 (Ca2+ predominant), at a higher α-dose (1.5 and 0.47 dpa correspondingly), with a collapse of the interlayer space from 15.7 and 15.3 to 9.5 and 9.6 Å respectively; the 228 charge was located principally in the octahedral layer in montmorillonites accounting for the difference in the values in comparison to beidellite (Stanković et al., 2011; Villar, 2002). The NAu-2 starting material (Ca2+ predominant) showed very little change in the interlayer space (14.8 to 14.4 Å) with a dose of 0.15 dpa. The higher Fe content (≈30%) and divalent interlayer cation may be the reason for the resilience of the sample when compared with the mono-valent, lower Fe beidellite. 5.5. Discussion 5.5.1. Effect of composition on clay interactions with α-irradiation Micro focus XRD analysis revealed that α-irradiated clays were more resistant to damage, in the form of interlayer (d001 spacing) collapse, if the interlayer cation was divalent (NAu-1 and STx-1b Ca2+/Mg2+) than if the interlayer cation was monovalent (SWy-1 and STx-1b Na+) at similar doses (0.47 and 0.16 dpa respectively). This was likely due to the increased hydration associated with divalent cations, and was in agreement with previous results showing enhanced resilience to damage caused by γ-irradiation of clays with Ca2+ in the interlayer compared with Na+ (Sims et al., in review, Savage and Authur, 2012; Svensson and Hanson, 2013). The critical dose at which interlayer collapse occurs for the low-Fe montmorillonite STx-1b with a divalent interlayer cation was 1.5 dpa. Lower doses (0.16 dpa) had a limited effect on the clay interlayer, and structural order in the tetrahedral and octahedral sheet was maintained in all clay samples even at the highest doses shown in the longer range XRD results and EXAFS analysis. The presence of Fe in the structure provided resistance to complete interlayer collapse due to α-particle damage in some of the clays analysed (SBld-1, SCa-3, NAu-1, Table 5.7). Differences between behaviour of the high-Fe containing nontronites (NAu-1 and NAu-2) was most likely due to differences in doses (1.5 vs 0.15 dpa) as the only overall differences between the two was the higher Al3+ content of NAu-1 (Keeling et al., 2000). This localized resistance to radiation damage was in contrast to previous bulk studies on smectites, showing some amorphisation at doses as low as 0.13-0.16 dpa (Sorieul et al., 2008), with complete amorphisation at higher doses of ≈0.3-0.5 dpa (Beall, 1984; Grauer, 1986; Meunier et al., 1998); although damage was also dependent on material density and crystallite size (Lu et al., 2013; Lieser and Kratz, 2008). 229 Ca K-edge XAS (Table 5.3, Table 5.5, Table 5.2SI) suggested an increase in disorder of the hydrated montmorillonites (STx-1b, SWy-1, and SCa-3) with α-irradiations at a higher dpa (1.5, 0.47 and 1.5 dpa respectively), but no change in structural order at the lower dose of 0.15 dpa received by the nontronite, NAu-2 (higher Fe content), and beidellite (weakly hydrated K+ in the interlayer). Fe K-edge XAS (Table 5.4, Table 5.5, Table 5.2SI) demonstrated an increase in structural disorder around the Fe, which was consistent with localised structural α-particle induced damage seen previously (Bower et al., 2015; Mosselmans et al., 2016; Farnan et al., 2007; Nasdala et al., 2006). The changes seen in the XAS data from the experiment only show very small changes in most cases suggesting the effect of the damage was localised as shown in the XRD and XCT results where damage was confined to the interlayer and a small fraction of the material at the surface. XCT analysis of the α-particle damaged montmorillonite (STx-1b) showed an increase in porosity and cracking in the surface material. This could lead to migration of α-emitting radionuclides away from the waste package through propagation the α-particle induced damage through the ECB, if a sorption-desorption cycle of α-emitting radionucledes were to occur. The proposed cause of the interlayer collapse seen in the smectites at higher α-irradation doses was due to a number of factors. Radiolysis (and hydrolysis) within the interlayer from α-irradiation caused redox reactions (increasing charge on layers), the local loss of structure through the displacement of atoms (and cations in the interlayer), associated ionisation, and knocking out of atoms (spacer) were postulated to contribute to the collapse of interlayer within clays during α-irradiation. These effects at higher doses lead to the dehydration of the interlayer (local) giving materials with an interlayer spacings resembling that of an illite- or biotite-like species, visualised through XRD. Preceding this effect, due to the local disordering, some re-hydration may be possible but it would be assumed that this would be to a lesser extent than prior. Parallels with heat treatment of expandable clay could be drawn, with the possibility of reversible dehydration/rehydration of the interlayers at low α-damage doses (i.e. low temperature heat treatment), and irreversible collapse at higher α-damage doses due to complete loss of local structure. 230 5.5.2. Effect of heat on a model montmorillonite under α-irradiation Heat has been previously shown to cause minimal changes to the clay structure below 160 °C, with a slight decrease in the d001 basal spacing in montmorillonite (Sims et al., In review; Honty et al., 2012; Johnson et al., 1994a; Johnson et al., 2002; Wersin et al., 2006; Hicks et al., 2009). Na+-exchanged montmorillonite does dehydrate at lower temperatures than divalent montmorillonite, but a semi-layer was still present at 140 °C and complete loss of water does not occur until 170 °C (Wang et al., 2012). However, heat can have an annealing effect and suppress the radiation-induced structural disorder caused by α-irradiation (Sorieul et al., 2008; Wang et al., 2004; Gu et al., 2001; Meldrum et al., 1998). Here, analysis by micro-focus XRD, and bulk Ca and Fe K-edge XAS, confirms that montmorillonite (STx-1b) response to α-particle damage was largely unaffected by heating to elevated temperatures (120 °C), with similar results obtained with and without a prior heating step. This represents the conditions experienced by the ECB during initial storage of HLW, prior to α-irradiation. Minor changes in the pre-heated clay response to α-irradiation included: (i) a slightly higher resistance to α-particle-induced structural disorder, likely due to the reduced hydration in the clay; and (ii) a small increase in porosity in the α-damaged region, with larger cracks in the pre-heated clay after α-irradiation. 5.6. Conclusions In conclusion, pre-heating the clay (120 °C) prior to α-irradiation has minimal effects on clay structure and response to α-particle induced damage. The demonstrated structural integrity of the clay at this temperature would allow for the earlier encapsulation of the wastes, and closer packing, which would reduce the time that the wastes are above ground and the GDF footprint, respectively. Localised interlayer collapse occurred in the clay as a result of α-particle damage (XRD), with the extent of collapse dictated by: (i) the valency and hydration state of the interlayer cations; and (ii) the amount of Fe in the clay structure. However, this damage was constrained locally to the interlayer and damaged surface of the clays, shown through the maintenance of the tetrahedral-octahedral-tetrahedral structure in the EXAFS data and longer range XRD scans. A lower α-particle dpa was required to cause complete collapse with monovalent interlayer cations (<0.47 dpa) than with divalent interlayer 231 cations (1.5 dpa). The extent of interlayer collapse in high-Fe nontronites was dose- dependent and less than in low-Fe containing clays, likely due to a reduction in the effects of radiolysis through electron transfer pathways provided by Fe in the structure. The doses to which the clays were exposed were representative of the expected total lifetime dose in the GDF, but at substantially higher dose rate, and the effect of dose rate should be examined further as this could vastly impact the extent of damage observed. α-irradiation resulted in increased porosity through crack expansion and crack formation at the surface of the clay locally, but this could have implications for the GDF safety case. If this effect was to be coupled with a self-propagating sorption-desorption cycle of α-emitting radionuclides, this could represent an advective flow path out of the GDF, releasing the radionuclides into the environment. Research into the sorption-desorption properties of the clay with respect to key radionuclides, before and after irradiation, is on-going. This would be ensure the possibly of transport away from the contaminant area would be evaded, showing if sorption-desorption cycles of α-emitting radionuclides onto the clays occur at all or if radionuclides display favourable desorption kinetics (slow) after uptake. Overall, the effect of α-irradiation on the clays was shown to be localised, with the longer range structure of the clays maintained even at higher doses (1.5 dpa) and damage limited to the surface layers of the clay (26 μm). This would benefit the GDF safety case as it shows a worst case scenario simulated here would be contained within the close vicinity of the canister (excluding sorption-desorption cycles) with minimal damage to the larger buffer area giving a positive outlook for the larger safety case for radioactive waste disposal. 5.7. Acknowledgements We acknowledge the Dalton Nuclear Institute, University of Manchester for funding this project. We acknowledge the Engineering and Physical Science Research Council (EPSRC) for funding the Henry Moseley X-ray Imaging Facility which has been made available through the Royce Institute for Advanced Materials through grants (EP/F007906/1, EP/F001452/1,EP/I02249X, EP/M010619/1, EP/F028431/1, EP/M022498/1 and EP/R00661X/1). We acknowledge the support of The University of Manchester’s Dalton Cumbrian Facility (DCF), a partner in the National Nuclear User 232 Facility, the EPSRC UK National Ion Beam Centre and the Henry Royce Institute. 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Pearce*a,e a School of Chemistry, University of Manchester, M13 9PL UK ; b School of Earth and Environmental Sciences, University of Manchester, M13 9PL UK; c Dalton Cumbrian Facility, University of Manchester, M13 9PL UK; d Henry Moseley X-ray Imaging Facility | School of Materials, University of Manchester, M13 9PL UK; e Pacific Northwest National Laboratory, Richland, WA, USA; f Diamond Light Source, Harwell Campus, Didcot, OX11 0DE UK. 5.9 Supplementary information Additional data and figures Figure 5.1SI. Showing the effect of α-irradiation on the high and low energy (6.7 and 12 keV) XRD measurements on monovalent Na+ substituted STx-1b and divalent Mg2+ substituted STx-1b both heated prior to α-irradiation 120 °C with d-spacings in Angstroms (Å). The X-axis is d-spacing in all graphs and the y-axis/z-axis is the transect of the left/right graphs respectively (y-axis is the intensity on all the 3D graphs). 241 Figure 5.2SI. Showing untreated STx1b, both unheated and heated at high and low energy (6.7 and 12 keV) XRD scans after α-irradiation (5 MeV). The X-axis is d-spacing in all graphs and the y-axis/z-axis is the transect of the left/right graphs respectively (y-axis is the intensity on all the 3D right-hand graphs). 242 Figure 5.3SI. Showing the Ca K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for Mg STx-1b before (green) and after (blue) α-irradiation; effect of prior heating (120 °C) also shown (red). Figure 5.4SI. Showing the Fe K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for Na STx- 1b before (green) and after (blue) α-irradiation; effect of prior heating (120 °C) also shown (red). Table 5.1SI: Ca K-edge EXAFS fit results for Mg treated STx-1b after irradiation (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r STx-1b Mg α Ca–O 6* 2.29 ± 0.014 ± 0.80* 0.014 0.03 0.002 STx-1b Mg heated α Ca–O 6* 2.38 ± 0.011 ± 0.80* 0.014 0.03 0.002 243 Figure. 5SI. Tomography image cross sections (XY plane) for Mg STx-1b unheated (top) and heated (bottom) before (black) and after (red) α-irradiation with corresponding pore density plots from small sections within the clays (yellow boxes). Samples were ≈0.5 mm in diameter and 100 μm thick. Figure. 5.6SI. Tomography cross section images (XY plane) for Mg STx-1b unheated (top) and heated (bottom) before (black) and after (red) α-irradiation with corresponding pore density plots from small sections within the clays (yellow boxes). Samples were ≈0.5 mm in diameter and 100 μm thick. 244 Figure 5.7SI. Showing a selection of images of Mg treated STx-1b (heated) from before (left, green) and after α-irradiation (right, orange). Top images are cross sections (XY plane) and the bottom images (2D) are slices through the samples (0.5 mm in diameter and 100 μm thick). Figure 5.8SI. Showing a selection of images of STx-1b from before (left, green) and after α-irradiation (right, orange). Top images are cross sections (XY plane) and the bottom images (2D) are slices through the samples (0.5 mm in diameter and 100 μm thick). 245 Figure 5.9SI. Showing a selection of images of STx-1b (heated) from before (left, green) and after α-irradiation (right, orange). Top images are cross sections (XY plane) and the bottom images (2D) are slices through the samples (0.5 mm in diameter and 100 μm thick). Figure 5.10SI. Showing a selection of images of Na STx-1b from before (left, green) and after α-irradiation (right, orange). Top images are cross sections (XY plane) and the bottom images (2D) are slices through the samples (0.5 mm in diameter and 100 μm thick). 246 Figure 5.11SI. Showing a selection of images of Na STx-1b (heated) from before (left, green) and after α-irradiation (right, orange). Top images are cross sections (XY plane) and the bottom images (2D) are slices through the samples (0.5 mm in diameter and 100 μm thick). 247 14.1 Å 14.1 Å 9.5 Å Figure 5.12SI. Showing the low energy (6.7 keV) XRD d-spacing scans of SCa-3, as 2D diffraction maps and as 3D line plots, initially in an untreated clay (top) and then after α-irradiation (bottom); mapping from an area of low α-irradiation into an area to an area of high α-damage. A collapse of the d001 basal plane from 14.1 to 9.5 Å. Images show the clay pellet (13 mm) and thinned section (0.5 mm diameter, 100 μm) before and after α-irradiation. The X-axis is d-spacing in all graphs and the y-axis/z-axis is the transect of the left/right graphs respectively (y-axis is the intensity on the 3D right-hand graphs). 248 Figure 5.13SI. Showing the low energy (6.7 keV) 2D XRD d-spacing scans of SBld-1, before (left) and after α-irradiation (right); a slight reduction of the d001 basal spacing from 12.9 to 12.7 Å. Images show the clay pellet (13 mm) and visible thinned section (0.5 mm diameter, 100 μm) after α-irradiation. The X-axis is d-spacing in both graphs and the y-axis is the transect in each graph. Figure 5.14SI. Showing the low energy (6.7 keV) 2D XRD d-spacing scans of NAu-2, before (left) and after α-irradiation (right); no change of the d001 basal spacing was observed (14.3 Å). Images show the clay pellet (13 mm) and visible thinned section (0.5 mm diameter, 100 μm) after α-irradiation. The X-axis is d- spacing in both graphs and the y-axis is the transect in each graph. 249 Figure 5.15SI. Showing the low energy (6.7 keV) 2D XRD d-spacing scans of SWy-1, before (left) and after α-irradiation (right); a collapse of the d001 basal spacing was observed from 12.3 Å in the area of high α- damage. The X-axis is d-spacing in both graphs and the y-axis/z-axis is the transect of the left/right graph respectively (y-axis is the intensity on the 3d right-hand graph). Figure 5.16SI. Showing the Ca K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for STx- -1b along a damage track from an area of no visible α-damage (green), to an area of low α-damage (blue), to an area of high α-damage (red). Figure 5.17SI. Showing the Ca K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for STx- -1b along a damage track from an area of no visible α-damage (blue), to an area of low α-damage (red), to an area of high α-damage (green). 250 Figure 5.18SI. Showing the Fe K-edge XANES, EXAFS (K-space), and Fourier transform (R-space) for NAu-1 before (blue) and after (red) α-irradiation. SWy-1 NAu-2 SBld-1 SCa-3 Figure 5.19SI. Showing the Fourier transform (R-space) of the Ca K-edge EXAFS for SWy-1, NAu-2, and SBld-1; before (blue) and after (red) α-irradiation. SWy-1 NAu-2 SBld-1 SCa-3 Figure 5.20SI. Showing the Fourier transform (R-space) of the Fe K-edge EXAFS for SWy-1, NAu-2, and SBld-1; before (blue) and after (red) α-irradiation. 251 Table 5.2SI: Ca and Fe K-edge EXAFS fits for NAu-1, NAu-2, SBld-1, SCa-3, and SWy-1 before and after α-irradiation (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r NAu-1 Ca–O 6* - - n/a n/a NAu-1 α Ca–O 6* - - n/a n/a NAu-2 Ca–O 6* 2.41 ± 0.008 ± 0.80* 0.003 0.01 0.001 NAu-2 α Ca–O 6* 2.41 ± 0.008 ± 0.80* 0.007 0.01 0.001 SBld-1 Ca–O 6* 2.40 ± 0.009 ± 0.80* 0.005 0.01 0.001 SBld-1 α Ca–O 6* 2.41 ± 0.009 ± 0.80* 0.016 0.02 0.002 SCa-3 Ca–O 6* 2.43 ± 0.007 ± 0.80* 0.004 0.01 0.001 SCa-3 α Ca–O 6* 2.41 ± 0.011 ± 0.80* 0.007 0.01 0.001 SWy-1 Ca–O 6* 2.39 ± 0.011 ± 0.80* 0.007 0.02 0.001 SWy-1 α Ca–O 6* 2.36 ± 0.016 ± 0.80* 0.013 0.02 0.002 NAu-1 Fe-O 6 * 2.00 ± 0.004 ± 0.80* 0.009 0.01 0.001 Fe-Fe/Al 3 * 3.06 ± 0.003 ± 0.01 0.001 Fe-Si 4 * 3.16 ± 0.014 ± 0.04 0.009 NAu-1 α Fe-O 6 * 1.99 ± 0.006 ± 0.80* 0.008 0.01 0.001 Fe-Fe/Al 3 * 3.04 ± 0.005 ± 0.01 0.001 Fe-Si 4 * 3.23 ± 0.018 ± 0.02 0.009 NAu-2 Fe-O 6 * 2.01 ± 0.003 ± 0.60* 0.017 0.01 0.001 Fe-Fe/Al 3 * 3.07 ± 0.003 ± 0.01 0.001 Fe-Si 4 * 3.26 ± 0.007 ± 0.05 0.005 NAu-2 α Fe-O 6 * 2.00 ± 0.009 ± 0.60* 0.016 0.01 0.001 Fe-Fe/Al 3 * 3.04 ± 0.019 ± 0.05 0.010 Fe-Si 4 * 3.26 ± 0.003 ± 0.01 0.001 SBld-1 Fe-O 6 * 2.01 ± 0.005 ± 0.80* 0.005 0.01 0.001 Fe-Al 3 * 3.05 ± 0.008 ± 0.02 0.002 Fe-Si 4 * 3.23 ± 0.006 ± 252 0.02 0.001 Fe-O6 4* 3.75 ± 0.003 ± 0.02 0.001 SBld-1 α Fe-O 6 * 2.01 ± 0.010 ± 0.80* 0.011 0.01 0.001 Fe-Al 3 * 3.13 ± 0.014 ± 0.05 0.006 Fe-Si 4 * 3.36 ± 0.012 ± 0.04 0.004 Fe-O6 4* 3.83 ± 0.010 ± 0.05 0.006 SCa-3 Fe-O 6 * 2.01 ± 0.005 ± 0.77* 0.006 0.01 0.001 Fe-Al 3 * 3.10 ± 0.007 ± 0.02 0.002 Fe-Si 4 * 3.26 ± 0.004 ± 0.01 0.001 Fe-O6 4* 3.77 ± 0.016 ± 0.06 0.010 SCa-3 α Fe-O 6 * 1.99 ± 0.014 ± 0.77* 0.006 0.01 0.001 Fe-Al 3 * 2.85 ± 0.027 ± 0.07 0.009 Fe-Si 4 * 3.12 ± 0.030 ± 0.02 0.013 Fe-O6 4* 3.64 ± 0.037 ± 0.14 0.025 SWy-1 Fe-O 6 * 2.01 ± 0.004 ± 0.80* 0.006 0.01 0.001 Fe-Al 3 * 3.06 ± 0.006 ± 0.02 0.002 Fe-Si 4 * 3.23 ± 0.006 ± 0.02 0.001 Fe-O6 4* 3.77 ± 0.006 ± 0.02 0.002 SWy-1 α Fe-O 6 * 2.01 ± 0.008 ± 0.80* 0.007 0.01 0.001 Fe-Al 3 * 3.08 ± 0.011 ± 0.03 0.003 Fe-Si 4 * 3.24 ± 0.007 ± 0.02 0.001 Fe-O6 4* 3.76± 0.008 ± 0.03 0.003 Notes: Path represents the scattering path, all single scattering paths; CN denotes degeneracy; R gives the interatomic distance; σ2 is the Debye-Waller factor; S02 the amplitude reduction factor; and r the fit factor a measure of fit quality. * denotes a fixed parameter. 253 Dose estimation The α-irradiation dose was estimated during the experiment to give displacements per atom (dpa); this was achieved using a number of ion beam parameters (time, current, energy, beam size and distribution), modelling values (SRIM, particle range and displacements), literature values (amorphisation dose, grams per mole of material, and columbic scaling factor) and calculated values (displacements per ion, atomic density, coulombs, and α-threshold) shown in Table 5.4SI. Number of ions incident at the sample per second was calculated from equation 1SI. Where I is the sample current and e is the electronic charge of the ions (2 required as the He2+ ions used had 2 charges per ion). 푁 = 퐼/2푒 (eq. 1SI) This can be expanded on to give an approximate total ion flux, calculated from measurements taken throughout the experiment on the sample and is dependent on the number of particles (N), irradiation time (t), current (I in C), and elementary charge of He2+ ion (2e=3.2x10-19 C) shown in Equation 2SI. 푁 퐼 = (eq. 2SI) 푡 2푒 The beam followed a Gaussian distribution (Figure 5.21SI) meaning that a higher ion density was received at the centre of the beam, resulting in a radiation gradient, and average dpa values were calculated for specific areas relating to this beam profile with the majority of the ion flux centred around 1 cm2 (95%). Figure 5.21SI. Beam profile image (focused using a quartz screen of dimension 25 x 25 mm2) used to derive a Gaussian fit using a high contrast image and right the image of the alpha damaged sample (tomography set) 254 The energy of the He2+ ions on the target material was 5.08 MeV, generated from an injection ion energy of 0.068 MeV (Bower et al., 2015). The average current used was 238 nA over runs of 204 minutes. The coulomb charge delivered (0.002679 C) was calculated from integrating the current collected using a beam profile image (Figure 5.21SI) and an approximate Gaussian fit (sigma =4.5mm), which generated a total flux of 8.36x1015 ions over the fit. This Gaussian was used to produce an ion beam flux integration value (Table 5.4SI) which was to be later used to give the required average dpa values. Table 5.3SI. Distribution of 8.36x1015 He2+ ions over a beam profile from the luminescence signal on a quartz screen (Figure 5.21SI) that follows a Gaussian distribution across the sampling region giving the percentage of ions that make up each 1 mm of the beam profile (black highlighted most relevant to study). X dimension (from beam Intensity (%) Integral (%) Flux (ions/mm2) centre mm) 10.5 0.0 0.0 0 9.9 4.4 0.3 2.12015E+13 8.9 6.4 1.1 8.88043E+13 8.0 14.3 1.8 1.53974E+14 7.0 20.8 3.5 2.89957E+14 6.0 35.4 5.5 4.63765E+14 5.0 49.2 8.4 6.98787E+14 4.0 63.9 11.2 9.34552E+14 3.0 75.0 13.7 1.14743E+15 2.0 90.9 16.4 1.37023E+15 1.0 97.8 18.6 1.55883E+15 0.0 100.0 19.5 1.63433E+15 Total 100.0 8.36E+15 The penetration of the beam into the sample and the displacements caused on average at by each 5 MeV α-particle was calculated using the Stopping and Range of Ions in Matter (SRIM) modelling software (Ziegler, 2013). This uses Monte Carlo based simulations to calculate an estimate penetration depth in montmorillonite to be 26 µm using a layer from an approximate sample formula (Si, O, Al, C, H, Mg, Ca, Fe, Na, K, and Ti) and thus density of sample to give Kinchin-Pease estimates for number of displacements per ion (164) (Figure 5.22SI). 255 Figure 5.22SI. SRIM pathway generated in montmorillonite type material. The Displacements per atom (dpa) was calculated using the following equation. 퐶 휙 퐷 = 푖 (eq. 3SI) 푑푝푎 퐴 D is the α-dose (displacements per atom), Ci is displacements per ion/per unit depth 2+ from SRIM calculations, φ is the 4He ion flux, and A is the atomic density of the sample (from Avogadro’s number). A full table of experimental, modelling, literature and calculated values is shown in Table 5.4SI for the montmorillonite sample used. 256 Table 5.4SI. Showing the values used to calculate dpa. Input/output values (energy of the alpha particles in MeV; C is coulombs of current in nano- amps; ϕ is the time in minutes), literature values (desired dose is Dc given in dpa; g/ mol and g/ cm2 are related to the STx-1b mass and density (montmorillonite); and Coulomb Scaling is the coulomb scaling factor for the beam line), modelled values (R is the particle range in μm; and Disp is the displacements per ion in STx-1b ), and calculated values (D/i/μm and D/i/cm are conversions of the displacements per ion per μm or cm; and α Thresh linking to the required number of α-particles to reach the Dc; Coul is the charge required in coulombs to give the required C), are included . Sample Energy R Disp D/i/μm D/i/cm Dc g/ mol g/ Atomic Atomic Coulomb α Thresh Coul C ϕ (MeV) (μm) (dpa) cm3 mass/ density Scaling (na) (mins) atoms STx-1b 5.01 26 164 6.31 63076.92 0.16 549.07 2 9.12 2.19 3.20 5.56 8.90 238 204 x10-22 x1021 x10-19 x1015 x10-4 257 Table 5.5SI. Showing the calculated area of α-damage across the Gaussian distribution of the beam profile. The integrated regions were used from Table 5.3SI and ion densities were normalised to 1 cm2 to give dpa over each ring of damage, with maximal α-damage occurring at the beam centre. From To Area of larger Area of smaller Difference Factor to Ion/ cm2 (density Atomic (cm) (cm) circle (cm2) circle (cm2) (cm2) 1 cm2 Norm) Displacements density DPA 1.05 0.99 3.46 3.08 0.38 2.65 5.60 x1013 63076.92 2.19x1021 0.0016 0.99 0.89 3.08 2.49 0.59 1.69 1.50 x1014 63076.92 2.19x1021 0.0043 0.89 0.8 2.49 2.01 0.48 2.09 3.22 x1014 63076.92 2.19x1021 0.0093 0.8 0.7 2.01 1.54 0.47 2.12 6.15 x1014 63076.92 2.19x1021 0.018 0.7 0.6 1.54 1.13 0.41 2.45 1.14 x1015 63076.92 2.19x1021 0.033 0.6 0.5 1.13 0.79 0.35 2.89 2.02 x1015 63076.92 2.19x1021 0.058 0.5 0.4 0.79 0.50 0.28 3.54 3.31 x1015 63076.92 2.19x1021 0.095 0.4 0.3 0.50 0.28 0.22 4.55 5.22 x1015 63076.92 2.19x1021 0.16 0.3 0.2 0.28 0.13 0.16 6.37 8.72 x1015 63076.92 2.19x1021 0.25 0.2 0.1 0.13 0.03 0.09 10.61 1.65x1016 63076.92 2.19 x1021 0.48 0.1 0 0.03 0 0.03 31.83 5.20 x1016 63076.92 2.19 x1021 1.50 Whole 8.36 x1015 63076.92 2.19x1021 0.24 258 Radiation damage in materials expressed as a dpa value signifies atoms in the structure have been displaced from their original sites at least once (i.e. 0.1 is 10% of atoms). Due to ion flux reduction with distance from the centre, the α-dose was calculated through the division of the Gaussian into eccentric rings and working out the normalised area of each, allowing an approximate dose range over for the clay pellet samples (used for XRD and XAS analysis) and a given dose for the multiple tomography samples (Figure 5.21SI, at about 0.3 cm from the beam centre) shown in Table 5SI. An average received dose, within the beam, over each sample was 0.24 dpa. Note on safety concerns of α-irradiation: The temperature over the run time was measured directly and shown not to exceed 75 °C limiting any annealing effects expected. The sample activation was also considered and shown to mostly be concerned with beta activities of the mounts but previous studies showed any activation was short lived and typically decayed to within background levels within 60 min. 259 6. The effect of γ-irradiation on the sorption capacity of a model montmorillonite and nontronite under simulated repository conditions with respect to U(VI) and Cr(VI) in solution (research article) This chapter is presented as a manuscript currently in preparation for submission to the Journal of Environmental Radioactivity. Supporting information for this manuscript is included immediately following the chapter. 260 The effect of γ-irradiation on the sorption capacity of montmorillonite and nontronite for U(VI) and Cr(VI) A. P. Simsa,b, K. Morrisb, J. F. W. Mosselmansc, S. Hayamac, F. R. Livensa, C. I. Pearce*a,d a School of Chemistry, University of Manchester, M13 9PL UK ; b Research Centre for Radwaste Disposal, School of Earth and Environmental Sciences, University of Manchester, M13 9PL UK; c Diamond Light Source, Harwell, Oxford, UK; e Pacific Northwest National Laboratory, Richland, WA, USA. 6.1 Abstract The UK has been operating a civil nuclear programme since 1956 (Grimston, 2008), resulting in a substantial amount of legacy radioactive wastes from past uses and on- going nuclear power generation which is now being stored at designated waste management facilities. The UK government has adopted for an open-fuel cycle approach, with the favoured route for the disposal of heat generating, higher-level waste (HLW) being a clay buffered geological disposal facility (GDF) (Wilson et al., 2010). A safety case for HLW typically considers uranium (U) as one of the most significant long-lived radionuclides; chemical contaminants, such as chromium (Cr), may also be present at nuclear sites and was used here as a redox active analogue species. These species have the potential to be remobilised as a consequence of canister failure and resultant groundwater infiltration. To allow a realistic estimation of the behaviour of these contaminants, the primary content of the buffer material, montmorillonite, and a likely alteration product nontronite, were examined over 12 months. Here, variations in behaviour with different carbonate concentrations and with γ-irradiation (5 MGy) are apparent. Clay minerals show small but measurable alterations in surface chemistry and charge with γ-irradiation which led to increases in the sorption of Cr(VI) primarily, with no significant changes seen in the U(VI) behaviour. Furthermore, solid-phase analysis of uranium oxidation state and co-ordination environment changes through extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) highlighted the impact of carbonate on uranium behaviour. Overall, there was negligible effects on the 261 2+ co-ordination environment and sorption of the U(VI) species (UO2 ) present after γ-irradiation with the overwhelming factor in uptake linked closely to the presence of 2- carbonate in solution. An increase in the sorption of Cr(VI) species (CrO4 ) was shown, postulated from an increase in the Fe(II) redox active portion of the clay with γ-irradiation. An increased understanding of the sorption mechanisms and more efficient attenuation of chemical and radioactive contaminants at waste storage facilities is required for an effective safety case to be built for a future GDF. 6.2. Introduction A substantial legacy of radioactive wastes, created from such processes as past weapon development, mining, and nuclear power generation exists (Brookshaw et al., 2015; Williamson et al., 2014; Li and Kaplan, 2012). Not all wastes currently lie within the remit if the inventory of legacy wastes, however, uranium wastes would be the most significant radionuclide by mass within the final inventory (UK) and chemical contaminants including those such as chromium (Hanford, USA) would also be present (Riley and Zachara, 1992; Beaumont et al., 2008; Smith, 2008; Cheng et al., 2012). Attenuation of chemical and radioactive contaminants is of paramount importance for sustainable radioactive waste storage safety policies, processes and procedures at sites internationally (DECC, 2014; Riley and Zachara, 1992; Cheng et al., 2012). Long-lived (long half-life), environmentally mobile and radiotoxic radionuclides, such as U and Tc (a redox active analogue of Cr) are to be stored at geological disposal facilities (GDF) and have both cationic and anionic mobile phases. To address the issue of future canister breaches in such facilities, high level wastes would need to be sequestered effectively to isolate them from the surrounding environment preventing the release of radionuclides (Greathouse and Cygan, 2006; Brookshaw et al., 2015; Sellin and Leupin, 2013; Apted et al., 2010). The most likely cause that can lead to the release of radionuclides and contaminants in a geological formation is the ingress by, and transportation in, groundwater (Dozol and Hagemann, 1993). Therefore a sound knowledge of the behaviour of radionuclides and containments, under conditions relevant to geological disposal, such as high accumulated γ-irradiation doses and interactions with hydrated mineral phases are key (Sylwester et al., 2000; Bernier et al., 2007; Holton et al., 2012; Morris et al., 2011). 262 The fate of mobile radionuclides, such as U(VI), and contaminants, such as Cr(VI), is primarily governed by their solubility and adsorption to geological materials (Silva and Nitsche, 1995, Pabalan and Turner, 1997; Bots et al., 2014). To address the challenge of migration in the geological structure most GDF solutions include an engineered barrier system which comprise of a secondary mineral containment structure around a primary containment canister (Baldwin et al., 2008). Smectite clay minerals, such as montmorillonite, make up the main constituent of the engineered barrier system (EBS), bentonite, which is the preferred material for heat generating waste storage and plays a key role in retarding radionuclide migration in the GDF environment (Posiva, 2009; Teich-McGoldrick et al., 2015; Baldwin et al., 2008). Clay minerals have shown a number of advantageous properties: hydration causes swelling reducing crack formation and making diffusion the main means of transport (Nagra, 2002; Missana et al., 2008; Beattie and Williams, 2012); high ion exchange and sorption capacity for radionuclides (Grauer, 1986; Hicks et al., 2009; Sylwester et al., 2000); low hydraulic conductivity, as well as a wide availability naturally and stability over geological timescales (Sellin and Leupin, 2013; Wilson et al., 2011). The chemical composition of the interlayer species and accessory minerals for different montmorillonites significantly affect their behaviour (Savage and Authur, 2012; Brigatti et al., 2011; Karnland, 2010; Stanković et al., 2011; Greathouse and Cygan, 2006). Iron content has a substantial effect on redox chemistry (Skomurski et al., 2011; Li and Kaplan, 2012; Jeon et al., 2005) where iron incorporation into clay minerals from canister decay can form Fe-rich phases. These higher Fe phases formed reduce swelling in clays, but an increased charge allows for a higher affinity for positively charged species and an increased redox potential, leading to the possibility of reductive precipitation or incorporation of contaminants (Gorski et al., 2013; Neumann et al., 2015; Liger et al., 1999; Du et al., 2011; Latta et al., 2012b). Functional groups present (AlOH/ SiOH/ FeOH) also affect sorption of contaminants and the type of bonding seen (Gückel et al., 2012; Greathouse and Cygan, 2005; Denecke et al., 2003; Hudson et al., 1999). 263 6.2.1. γ-irradiation Mineral phases would be subjected to α-, β-, and γ-ionising radiations as well as heavy ion (elastic) effects within the confines of the GDF (Allard and Calas, 2009; Ewing et al., 2000; Pusch, 1994; Ewing, 1995). The engineered clay barrier would be exposed to γ-radiation from the HLW with dose rate estimations ranging from 72 Gy h-1 to 0.5 Gy h-1 (SKB, 2006; Stroes-Gascoyne et al., 1994; Allard and Calas, 2009), with cumlative doses above 5 MGy after the first 1000 years. In clays, the effects of γ-irradiation are predominantly changes to physiochemical properties, through large charge defects (electronic) resulting from interactions of highly energetic electrons causing an increased surface potential of the clays (Sims et al., In Review; Allard et al., 2012; Ewing et al., 1995). There are also minor structural changes in Fe2+ concentration through partial reduction of Fe3+ in minerals, as well as increased colloid stability and a small amount of radiolysis casusing a minor interlayer disorder (Sims et al., In Review; Allard et al., 2012; Gournis et al., 2001; Holmboe et al., 2009; Missana et al., 2008; Kunze et al., 2008). The small alterations to the surface and structural properties of clays with γ-irradiation may play a key role in the retardation of radioactive contaminants and thus must be examined further. 6.2.2. Chromium (CrO42-) Chromium contamination has been recorded at a number of sites worldwide from industrial processes (Khan et al., 1995; Jemima et al., 2018; Dhal et al., 2013). A number of Department of Energy (DOE) sites in America, such as Hanford, relating to nuclear projects have significant quantities of chromium in the soil sediments that show a mixture of mineral phases including clay (Riley and Zachara, 1992; Truex et al., 2015). Chromium has two main oxidation states depending on environmental conditions; a 2- mobile, anionic Cr(VI) species in solution (CrO4 ) is seen in oxic conditions, and an immobile, insoluble Cr(III) species (Cr2O3) is present in reducing conditions (Bishop et al., 2014; Richard and Bourg, 1991). The high mobility and toxicity of Cr(VI) makes it an environmentally hazardous contaminant; with research showing an increase in cancer rates in populations where Cr(VI) is present in drinking water (Beaumont et al., 2008). One approach used for the remediation of Cr(VI) was through reduction to the 264 immobile, safer, Cr(III) (Richard and Bourg, 1991; Beaumont et al., 2008; Rai et al., 1988; Rai et al., 1989). 6.2.2.1. Chromate Sorption The main uptake mechanism for heavy metal contaminants in soil, such as Cr(VI), is adsorption through ion exchange, precipitation, co-precipitation and organic binding (Arnfalk et al., 1994). Previous research has shown that Cr(VI) reduction to Cr(III) occurs from a redox reaction with ferrous iron, either in aqueous solution or from Fe(II) bearing minerals (Pettine et al., 1998; Chon et al., 2006; Jung et al., 2007). Clays contain Fe(II) structurally or as an absorbed species and this reduces Cr(VI) to Cr(III) with a relationship between Fe(II) concentration in clays and extent of Cr reduction observed (Gan et al., 1996; Taylor et al., 2000; Zhuang et al., 2012; Brigatti et al., 2000; Parthasarathy et al., 2007; Bishop et al., 2014). The reductive transformation of anionic Cr(VI) on anionic clay surfaces (weak negative charge), such as montmorillonite (Brigatti et al., 2011; Wolters et al., 2009; Savage and Authur, 2012), requires reduction of Cr(VI) to Cr(III) by Fe(II) within the clay or modification of the clay, including the substitution of the interlayer cations with more favourable cationic species (hydrophobic surfactants), alteration of surface groups, or addition of anionic ‘getters’ such as argentite (Bishop et al., 2014; Jemima et al., 2018; Rai et al., 1989; Qafoku et al., 2015). In a study by Taylor et al. (2000), the adsorption of Cr(VI) in clays was shown to require the presence of Fe(II) prior to reaction, allowing a sorption-reduction reaction not seen with the insitu reduction of Fe(III). Natural samples can be reactive towards reductive precipitation of Cr(VI). Fe(II)-rich saponite, is a possible alteration product from canister corrosion (Svensson, 2015) and was shown to be good in the reduction of hexavalent chromium with an efficiency of 75% with no other treatments (Parthasarathy et al., 2007). Qafoku et al. (2017) showed naturally reduced field samples from the Hanford site (USA), to have minimal - 2- reactivity towards anionic species TcO4 and CrO4 under anoxic conditions, with slow reaction kinetics in comparison to experimentally reduced clays. However, this was shown to increase with the addition and sorption of Fe(II) on the clays through enhanced electron transfer reactions allowing for the surface mediated reduction of weakly associated oxyanions. 265 The sorption of Cr(III) and Cr(VI) in aqueous solutions on bentonite has been shown to occur through exothermic and endothermic reactions, respectively, showing preference for cationic species (Khan et al., 1995; Chalermyanon et al., 2008). Arnfalk et al. (1994) showed very limited sorption on bentonites (Ca/Na) with a maximum of 20% sorbed due to the anionic nature of the species. Microbial-induced reduction of structural Fe(III) in a nontronite (NAu-2) and a montmorillonite (SWy-2) conducted by Bishop et al. (2014) was effective at reducing Cr(VI) to Cr(III), and was shown to sorb to the clay surfaces through SEM, TEM, and EDS techniques. High doses of γ-irradiation (>50 kGy) sterilize clays, eliminating all viable organisms, so this effect would not be prominent in this study (Bank et al., 2007). 6.2.2.2. γ-irradiation effects Radiolysis products (·e or ·OH) from γ-irradiation have been shown to convert soluble Cr(VI) to nano-particulate Cr(III) oxides and hydroxides through multi-stage processes, controlling particle size through solution-solid interfacial reactions on the colloids (Alrehaily et al., 2015; Alrehaily et al., 2013). Cr(VI) interactions with a γ-irradiated clay minerals may help predict future behaviour of other pertinent oxyanions present at geological disposal sites, such as Tc(VIII), Se(IV)/Se(VI), and I(V), that are all redox active species relevant to GDF storage (Hjerpe et al., 2010; Bower, 2015; Pezzarossa, 1999; Qafoku et al., 2017; Whitehead 1974; Um et al., 2004). 6.2.3. Uranyl Studies (UO22+) Uranium, the dominant radionuclide by mass in many radioactive wastes, is a long lived radionuclide with a number of radioactive daughter products and multiple stable oxidation states in the environment (Barnett et al., 2002; Pabalan and Turner, 1997; Pourcelot et al., 2011; Hudson et al., 1999; Duff et al., 1997; Giaquinta et al., 1997). 2+ The hexavalent aqueous uranyl ion (UO2 ), is the dominant uranium species found in contaminated groundwater systems (Greathouse and Cygan, 2006; Langmuir, 1978) and its mobility is controlled through absorption onto mineral surfaces, mitigating the transport of U(VI) in the subsurface (Langmuir, 1978; Williamson et al., 2014; choppin, 2007; Barnett et al., 2002; Dent et al., 1992). 266 Retention of uranyl occurs through a number of processes, such as: adsorption of U(VI) at clay mineral surfaces (Dent et al., 1992; Giaquinta et al., 1997; Hudson et al., 1999; McKinley et al., 1995; Wasserman et al., 1997), precipitation of U(VI) minerals (Catalano and Brown, 2004), reductive precipitation to U(IV) (Shoesmith, 2018; Liger et al., 1999), or incorporation/co-precipitation in silicates (Allard et al., 1999; Soderholm et al., 2008). 2+ Uranyl ions (UO2 ) have been shown to complex with carbonate and other ligands in aqueous solution, affecting sorption on to materials and mobility across a range of pH values (Bargar et al., 1999; Li and Kaplan, 2012; Waite et al., 1994; Brookshaw et al., 2015). 6.2.3.1. Uranyl Sorption U(VI) mobility in subsurface environments is influenced by phosphate, carbonate, and pH (Li and Kaplan, 2012; Waite et al., 1994 Cheng et al., 2012); adsorption onto minerals, incorporation and precipitation as U(VI) or U(IV) phases provide a remediation route for highly mobile U(VI), which is present in the environment from anthropogenic releases from the nuclear sector (Morris et al., 2011). However, desorption or re-oxidation under oxic conditions could allow for re-mobilisation of U(VI), and transport in the environment (Massey et al., 2014; Brookshaw et al., 2015; Sellin and Leupin, 2013; Wilkins et al., 2007). Sorption of uranyl to a number of materials, including montmorillonite, γ-alumina, hematite, magnetite, goethite, and ferrihydrite, has been shown to be at a maximum around near-natural pH (5-7) and a steady state reached within 48 hrs. Sorption capacities decreased with variations from natural pH, forming uranyl carbonate complexes at higher pH, suppressing ion exchange, which is influenced through surface area, particle size, and porosity of each material (Zeng et al., 2009; Zuyi et al., 2004; Missana et al., 2003; Li and Kaplan, 2012; Pabalan et al., 1998; Prikryl et al., 2001; Bachmaf and Merkel, 2010; Rotenberg et al., 2010; Barnett et al., 2000). These studies also showed the charge location to have minimal effect on sorption, with similarities seen for beidellite (tetrahedral) and montmorillonite (octahedral). Sorption was not largely impacted by the different mass to volume ratios, although under some geological conditions this did show small effects (Li and Kaplan, 2012; Cheng et al., 267 2012). The selectivity of montmorillonite ion-exchange sites for uranyl was shown to be between that of divalent and monovalent metal cations (Tsunashima et al., 1981). A number of adsorption environments were shown for uranyl uptake onto minerals; 2+ pentaaquouranyl complexes (UO2 ) dominated at low uranyl concentration, showing adsorption at the clay basal plane (interlayer) through outer-sphere mechanisms. Mono(carbonato) complexes formed at higher concentrations of uranyl due to higher + ionic strength and carbonate concentration in solution. Formation of mono-(UO2OH ), 3− bi-(UO2(OH)2), or tri-dentate (UO2(OH) ) hydrolysis species, such as schoepite or co- precipitated uranyl silicate (soddyite), reduced adsorption with a dependence on environmental factors (Greathouse and Cygan, 2006; Li and Kaplan, 2012; Denecke et al., 2003; Michard et al., 1996). U(VI) sorption decreases in the presence of carbonate at higher pHs, above 8, due to the stability of uranyl carbonate complexes (Barnett et al., 2002; Greathouse and Cygan, 2005; Waite et al., 1994). Inner sphere uranyl-carbonato or uranyl surface complexes were shown to form in carbonate systems (Bargar et al., 1999: Ilton et al., 2006; Rossberg et al., 2009 Pabalan and Turner, 1997); XAS studies showed uranyl carbonate species and inner-sphere interactions through surface O atoms acting as equatorial ligands at neutral or high pH, relevant to geological disposal (Dent et al., 1992; Bargar et al., 1999; Allen et al., 1995; Denecke et al., 2003). Sorption on Na+ montmorillonite decreased with increasing uranyl carbonate concentration, as a factor of ionic strength, due to the formation uranyl carbonato-oligomers (Greathouse and Cygan, 2005; Sylwester et al., 2000; Greathouse and Cygan, 2006). However, ionic strengths of 0.001 M to 0.1 M, in the presence of Mg2+, Ca2+ cations (10-3 M), were examined by Li et al. (2012) and showed insufficient effects on sorption. Carbonate free systems revealed the opposite trend, with increased sorption seen at higher pH up to pH 10 (Greathouse and Cygan, 2005; Sylwester et al., 2000). This was exemplified in the sorption of U(VI) on microbially reduced Fe-silicates by Brookshaw et al. (2015), showing very high soprtion at a low carbonate concentration (0.2 mM), proceeding through reductive precipitation to an insoluble U(IV) species and decreasing significantly at higher concentrations of carbonate (30 mM). 268 A number of studies have shown the adsorption and reduction of U(VI) to insoluble U(IV) to be possible, even under aerobic conditions, through the use of Fe(II)-bearing minerals such as siderite, aqueous Fe(II), or prior reduced Fe(III) minerals for example nontronite. Fe(II) clay minerals showed above 95% sorption at natural pHs with reduction dependent on the Fe(II) and Fe(III) content of the minerals, pH of the solutions, and electron catalysed transfer processes at the mineral surfaces (Neumann et al., 2015; Ilton et al., 2006; Skomurski et al., 2011; Duff et al., 2001; Jeon et al., 2005; Brookshaw et al., 2015; Chakraborty et al., 2010; Dreissig et al., 2010). This immobilization of U(VI) in clay minerals at depth was strongly linked to the reduction of U(VI) to U(IV), coincident with the Fe(II) to Fe(III) redox reaction, and revealing Fe(II) as the active reducing agent (Stetten et al., 2018). The stability of these U(IV) phases was shown to be dependent on the presence of other oxides in the clays, and if the clay mineral was absent then nano-particulate uraninite was shown to form (Boyanov et al., 2017). Incorporation of U(VI) was favoured at higher carbonate concentrations, forming phases such as schoepite (Dreissig et al., 2010; Duff et al., 2001; Massey et al., 2014; Williamson et al., 2014). A large body of work has been carried out on the bonding environment of uranyl, through experimental XAS studies and theoretical simulations, showing outer- and inner-sphere complexes dependent on environmental conditions, such as pH, carbonate presence, and mineral surface groups. Bonding interactions in montmorillonite and bentonite showed ion-exchanged uranyl ions to form outer- sphere complexes, resembling the aqueous uranyl species with no preferential 2+ o o orientation of the [O=U=O] axis (between 0 and 90 ) in the basal plane (d001 basal spacing of 14.58 Å) as a result of XAS, XRD and Monte Carlo simulation studies (Denecke et al., 2003; Zaidan et al., 2003; Hudson et al., 1999; Greathouse et al., 2005; Giaquinta et al., 1997; Wasserman et al., 1997; Waite et al., 1994). Outer-sphere complexation was favoured in lower pH systems with monatomic U(VI) cations due to the maximisation of H-bonding, giving 2 axial oxygens and 5-6 equatorial oxygens in EXAFS, and no Fourier transform peak of U-Si interactions expected with inner-sphere bonding (Greathouse and Cygan, 2006; Sylwester et al., 2000). A tris-carbonato complex, [UO2(CO3)3], was modelled by Greathouse et al. (2005) on montmorillonite featuring six-fold equatorial coordination about the 269 uranium atom at higher pHs (8). This species was also seen within other clay mineral systems (Brookshaw et al., 2015; Zhang et al., 2011). Loss of the symmetry of equatorial oxygens through surface group interaction at higher pH caused splitting of the equatorial oxygens in the EXAFS, and interactions from U-Si and U-U seen within the Fourier transform of the EXAFS suggest inner-sphere interactions on clays at pH ranges relevant to disposal (Sylwester et al., 2000; Kowal-Fouchard et al., 2004; Greathouse and Cygan, 2005; Hudson et al., 1999). Similar effects were observed on dehydration at a neutral pH with a less symmetric inner-sphere complex formed at amphoteric surface hydroxyl sites, and a collapse of the interlayer to 10 Å, showing a highly disordered equatorial shell with neighbouring U-U interactions suggesting the formation of precipitates (Greathouse and Cygan, 2005; Hudson et al., 1999). Surface complexation of uranyl and associated complexes was highly dependent on pH and competition at edge sites, Al-OH and Si-OH. The presence of other cations in solution caused exposed sites to be less readily available in 2:1 minerals over 1:1 minerals (Chisholm-Brause et al., 1994; Dent et al., 1992; Hudson et al., 1999; McKinley et al., 1995: Bachmaf and Merkel, 2010; Kowal-Fouchard et al., 2004; Barnett et al., 2000). 6.2.3.2. γ-irradiation effects The influence of γ-irradiation on the properties of clay minerals has been shown to be modest. Minor structural changes at large doses of γ-irradiation (≈1 MGy) have been reported thus making clays a suitably resistant material for backfill use in GDFs (Plötze et al., 2003; Negron et al., 2002: Fidalgo et al., 2014; Galamboš et al., 2012; Grambow, 2016). A further consequence of γ-irritation was the reduction of structural Fe(III) to Fe(II) which mitigates the associated decrease in clay cation exchange capacities through enhanced redox potential, with a general increase shown in the sorption of U(VI) after γ-irradiation (Bonne and Heremanns, 1981; Gournis et al., 2001; Bank et al., 2007). Bank et al. (2007) showed the clay fraction of γ-irradiated sediments were better sorbents of U(VI), in spite of the fact that reduced biological activity was shown, which was known to aid in the cycling of Fe(III) to Fe(II) and subsequent reduction of U(VI) to U(IV) (Brookshaw et al., 2015; Williamson et al., 2014). A slight reduction in pH, a significant decrease in the cation exchange capacity (CEC), a change in solubility (lattice 270 cations and dissolved organic content), a decrease in plasticity (swelling strain), an increase in Eh, and an increase in Fe(II)/Fe(III) ratio were all shown to be caused by γ-irradiation of geological materials (Huang and Chen, 2004; Bank et al., 2007; Kreller et al., 2005; Pushkareva et al., 2002; Gournis et al., 2001). The small decrease in pH may shift the adsorption edge into a more favourable range between pH 7.5-8 for U(VI) adsorption (Barnett et al., 2000; Barnett et al., 2002). This enhanced sorption was due to γ-induced Fe(III) reduction, a slight increase in the surface charge making the solids more conducive to sorption of any cationic U(VI) species, and increased surface area of this fraction in comparison to coarser grains (Barnett et al., 2000; Lee et al., 2016). The majority of U(VI) sorption was controlled by Fe(II) oxides or organic matter present in the materials studied through electron transfer processes at the clay surface; Kd values showed a marked increase after γ-irradiation (25 Vs 14) suggesting increased uptake from solution of U(VI) (Stubbs et al., 2006; Ilton et al., 2006; Skomurski et al., 2011; Jeon et al., 2005). Holmboe et al. (2011) showed γ-irradiation induced variations in the surface chemistry, showing small changes in the behaviour of clay minerals with differing cations (Cs+, Co2+). An increased colloid stability, through increased surface potential, from aqueous radiolysis product interactions could have large implications on U(VI) transport in a GDF environment (Holmboe et al., 2009; Laine et al., 2017). Fidalgo et al. (2014) showed bentonite to play a key role in the delay of the dissolution of U(VI) from a UO2 pellet primarily through scavenging of radiolytic oxidants present, as a by-product of radiolysis from γ-irradiation, through interaction with Fe(III) present in the structure (Laine et al., 2017). Zuo et al. (2017) showed the interaction of montmorillonite with U(VI) (9.96×10−5 mol/L) over a range of γ-irradiation doses (0-200 kGy) to be maintained. U(VI) sorption through inner-sphere and outer-sphere mechanisms was dominated by ion exchange. Thermal pre-treatment (600 °C) was shown to enhance the sorption at activated hydroxyl sites (Zuo et al., 2016). It was shown that the U(VI) sorption increased quickly within the first few hours (surface sorption), gradually reaching a steady state after 24—48 hrs (Aytas et al., 2009; Zuo et al., 2017; Huang and Chen, 2004). Adsorption of uranium fission products, 137Cs and 90Sr, after γ-irradiation of bentonite was shown to 271 remain unchanged in the case of Cs(I) and increase for Sr(II) due to an increased adsorption capacity (Galamboš et al., 2012). 6.2.4. Current Cr(VI) and U(VI) sorption study Initial tests were used to assess the viability of a longer time scale study (Suplamentary information, SI 6.9) and a set of experiments were performed at The University of Manchester. Experiments aimed to fully examine the behaviour of both U(VI) and Cr(VI), in artificial ground water (AGW; Wilkins et al., 2007), through sorption studies on montmorillonite and nontronite. This allowed for a more realistic estimation of behaviour of the buffer material after a large dose of γ-irradiation (5 MGy), probing 2+ the interactions of both a positively charged species (UO2 ) and negatively charge 2- species (CrO4 ), after canister failure, in a GDF environment related to possible early surface releases. 6.3. Methods and Materials 6.3.1. Chemicals Analytical grade chemicals were used throughout with 19 mohm deionised water. 6.3.2. Clay minerals Two smectitic clays were purchased from the Source Clays Repository of the Clay Minerals Society (West Lafayette, IN); a montmorillonite, STx-1b, with a low Fe content and a nontronite, NAu-1 with 30% structural Fe(III) content acting as a pseudo alteration product (Neumann et al., 2013; Gorski et al., 2012). The STx-1b montmorillonite sample was shown to have chemical formula (Castellini et al., 2017); (Si7.753Al0.247) (Al3.281Mg0.558Fe0.136Ti0.024Mn0.002) (Ca0.341Na0.039K0.061)O20(OH)4 Here, the layer charge was primarily in the octahedral sheet from isomorphous substitution of lower charge cations into the octahedral sites (Vantelon et al., 2003). The NAu-1 nontronite sample was shown to have the chemical formula: + M 1.05(Si6.98Al1.02)(Al0.29Fe3.68Mg0.04)O20(OH)4 272 Where the balancing cation was Ca2+ or Mg2+ and the layer charge was predominantly in the tetrahedral sheet from substitution of lower charged cations into the tetrahedral sites (Keeling et al., 2000). The clays were milled in a TEMA ball mill and sieved to obtain a size fraction below 53 μm relating to clay and silt-sized particles before use. 6.3.3. γ-irradiation The clay samples (1.0 g x 12) were weighed into quartz tubes (15 mm OD x 13 mm ID), approximately 10 cm long, and flame sealed following evacuation (2.5 x 10-3 mbar). Samples were then γ-irradiated using a Foss Therapy Services Model 812 60Co γ-irradiator at The University of Manchester’s Dalton Cumbrian Facility (DCF) (Leay at al., 2014). Absorbed dose rates from 4 Gy/min up to 450 Gy/min were used to yield a dose of 5 MGy in each sample, determined by Fricke dosimetry (Leay at al., 2015). 6.3.4. Sorption experiments 6.3.4.1. Experimental set up 6.3.4.1.1. Uranyl stock solution A uranyl(VI) stock solution was made up from 3.49 mL of uranyl chloride (17,200 ppm, 0.072 moles) dispensed in 16.51 mL of 0.001 M HCl (diluted from 1 M stock solution) to give a 20 mL uranyl sub-stock solution (3010 ppm, 0.63 M, 1.2 x 10-2 moles). 6.3.4.1.2. Chromate stock solution -5 A chromate stock solution was made up from K2Cr2O7 (0.019 g, 6.46 x 10 moles) in deionised water (25 mL) to give a 25 mL chromate solution (Cr: 6.7 ppm, 5.16 x 10-3 M, 1.29 x 10-4 moles). 6.3.4.1.3. Artificial groundwater (AGW) Artificial groundwater (AGW) representative of the Sellafield area (shallow groundwater chemistry) was made up using the method given by Wilkins et al. (2007): a normal AGW system (5 mM carbonate) and a low carbonate system (0.5 mM) were set up for comparison. The solutions were all de-gassed and used anaerobically throughout the experiment. However, in a deeper geological setting the groundwater make-up would more likely be that of a saturated NaCl brine but this was not used here and surface contaminant release was simulated. 273 The following chemicals were added to deionised water (500 mL); NaHCO3 (0.2424 g), NaNO3 (0.0275 g), CaCO3 (0.1672 g), MgCl2.6H2O (0.081 g), MgSO4.7H2O (0.0976 g), Na2SiO3 (0.0829 g), and NaCl (0.0094 g). The solution was then made up to 1 L and the pH was measured (pH 7.38). 6.3.4.1.3.1. Low carbonate AGW The procedure followed the same method (6.3.4.1.3) but reduced the carbonate concentration in solution (0.5 mM) through adding less NaHCO3 (0.03 g) and CaCO3 (0.017 g). The solution was again made up to 1 L and the pH was measured (pH 7.36). 6.3.4.2 Chromate sorption STx-1b, NAu-1, STx-1b γ-irradiated, and NAu-1 γ-irradiated (0.5 g) were weighed and added to degassed solutions of AGW (9.95 mL), in triplicate, in microcosms within the anaerobic chamber (AC) and left to equilibrate (48 hrs). To start the experiment the solutions were spiked with the chromium stock solution (0.05 mL, 5.16 x 10-3 M) in the AC to give a constant solid to solution ratio (1:20) and concentration (5.16 x 10-5 M, pH 8.61-8.95). The experimental samples were then sealed and moved to the bench top, any further work was done anaerobically under an argon atmosphere. Sampling points (0.2 mL) were taken at 1 hr, 4 hr, 24 hrs, 1 wk, 3 wks, 5 wks, 6 wks, 11 wks, 24 wks, 38 wks and 51 wks. The sample was centrifuged (14000 rpm, 5 mins) and solution phase diluted (ppb) for ICP-MS analysis. 6.3.4.3. Uranyl(VI) sorption STx-1b, NAu-1, STx-1b γ-irradiated, and NAu-1 γ-irradiated (0.1 g) were weighed anaerobically and added to degassed solutions of AGW (9.9 mL), in triplicate, in microcosms within the AC and left to equilibrate (48 hrs). A set of U(VI) solutions (10 mL, 30.1 ppm, 1.26 x 10-2 M, pH 8.16-8.88) were made up from the stock U solution (0.1 mL, 3010 ppm) to start the sorption experiment in the AC and the samples were then sealed and moved to the bench top, any further work was done anaerobically under an argon atmosphere. Sampling (0.2 mL) was undertaken at 1 hr, 4 hr, 24 hrs, 1 wk, 3 wks, 5 wks, 6 wks, 11 wks, 24 wks, 38 wks and 51 wks. Solution phase was extracted using micro-centrifuge (14000 rpm, 5 mins) and acidified with nitric acid (0.02 mL, 8 M) before making up samples (ppb) up for ICP-MS. 274 Solid samples for XAS analysis were separated under anaerobic conditions in the AC, using centrifugation (14000 rpm, 10 mins), decanting the solution and extracting the solid (≈2000-3000 ppm on solid) into XAS sample holders and flash freezing (-80 °C). 6.3.5. Solid and solution phase characterisation 6.3.5.1. Solution phase characterisation: Inductively Coupled Plasma-Mass Spectrometry Samples (0.2 mL) taken under an argon environment and the U(VI) and Cr(VI) extractions were diluted in 2 % nitric acid (made up to 10 mL) for ICP-MS analysis. Samples were measured using a 7700x inductively coupled plasma-mass spectrometer at the School of Earth and Environmental Sciences (SEES) at the University of Manchester. Data was run in triplicate to allow for error calculation (1 σ) and plotted in excel. In the event a sampling point was missed (due to extraction for XAS analysis) errors were calculated from 2 results. 6.3.5.2. Solid phase characterisation: X-Ray Absorption Spectroscopy (XAS) Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data was collected on beamline I20 at Diamond Light Source (Diaz- Moreno et al., 2018). Samples were collected as slurries in XAS holders, sandwiched between wax to give an evenly distributed sample before flash freezing (-80 °C). Uranium (U) L3-edge X-ray absorption spectra were measured in fluorescence mode, with samples diluted with a Y window to reduce elastic scattering, the detector was moved in and out to give the necessary absorption coefficient, run under cryogenic conditions (liquid nitrogen). A Y foil was used as an energy calibration standard. This allowed detailed analysis of U L3-edges to examine oxidation state and co-ordination geometry of uranium sorbed in the clays. All data was processed and analysed using the Demeter software package (Ravel and Newville, 2005). 6.3.5.2.1 XAS analyses Two samples were taken on U(VI) STx-1b samples (un-irradiated vs. γ-irradiated) at 5 weeks in AGW systems (≈800 ppm on the solid), to examine the bonding environment of U(VI) and oxidation state of uranium. Four U(VI) samples, STx-1b and NAu-1 (un- irradiated vs. γ-irradiated), were taken at 9 months in the low carbonate AGW solution systems (≈2500 ppm on the solid) to examine the effects of the lower carbonate 275 systems on the bonding environment. One U(VI) sample (γ-irradiated) at 12 months in AGW (≈800 ppm on the solid) was taken to explore ageing effects (in comparison to 5 week sample). 6.4. Results 6.4.1. Characterisation The two smectite clays; a montmorillonite, STx-1b, with a low Fe content, and a nontronite, NAu-1, a pseudo alteration product with a higher Fe content (≈30%) were used and have been previously characterised by Castellini et al. (2017) and Keeling et al. (2000). Sims et al. (in review) characterised the clays showing STx-1b to have higher Si and Al oxide contents (64.7% and 13.3%) and a lower Fe oxide content (0.96%) in comparison to NAu-1 (42.8, 2.61%, 31.9% respectively), presumably leading to a higher layer charge in nontronite with increased Fe(III) substitution into the octahedral sheet in place of Al(III) (Savage and Authur, 2012; Gorski et al., 2012; Brigatti et al., 2011; Wolters et al., 2009). The effect of γ-irradiation (5 MGy) on the bulk STx-1b and NAu-1 clays showed minimal changes to the clay structure on a macro-scale (XRD, IR, CEC, extractable Fe2+) but were manifested as molecular-scale effects through the formation radiation induced defects associated within the electronic structure of the clay evident in EPR, zeta potential, and XAS analyses. These may have implications for the sorption and interactions with both anionic and cationic species (Sims et al., in review; Holmboe et al., 2009; Allard et al., 2012; Bank et al., 2007). Two sets of sorption experiments on a STx-1b and NAu-1 were run in parallel to examine the effect of γ-irradiation (5 MGy) on the sorption of an anionic species 2- 2+ (CrO4 ) in AGW and cationic species (UO2 ) in AGW (and low carbonate AGW). All the reactions were tracked using solution phase analysis (ICP-MS) and further solid phase analysis of U(VI) oxidation state changes and alterations to the co-ordination environment were observed through XAS analysis. The use of a low carbonate AGW in the U(VI) sorption experiment was intended to explore the impact of carbonate on U(VI) sorption behaviour. 276 6.4.2. Chromium Sorption 6.4.2.1. Solution phase chemistry Cr(VI) groundwater only controls showed a constant concentration of ≈1.5 ppm in solution throughout the experiment (12 months, Figure 6.1SI). The pH of the exprimental solutions were taken at the start of the experiment and at the end and showed a slight increased over the complete experiment from a pH of 8.61 to 8.95. Figure 6.1. A comparison of chromium sorption onto a montmorillonite (STx-1b, low Fe) and nontronite (NAu-1, higher Fe) with and without prior γ-irradiation (G, 5 MGy) over 12 months in AGW (error 1 σ). For both experiments with clay, sorption increased over time with a steady state, reached after ≈500 hours. The higher Fe-bearing nontronite (NAu-1) showed between 10-30% sorption and the Fe-poor montmorillonite (STx-1b) showed ≈10% uptake onto the samples at 500 hours. The γ-irradiated samples (5 MGy), STx-1b G and NAu-1 G, showed a higher uptake of Cr(VI). Fe-bearing NAu-1 G showing sorption of ≈80-95% Cr(VI) by 500 hours. Fe-poor STx-1b showed between ≈50-75% sorption after 500 hours. The γ-irradiation appears to increase the reactivity of the clays towards the Cr(VI). This could be via γ-irradiation causing partial reduction of the Fe(III) in the clays to Fe(II). In turn this could cause the slow reduction of Cr(VI) to insoluble Cr(III). Large variations apparent in the results may be due to the small sampling concentration permitted by ICP-MS (as Cr is a toxic chemical), but may be associated with small changes in pH effecting the environment of the clay samples. 277 6.4.3. Uranium Sorption Study 6.4.3.1. Solution Phase chemistry: The U(VI) only groundwater control showed ≈30 ppm U in the solution throughout the experimental period (12 months, Figure 6.2SI). The results from the U(VI) experiment are shown enlarged in Figure 6.2 below. The pH of the exprimental solutions were taken at the start of the experiment and at the end and showed a slight increased over the complete experiment from a pH of 8.16 to 8.88. 278 Figure 6.2. Showing the uptake of uranium (%) onto a montmorillonite (STx-1b, lower Fe) and nontronite (NAu-1, higher Fe) with and without prior γ-irradiation (G, 5 MGy) and in low and normal level carbonate solutions of AGW (L-low carbonate) over 12 months. 279 Figure 6.2 shows the main influlence on U behaviour was carbonate concentration with γ-irradiation having little discernable effect of U sorption behaviour. The high carbonate experiments (5 mM) showed far lower sorption than the experiments at lower carbonate concentrations (≈20-30% and ≈800 ppm on the solid). The lower carbonate solutions (0.5 mM), showed showed far greater sorption across all samples (≈70-95% and ≈2400 ppm on the solid). The lower sorption of uranyl was seen with carbonate presence due to the presumed ligation to carbonyl groups present; the formation of a tris-carbonyl complex has been shown previously and enhances the stability of U(VI) in solution under these environmental conditions (Brookshaw et al., 2015; Greathouse and Cygan, 2005). A steady state for U sorption appears was reached raplidly with the slowest uptake seen in the NAu-1 lower carbonate system with steady state reached at 48 hours. In the higher carbonate experiment the main factor influenceing the sorption was the prescence of the carbonate in the sytem with minimal differecnes shown after γ-irradiation. The lower carbonate systems also showed very little difference after γ-irradiation displaying a slight increase in the sorption although quantifing this would be hard due to the varition in sorption seen. This variation in the results shown was postulated to possibily be from small changes in pH of the solutions, a highly influencing factor in soprtion onto mineral, and conceivably evidient throughout the experiment due to the natural samples used, array of sampling points taken, and measurement of the pH only at starting and commencment, which showed a fair increase over the duration (pH 8.16-8.88). If increased sorption within the system was evident it may have been due to the increase in charge on the γ-irradiated clay surfaces or from γ-irradiation related reduction of Fe(III) in the clay struture (partial), previously shown to activate the clay towards redox reactions (Gorski et al., 2012; Neumann et al., 2013; Gournis et al., 2001) and allowing reductive precipitation to poolry soluble U(IV) phases. 6.4.3.2. Solid phase analysis Selected samples from the U(VI) sorption experiment were extracted and run on I20 at the Diamond light source. STx-1b before and after γ-irradiation were looked at in normal AGW (≈800 ppm on the solid: 20-30% sorption; STx-1b 5Wk, STx-1bG 5Wk, and STx-1bG 12M) and low carbonate AGW (≈2400 ppm on the solid: >80% sorption; STx-1b 9M and STx-1b G 9M). A NAu-1 sample before and after γ-irradiation at 9 280 months in the low carbonate AGW (NAu-1 9M and NAu-1G 9M) were also studied to examine the effect of a higher Fe content on the sorption environment. Samples were collected as slurries in XAS holders, flash frozen and run at liquid nitrogen temperature (77 K), under vacuum. 6.4.3.2.1. XANES analysis Initially uranium L3-edge XANES analysis of the samples was undertaken to measure the oxidation state changes between the samples. The shape and position of the U L3- edges were compared with two known reference samples of schoepite (U(VI)) and uraninite (U(IV). Uranium sorption in the samples has been shown to be mainly influenced by the axially co-ordinated oxygens; a white line peak centred at ≈17175-6 eV was seen in spectrum with a variable shoulder centred at ≈17190 eV due to multiple scattering of the linear uranyl ion structure. Figure 6.3 shows an overlaid plot of the spectra, these link well with previously results on U(VI) sorbed silicates, like montmorillonite with an initial peak centred at 17175 eV and a shoulder at 17190 eV (Allen et al., 1996; Farges et al., 1992; Hudson et al., 1999). Interestingly, the high carbonate samples showed evidence for U(IV) reduction, but this was most likely enhanced through beam damage and low concentration on the solid (STx-1b 5Wk, STx-1bG 5Wk, and STx-1b 12M). The γ-irradiation of samples appeared to have little effect on the XANES (G, γ-irradiated) in compassion to the unirradiated samples. Additional examination of the oxidation states of the samples were examined through linear combination fitting, carried out in Athena (Ravel and Newville, 2005), to give an approximation of U(VI) and U(IV) ratios in each of the measured systems shown below in Table 6.1. 281 Figure 6.3. Normalised uranium L3-edge XANES analysis of STx-1b at 5 weeks (STx-1b5Wk, higher carbonate), NAu-1 and STx-1b at 9 months (NAu-19M and STx-1b9M, lower carbonate) and STx-1b at 12 months (STx-1bG12M, higher carbonate) for both natural and γ-irradiated samples (G in these samples names). A standard U(VI) schoepite and U(IV) uraninite are shown for clarity (Black and yellow). Table 6.1. Showing the linear combination fitting from Athena, comparing the U(VI)/U(IV) ratios in each of the measured systems, with and without γ-irradiation (5 MGy). Sample Time Carbonate Gamma Schoepite Uraninite (Months) Concentration Dose U(VI) % U(IV) % (mM) (MGy) STx-1b (5Wks) 1.5 5 0 49.8 50.2 STx-1b γ (5Wks) 1.5 5 5 48.2 51.8 STx-1b γ (12M) 12 5 5 53.7 46.3 STx-1b 9 0.5 0 80.2 19.8 STx-1b γ 9 0.5 5 80.7 19.3 NAu-1 9 0.5 0 78.7 21.3 NAu-1 γ 9 0.5 5 77.7 22.3 It should be noted that there was a small amount of beam reduction (≈10%) during each measurement taken (5-6 scans), due to the high flux on the beamline, with the same effect realized in each subsequent sample and the pH from which the samples were taken was not noted, but assumed to be within the range of the experimental values of pH 8.16-8.88. The scans were averaged to help mitigate the effects of this 282 reduction across measurements to ≈5% in the results overall (Figure 6.8SI). The higher carbonate systems have shown enhanced effects of reduction due to the lower overall sorption of U(VI) (≈20%) on the samples (lower concentration overall), although partial reduction appears to be evident in both high and low carbonate systems. After taking account of beam reduction and error indicative of linear combination fitting, only a small amount of reduction can be postulated (10-20%) with no significant change with γ-irradiation or over time. 6.4.3.2.2. EXAFS analysis Analysis of Uranium L3-edge EXAFS on each of the selected samples allowed for the co-ordination environment of uranium sorbed to the clays to be examined, looking at nearest neighbours and giving an idea of phase and possible form of the uranium species present. Sample data was initially computed in Athena and then fit, in Artemus, using known crystal structures of schoepite, uraninite, weeksite, and goethite (Ravel and Newville, 2005; Finch et al., 1996; Stohl and Smith, 1981; Zemann, 1965; Hazemann et al., 1991). A similar binding state was seen in all samples examined with the axial and equatorial 2+ oxygen shells of UO2 dominating the initial part of the spectrum and splitting slightly differently between the samples with some U(IV) interactions also present. The mixed oxidation seen in the XANES above was also shown in the EXAFS as the best fit U-O axial co-ordination number was lower than 2, which would be expected in a purely U(VI) system. The U-O axial distance was measured at ≈1.79 Å in the Fourier transform (Figure 6.4 and 6.5). The equatorial oxygens also needed multiple shells to be fit effectively at ≈2.28 and ≈2.43 Å; all samples were fit using this process above and with paths from a U(VI) and U(VI) species initially. The interaction of the central uranium atom with other surrounding species was detected in the Fourier transforms shown in Figures 6.4 and 6.5. Peaks from U-Si (≈3.08 Å) and U-Fe (≈3.48 Å) interactions suggest that inner-sphere bonding between the uranium species and surface siloxane and FeOH groups was occurring (Greathouse and Cygan, 2005; Denecke et al., 2003); if paths past the equatorial oxygen interaction were not seen then the uranium species present would have been solely in the interlayer space as a cation exchanged species 2+ much like that of the aqueous uranyl cation (UO2 ). There was also a number U-O-U multiple scattering interactions, included from the U(VI) schoepite structure, at 283 ≈3.60 Å. A U-U interaction at ≈3.93 Å was assigned to a complexation of a carbonate bi-dentate species on the surface (Chakraborty et al., 2010; Greathouse and Cygan, 2005) or to the formation of a U(VI) or U(IV) precipitate on the surface of the clay (Sylwester et al., 2000; Greathouse and Cygan, 2005). All the fitting data was included in Table 6.2 and 6.1SI and it should be noted that a U-C interaction at ≈2.9 Å and either a U(VI) and U(IV) species at ≈3.6 Å could have also been included, but due to a limitation on the parameters that could be used these were left out, as the fit was only marginally improved with inclusion of each. As an example of the spectra produced, the 9 month low carbonate STx-1b systems are shown below in Figure 6.4. Characteristics were resembled in the higher carbonate systems indicating similar bonding within each STx-1b system. Figure 6.4. U L3-edge EXAFS data and fits from Artemis in both K-space and R-space (Fourier transform) for STx-1b in contact with U(VI) solution (low carbonate) for 9 months; samples without prior γ-irradiation (black) with fit (red), and after γ-irradiation (blue) with fit (green). Individual data and fits found in supplementary information. There was very little differnce between the STx-1b samples before and after γ-irradiation in both low and high carbonate conditions; both showed U-O axial and equatorial interactions mentioned above as well as U-Si, U-Fe, U-O-U and U-U interactions. The U-U interaction fit was from the path in uraninite (≈3.93 Å); the peak at 3.6 Å was left unfit but could be assigned to a U(VI) schoepite U-U interaction or a multiple scattering species from U-Si-O interactions in a uranium silicate. The NAu-1 low carbonate 9 month samples are shown in Figure 6.5 and did show a change in bonding interaction after γ-irradiation. 284 Figure 6.5. U L3-edge EXAFS data and fits from Artemis in both K-space and R-space (Fourier transform) for NAu-1 in contact with U(VI) solution (low carbonate) for 9 months; samples without prior γ-irradiation (black) with fit (red), and after γ-irradiation (blue) with fit (green). Individual data and fits found in supplementary information. The same paths were fit at similar distances for the NAu-1 samples but an increased U(VI) interaction was prominent after γ-irradiation (Table 6.2) and from the spectrum the loss of a U-U interaction was observed with a reduction in the U-Si peak intensity and increase in the U-Fe peak interaction. A shorter U-U interaction at 3.77±0.5 Å was assigned and may have been from a precipitate formed on the surface of the mineral due to enhanced reduction of Fe(III) previously observed with γ-irradiation (Gournis et al., 2001). The EXAFS fits for the all the STx-1b samples resembled one another showing similar co-ordination numbers and bond lengths for all the paths included (Table 6.2 and Table 6.1SI); showing both U(VI) and U(IV) paths (U-Oax, U-Oeq, U-Si, U-Fe, U-U). Un- irradiated NAu-1 showed similar paths but distances were slightly different, most likely due to differences in the composition of the clays; after γ-irradiation NAu-1 showed a change in the bonding fit shown in Table 6.2. Table 6.2: U L3-edge EXAFS fit results for STx-1b with and without γ-irradiation (9 months-low carbonate) and NAu-1 with and without γ-irradiation (9 months-low carbonate) (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r STx-1b 9M U-Oax 1.5* 1.79± 0.004± 1.00* 0.012 (0.5 mM – CO3) 0.01 0.001 U-Oeq1 3* 2.43± 0.013± 0.04 0.004 U-Oeq2 3* 2.24± 0.005± 285 0.02 0.001 U-Si 1* 3.09± 0.006± 0.03 0.003 U-Fe 1* 3.43± 0.010± 0.04 0.005 U-O-U 1* 3.56± 0.017± rattle 0.02 0.004 U-O-U 2* 3.58± 0.009± non- 0.02 0.002 forward linear U-O-U 2* 3.58± 0.009± forward 0.02 0.002 through absorber U-U 1* 3.93± 0.006± 0.04 0.004 STx-1b 9M G U-Oax 1.5* 1.80± 0.003± 1.00* 0.009 (0.5 mM – CO3) 0.01 0.001 U-Oeq1 2.5* 2.45± 0.008± 0.03 0.002 U-Oeq2 3* 2.25± 0.004± 0.02 0.001 U-Si 1* 3.08± 0.005± 0.02 0.002 U-Fe 1* 3.39± 0.009± 0.03 0.004 U-O-U 1* 3.58± 0.013± rattle 0.02 0.004 U-O-U 2* 3.60± 0.007± non- 0.02 0.002 forward linear U-O-U 2* 3.60± 0.007± forward 0.02 0.002 through absorber U-U 1* 3.94± 0.005± 0.03 0.003 NAu-1 9M U-Oax 1.3* 1.81± 0.002± 1.00* 0.008 (0.5 mM – CO3) 0.01 0.001 U-Oeq1 3* 2.50± 0.014± 0.03 0.003 U-Oeq2 3* 2.28± 0.007± 0.02 0.001 U-Si 1* 3.12± 0.003± 0.02 0.002 U-Fe 0.5* 3.52± 0.007± 286 0.04 0.005 U-O-U 1* 3.60± 0.006± rattle 0.02 0.004 U-O-U 2* 3.62± 0.003± non- 0.02 0.002 forward linear U-O-U 2* 3.62± 0.003± forward 0.02 0.002 through absorber U-U 1* 3.99± 0.004± 0.02 0.002 NAu-1 9M G U-Oax 1.5* 1.79± 0.004± 1.00* 0.005 (0.5 mM – CO3) 0.01 0.001 U-Oeq1 3* 2.46± 0.007± 0.02 0.002 U-Oeq2 3* 2.27± 0.007± 0.01 0.001 U-Si 1* 3.10± 0.010± 0.04 0.007 U-Fe 0.5* 3.46± 0.005± 0.02 0.003 U-O-U 1* 3.55± 0.015± rattle 0.02 0.004 U-O-U 2* 3.57± 0.008± non- 0.02 0.002 forward linear U-O-U 2* 3.59± 0.008± forward 0.02 0.002 through absorber U-U 1* 3.77± 0.013± 0.05 0.006 Notes: Path represents the scattering path; CN denotes occupancy; R gives the interatomic distance; σ2 is the Debye-Waller factor (a measure of disorder in the structure); S02 the amplitude (from 1); and r the fit factor, a measure of fit quality (0 being perfect). * denotes a fixed parameter. 6.5. Discussion The γ-irradiation (5 MGy) of both the low Fe montmorillonite (STx-1b) and the higher 2- Fe nontronite (NAu-1) was shown to enhance the sorption of chromate (CrO4 ) from solution. This was postulated to be from the partial reduction of Fe(III) in the structure 287 of the clays to a redox active Fe(II) species; this has been shown previously through an increase in extractable Fe(II) after γ-irradiation (Sims et al., in review; Allard et al., 2013; Gournis et al., 2001) and would have allowed for a sorption-reduction reaction forming immobile Cr(III), most likely as a precipitate at the clay surface or edge sites (Parthasarathy et al., 2007; Taylor et al., 2000). This fits well with results reported through higher sorption after γ-irradiation onto both clays. Between the clays, higher sorption was seen in the nontronite only after γ-irradiation as the Fe speciation in NAu-1 was Fe(III) at octahedral sites (Gorski et al., 2012; Neumann et al., 2013). This was further implied from the unirradiated samples, that showed minimal sorption (≈10-20%) of Cr(VI) from solution which was also shown previously by Arnfalk et al. (1994). Consequently, the Fe content of the clay appeared to play a key role in the sequestration of Cr(VI) anionic aqueous species, most feasibly as redox active Fe(II) after γ-irradiation. Therefore, the possible formation of corrosion alteration products and reduction of Fe(III) with γ-irradiation may be may be beneficial to retarding migration of redox active species, especially in the case of less favoured anionic contaminants that may be present such as mobile Tc(VII) in the form of pertechnetate. All the U(VI) sorption experiments, regardless of total sorption, showed a steady state to be reached within the first 48 hrs comparing well with previous studies (Zeng et al., 2009). The position of charge had a minimal effect with similar behaviour between STx-1b (octahedral) and NAu-1 (tetrahedral) observed across all the unirradiated samples and similar to past observations Greathouse and Cygan (2006). The effect of γ-irradiation (5 MGy) on the sorption of U(VI) appeared to be over- shadowed in the higher carbonate solutions, both with STx-1b and NAu-1, through the complexation of carbonate with uranyl ions in solution forming stabile uranyl carbonate complexes and limiting the sorption seen (Barnett et al., 2002; Greathouse and Cygan, 2005; Waite et al., 1994). This factor was removed in the lower carbonate systems which allowed the exploration into the effect of γ-irradiation on each of the clays. Both STx-1b and NAu-1 showed no significant increase in the sorption of the U(VI) from solution after γ-irradiation. This was due to the high sorption capacity of the clays for cationic ions prior to γ-irradiation (Savage and Authur, 2012), present from the net negative charge on the layers and higher relative charge on uranium of +3.3 (Choppin, 2007). There were negligible differences between the montmorillonite 288 (STx-1b) and nontronite (NAu-1) before and after γ-irradiation, with high sorption still observed (75-90%) throughout under conditions used. Previous studies have also showed the maintenance of sorption properties (Zuo et al., 2017), and some higher Fe minerals showed increased sorption of U(VI) after γ-irradiation (Bank et al., 2007; Holmboe et al., 2011). It was observed across all of the sorption experiments (U and Cr) that a variation in the total sorption was observed even when a steady state of uptake was apparent (leaving off of uptake). This was hypothesized to be from small changes in pHs of the measured systems causing large effects on the sorption properties of the clay minerals; presence of carbonate was also shown to alter the favourable pH for sorption of specific cations onto minerals (Barnett et al., 2000; Brookshaw et al., 2015). A slow increase of the pH was observed overall in the experiment, from initial and final measurements taken from the samples (~pH 8 to 9), but experiments were not constantly measured and the effect of small variations in pH should be undertaken further, with future studies examining the effect of small elevations and decreases in pH. In the solid phase studies, the XANES spectrum from the U(VI) sorption on both the STx-1b and NAu-1 systems resembled a mix of U(VI) and U(IV) adsorption spectra in comparison to a U(VI) schoepite standard and U(IV) uraninite standard. The predominant features of U(VI) XANES were present with a major peak at 17190 eV from the linear uranyl ion structure and white line peak centred at 17175 eV (Allen et al., 1996; Farges et al., 1992). A small amount of beam reduction was observed, possibly due to the high flux of the beam (I20), but the changes were minimised using an average of the measurements taken (Figure 6.8SI) and were the concordant in all the samples measured. A small amount of reduction was observed within the samples that cannot be accounted for through beam reduction effects, this was most likely due to the interaction structural Fe(II) present in the clays with uranyl species present (Duff et al., 2001). A number of adsorption environments have been shown for U(VI) after uptake onto 2+ minerals; pentaaquouranyl ions (UO2 ) complexing through outer-sphere mechanisms, uranyl carbonate complexes forming through inner-sphere mechanisms, sorption at edge sites through siloxane or Fe-hydroxide groups, and the formation of 289 hydrolysis species (Greathouse and Cygan, 2006; Michard et al., 1996; Barnett et al., 2002; Bargar et al., 1999). U(VI) inner-sphere interactions in clay minerals occurred through surface oxygens acting as equatorial ligands (Dent et al., 1992); confirmed through EXAFS peaks from interactions of U with Si, Fe and C at greater distances and the formation of surface precipitates (U-U interactions) through adsorption or redox reactions at the clay surface (Greathouse and Cygan, 2005; Hudson et al., 1999; Sylwester et al., 2000; Stetten et al., 2018). The EXAFS measured on the all the STx-1b samples and the unirradiated NAu-1 samples all showed similar spectra, with no change in bonding over time with γ-irradiation. The NAu-1 sample after γ-irradiation showed differences with an increased U-Fe interaction and only one peak at ≈3.8 suggesting only one precipitate to be formed; reduced Fe(II) after γ-irradiation in the structure may have acted as a redox agent, forming a U(IV) phase through reductive precipitation at edge sites or on the surface of the NAu-1. Axial and equatorial oxygens dominated the spectra and further interactions were associated with inner-sphere complexation or the formation of precipitates (Sylwester et al., 2000). The main peak was from the axial oxygen interaction (Dent et al., 1992; Wasserman et al., 1997) at, ≈1.80 Å, the same value as 2+ UO2 sorbed on haematite, biotite, chlorite and montmorillonite, with a lower co- ordination number (<2) suggesting some reduction in the system (Bargar et al., 2000; Brookshaw et al., 2015). Equatorial associated oxygens were observed in the all STx-1b samples from combination of two split peaks (2.28 and 2.46 Å) close to those previously reported (Sylwester et al., 2000) and the NAu-1 samples showed values closer to those previously reported for higher charged biotite (2.32 Å and 2.51 Å), most likely from increased Fe(III) content. Equatorial peaks were not well resolved suggesting that reduction had occurred in the samples to some extent. Further peaks were attributed to nearest neighbours (Si, Fe, C) across all the samples and large interactions at distances of ≈3.6-3.9 Å were attributed to U-U interactions, mostly likely from a U(VI) or U(IV) surface precipitate(s). A number of studies have assigned this later interaction to bi-dentate coordinated carbonato groups but this could only be postulated here at best due to the simulatires between high and low carbonate system bonding (Brookshaw et al., 2015; Gückel et al., 2012; Chakraborty et al., 2010; Rossberg et al., 2009; Amayri et al., 2005). 290 6.6. Conclusions The overall effect of γ-irradiation (5 MGy) on the sorption of the cationic contaminant, 2+ UO2 , within both clays used was shown to be insignificant. Sorption was already very high within the low carbonate clay systems and was maintained after γ-irradiation; the higher carbonate clay systems showed an inhibited sorption presumably from the formation of uranyl-carbonato species. A negligible effect on the bonding environment for the low Fe clay was seen but changes in bonding were seen in the higher Fe, nontronite, postulated to be from the reduction of structural Fe(III) to Fe(II) after γ-irradiation and subsequent reduction of U(VI) to U(IV) or precipitation of aqueous U(VI) species. The difference between a high (NAu-1) and low (STx-1b) Fe clay systems bonding environment after γ-irradiation was mainly due to the content of Fe present, with the partial reduction of Fe(III) in the structure to active Fe(II) increasing the reductive potential of the clay (NAu-1), this was concordant with previous work by Sims et al. (In Review) showing the extractable Fe2+ to increase in clays after γ-irradiation. This interaction of U(VI) with Fe(II) would beneficial to the sorption of U(VI) in the GDF environment if canister corrosion were to occur, forming alteration products higher in Fe around the canister. The effect of γ-irradiation on the sorption of 2- anionic contaminants, CrO4 , was to increase the sorption capacity of both of the clays used, giving an improved material for the sequestration of redox active anionic contaminants such as Tc(VII) or Se(VI). The increase in sorption was linked to the amount of Fe(II) in the structure of the clay, with NAu-1 showing enhanced sorption over STx-1b after γ-irradiation; both showed very low sorption without any prior treatment due to Fe(III) predominating in the NAu-1 and STx-1b, inhibiting redox activity of the clays. The stability of the sorption in both sample sets after the initial uptake, fast in the case of U(VI) and a slower uptake of Cr(VI), which was still fast on a geological timescale. This ‘fast’ uptake was promising as sorption remained at a stable level after a steady state was reached, key to the retardation of contaminants within a GDF environment. Extended length studies on sorption of both cationic and anionic species, over longer timescales, under conditions relevant to disposal (pressure and temperature fluctuations) could be run on natural mineral deposits to allow the stability of the complexes formed over geological time periods to be observed and better influence the planning of GDFs. The effect of pH on the systems was suggested 291 to have a large impact on the stability of the sorption within clay mineral systems and as such this would also need to be considered in the future planning of long term studies taking a number of regular pH readings within the systems of study to examine this effect. In conclusion, this experiment allowed for the realistic estimation of behaviour of the buffer material after a large dose of γ-irradiation (5 MGy). These were expected to cause small but measurable changes over the initial stage of disposal (resilient over 2+ this time); the interactions of both a positively charged species (UO2 ) and a 2- negatively charge species (CrO4 ), after canister failure, in a GDF environment were examine in relation to early surface release (AGW). The γ-irradiation (5 MGy) appeared to have an advantageous effect on the main constituent of the buffer material, montmorillonite, as well as the possible pseudo alteration product, nontronite, which may be present around the canister as an alteration product. Enhanced sorption was observed for Cr(VI) species after γ-irradiation, assumed from the interaction of an active redox portion of the clay activated through γ-irradiation (Fe(II)). Negligible increases were seen in the uptake of U(VI), already high in clay minerals under low carbonate conditions, but the presence of carbonate was problematic and should be minimised in a GDF system. These effects suggest a positive effect in the buffer material from one of the key challenges faced (γ-irradiation) after the initial disposal of radioactive waste in a GDF. An increased understanding of the possible sorption mechanisms would allow for a clearer and more complete picture of the GDF environment after canister breach and the more efficient attenuation of chemical and radioactive contaminants at waste storage facilities worldwide. Further research into the coupled effect of heating and γ-irradiation, variations in pH, and effect of groundwater solution composition (brine) on the sorption of cationic and anionic species in relation to geological disposal would give further insight into the safety case required for the storage of radioactive waste as well as the remediation of contaminated land. 6.7. Acknowledgements We acknowledge the Dalton Nuclear Institute (University of Manchester) for funding this project. We would like to thank the Dalton Cumbria Facility (University of 292 Manchester) for the use of the 60Co γ-irradiator and Ruth Edge for assistance with γ-irradiation. Thank you to the Williamson Research Centre for Molecular Environmental Science and the School of Earth and Environmental Sciences (University of Manchester) for lab use and Paul Lythgoe for ICP-MS assistance. We thank Diamond Light Source for access to beamline I20 (SP17376) that contributed to the results presented here; Luke Townsend, Alana McNulty, and Prof. Samuel Shaw are thanked for their assistance in the collection of the XAS data and Kurt Smith for guidance on XAS processing. 6.8. References Allard, T. Ildefonse, P. Beaucaire, C. and Calas, G. (1999) Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine. Chemical Geology 158, 81-103. Allard, T. and Calas, G. (2009) Radiation effects on clay mineral properties. Applied Clay Science 43, 143-149. Allard, T. Balan, E. Calas, G. 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Colloids and Surfaces A Physicochemical and Engineering Aspects 242, 39–45. 304 The effect of γ-irradiation on the sorption capacity of montmorillonite and nontronite for U(VI) and Cr(VI) A. P. Simsa,b, K. Morrisb, J. F. W. Mosselmansc, S. Hayamac, F. R. Livensa, C. I. Pearce*a,d a School of Chemistry, University of Manchester, M13 9PL UK ; b Research Centre for Radwaste Disposal, School of Earth and Environmental Sciences, University of Manchester, M13 9PL UK; c Diamond Light Source, Harwell, Oxford, UK; e Pacific Northwest National Laboratory, Richland, WA, USA. 6.9. Supplementary Information Pre-experimental studies Initial tests at Pacific Northwest National Lab on bulk and colloidal STx-1b (montmorillonite) were run in deionised water under anaerobic conditions over 72 hours. Stock solutions The Uranyl stock solution was made up from a U(VI) ICP-MS standard solution (0.22 mL, 1000 ppm) with addition to slightly acidified deionised water (0.001 M, 14.78 mL) to afford a U(VI) stock solution concentration of 15 ppm (4.2x10-3 M). The Chromate(VI) solution was prepared in lab from the addition of sodium dichromate (0.0195 g, 1.3 x 10-4 M) to deionised water (15 mL) to give a chromate stock solution concentration (5.23x10-3 M). Two colloidal stock solutions of STx-1b (0.5 g) and γ-irradiated STx-1b (0.5 g) were made up with deionised water (1:100) and stirred for 7 days with a stirrer bar using a method adapted from Bouby et al. (2011). The solutions were centrifuged, decanted and replaced with fresh deionised water, re-suspending the solution. The process was repeated 4 times to remove particles larger than 500 nm and the 4th decanted samples were keep and used a colloidal stock solutions in each case (≈300 ppm). 305 Pre-study experimental Uranium: STx-1b and STx-1b γ-irradiated (0.1 g) were added to deionised water (9 mL) and colloid solutions were prepared as above to the same volumes (9 mL). The stock solution of U(VI) was subsequently added (1 mL, 4.2x10-3 M) before starting the experiment. Samples (0.5 mL) were taken at 1 hr, 5 hrs, 24 hrs and 72 hrs after addition of the the U(VI) stock solution. Samples were filtered and acidified and U(VI) concentration measure through ICP-MS measurements. Chromate: STx-1b and STx-1b γ-irradiated (0.1 g) were added to deionised water (9.95 mL) and colloid solutions were prepared as above to the same volumes (9.95 mL). The stock solution of Cr(VI) was subsequently added (0.05 mL, 2.6x10-3 M) before starting the experiment. Samples were taken at 1 hr, 5 hrs, 24 hrs and 72 hrs after addition of the Cr(VI) stock solution. Samples were filtered and acidified and Cr concentration measured through ICP-MS measurements. Pre-experimental findings Findings showed that a very low uptake of chromate (≈2%) from solution was seen over 72 hours; with a higher uptake of uranium observed (80% on average). A negligible change was seen between natural and γ-irradiated samples in the solution chemistry (ICP-MS). A small increase in the pH of the solution in both experiments was seen, with the chromate systems rising from pH ≈8.65 to pH ≈8.95, and the uranium system rising from pH ≈5.27 to pH ≈5.5. A study undertaken, under the same conditions, on montmorillonite colloid suspensions (STx-1b) found uptake of contaminants but to a lesser extent due to the lower concentration of particles in these systems (306 ppm). Further study at lower concentrations of contaminants would be required here to quantify changes between samples to a higher degree of accuracy. From the initial tests it was shown that uptake could be observed in the U(VI) systems at the solid to solution ratio used in the bulk samples (1:100), but this may need to be increased in the chromate experiments to obtain measurable results (1:20) over a longer time scale (12 months). 306 ICP-MS analysis Experimental acidification A 8 M nitric acid solution was made by decanting HNO3 (51 mL, 70%) into deionised water and making it up to 100 mL. This nitric acid solution (8 M) was used to acidify extracted samples (20 μL) after solution had been separated from the solid (≈0.2 mL). ICP-MS dilutions A 2% nitric acid for ICP-MS analysis was made by decanting HNO3 (28.5 mL, 70%) into deionised water and making it up 1 L. Supporting Figures and tables 0.30 0.25 0.20 Cr ppm 0.15 Control 0.10 0.05 0.00 0 10 20 30 40 50 60 Weeks Figure 6.1SI. Control results for Cr(VI) sorption experiment in AGW over 12 months. 307 40.00 35.00 30.00 25.00 20.00 U ppm Control 15.00 Control L 10.00 5.00 0.00 0.0 10.0 20.0 30.0 40.0 50.0 60.0 Weeks Figure 6.2SI. Control results for U(VI) sorption experiment, low ( ) and high ( ) carbonate systems in AGW over 12 months. Figure 6.3SI. U L3-edge EXAFS data (black) and fit (red) from Artemis in both K-space and R-space (Fourier transform) for STx-1b in contact with U(VI) solution for 9 months without prior γ-irradiation (5 MGy, Lower carbonate system). 308 Figure 6.4SI. U L3-edge EXAFS data (black) and fit (red) from Artemis in both K-space and R-space (Fourier transform) for STx-1b in contact with U(VI) solution for 9 months with prior γ-irradiation (5 MGy, Lower carbonate system). Figure 6.5SI. U L3-edge EXAFS data (black) and fit (red) from Artemis in both K-space and R-space (Fourier transform) for STx-1b in contact with U(VI) solution for 12 months with prior γ-irradiation (5 MGy, higher carbonate system). Figure 6.6SI. U L3-edge EXAFS data (black) and fit (red) from Artemis in both K-space and R-space (Fourier transform) for NAu-1 in contact with U(VI) solution for 9 months without prior γ-irradiation (5 MGy, Lower carbonate system). 309 Figure 6.7SI. U L3-edge EXAFS data (black) and fit (red) from Artemis in both K-space and R-space (Fourier transform) for NAu-1 in contact with U(VI) solution for 9 months with prior γ-irradiation (5 MGy, Lower carbonate system). Table 6.1SI: U L3-edge EXAFS fit results for STx-1b with and without γ-irradiation (5 weeks and 12 months - higher carbonate) (fixed CN and amplitude). Sample Path CN R (Å) σ2 (Å2) SO2 r STx-1b 5 Wk U-Oax 1.5* 1.79± 0.005± 1.00* 0.016 (5 mM – CO3) 0.01 0.001 U-Oeq1 3* 2.45± 0.002± 0.01 0.001 U-Oeq2 3* 2.28± 0.007± 0.02 0.002 U-Si 1* 3.10± 0.005± 0.03 0.003 U-Fe 0.8* 3.46± 0.009± 0.04 0.005 U-O-U 1* 3.57± 0.020± rattle 0.02 0.004 U-O-U 2* 3.59± 0.010± non- 0.02 0.002 forward linear U-O-U 2* 3.59± 0.010± forward 0.02 0.002 through absorber U-U 1* 3.94± 0.005± 0.03 0.002 STx-1b 5 Wk G U-Oax 1.5* 1.79± 0.004± 1.00* 0.016 (5 mM – CO3) 0.01 0.001 310 U-Oeq1 3* 2.46± 0.003± 0.01 0.001 U-Oeq2 3* 2.27± 0.007± 0.02 0.002 U-Si 1* 3.10± 0.006± 0.03 0.003 U-Fe 1* 3.46± 0.009± 0.03 0.004 U-O-U 1* 3.56± 0.017± rattle 0.02 0.004 U-O-U 2* 3.58± 0.009± non- 0.02 0.002 forward linear U-O-U 2* 3.58± 0.009± forward 0.02 0.002 through absorber U-U 1* 3.94± 0.005± 0.03 0.003 STx-1b 12M G U-Oax 1.5* 1.79± 0.005± 1.00* 0.016 (5 mM – CO3) 0.01 0.001 U-Oeq1 3* 2.45± 0.004± 0.02 0.001 U-Oeq2 3* 2.26± 0.007± 0.02 0.001 U-Si 1* 3.09± 0.006± 0.03 0.003 U-Fe 1* 3.46± 0.011± 0.03 0.006 U-O-U 1* 3.57± 0.020± rattle 0.02 0.004 U-O-U 2* 3.59± 0.010± non- 0.02 0.002 forward linear U-O-U 2* 3.59± 0.010± forward 0.02 0.002 through absorber U-U 1* 3.94± 0.005± 0.03 0.002 Notes: Path represents the scattering path; CN denotes occupancy; R gives the interatomic distance; σ2 is the Debye-Waller factor; S02 the amplitude; and r the fit factor, a measure of fit quality. * denotes a fixed parameter. 311 Beam damage observation: An example of the beam damage observed was shown in Figure 6.8SI of the U L3-edge XANES for the STx-1b 9M low carbonate system. The average, 1st scan and last scan are measured. Beam reduction was observed (≈10%). Although, XANES and Linear combination fitting both rely heavily on background subtraction of data, which should be taken into consideration. st Figure 6.8SI. U L3-edge XANES for the STx-1b 9M low carbonate system; average (blue), 1 (red) and last (green) scan. Table 6.2SI. Showing the linear combination fitting of the STx-1b 9M low carbonate system and evidencing beam reduction. Sample Time Carbonate Point Schoepite Uraninite (Months) Concentration U(VI) % U(IV) % (mM) STx-1b 9 0.5 Av. 80.2 19.8 STx-1b 9 0.5 1st 83.7 16.3 STx-1b 9 0.5 6th 72.2 27.8 312 7. Summary, conclusions and future work 7.1. Summary A number of clay samples based around the primary component of the engineered clay buffer, montmorillonite (>75% Posiva, 2009), were analysed to find a ‘designer clay’ that sustained physical and chemical properties under GDF conditions. These conditions were examined using octahedrally charged montmorillonites with varying compositions (STx-1b, SWy-1, SCa-3, SAz-2), a tetrahedrally charged a beidellite as an end member to the series (SBld-1), and two tetrahedrally charged nontronites (NAu-1 and NAu-2) used as pseudo-alteration products containing higher Fe contents. The number of clay samples used in experiments was reduced throughout the project to allow for specialised studies into the structure and reactivity using a low impurity model montmorillonite, STx-1b, and a higher Fe alteration type product, NAu-1. This investigation followed the initial challenges faced by the barrier directly after radioactive waste disposal; the coupled effects of heat and γ-irradiation (Chapter 4) on clay structure were examined. The response of homo-ionized clays (Na+, Mg2+, K+, Ca2+) to temperatures of 25, 90, 160, 500 and 1000 °C was studied, with subsequent γ-irradiation (2.7 MGy) studies on the clays proceeding this heat treatment. Homo- ionised samples of STx-1b and NAu-1 (Na+, Mg2+, K+, Ca2+) were exposed to temperatures of 25, 90, 160, 500 °C and doses of 0.2—5 MGy to investigate the effect of interlayer cation and dose on clay response. The key hypothesis tested was that the clay would withstand the challenges of heating (160 °C) and γ-irradiation (5 MGy) under conditions relevant to initial disposal of wastes and continue to perform its safety function. The nature of the interlayer cation played a significant role in the long-term performance of the clay barrier; divalent interlayer cations were preferable due to greater resistance to heating and γ-irradiation, as shown by larger d001 spacing (≈15-16 Å), higher CEC values, and reduced distortion of the structure as a function of γ-irradiation (EXAFS and EPR). Higher Fe clays showed a collapse of the d001 spacing with γ-irradiation if a monovalent cation was present (Na+) due to higher layer charge from the increased Fe substitution into the structure. Structural Fe could also act as an electron transfer pathway and sink for radiolysis products, explaining the partial 313 reduction of Fe3+ to Fe2+ seen within the clays. Ca EXAFS data revealed a very small increased disorder in interlayer species as a function of γ-irradiation, and negligible changes in the structural Fe, with fits in both sets showing very minimalistic alterations. However, a shift to a more crystalline Fe phase in EPR measurements was observed. Additionally, γ-irradiation results showed electrons to be trapped within the structure forming radiation-induced defects and increasing the charge on the clay surface (zeta potential and AFM measurements). Charge was induced in the clay structure through the knocking off of an electron from bonding oxygens within the structure (3 defect types EPR) via an interaction of a gamma ray (high energy electron) with a pi electron in the bonding system, forming defect sites and an oxygen radical. Zeta potential results showed that the γ-irradiated colloids had a slightly higher negative surface charge, resulting in greater charge repulsion and therefore increased colloid stability. These electrons are stored at the surface of the structure for colloids, manifesting itself as negative surface charge because the particles are so small that they are basically all surface. A similar interaction was proposed for the bulk which resulted in reactions with redox active species within the clay (i.e. Fe3+ to Fe2+ reduction). Relating back to the hypothesis above, the changes to the clay structure as a result of temperature (<160 °C) and γ-irradiation (5 MGy) were minimal on a macro- scale. However, measureable changes in surface properties after γ-irradiation could be beneficial in the sorption properties of the clays (Chapter 6), but may be problematic if colloidal suspensions are formed at buffer/host rock interfaces, due to the stabilisation of colloidal particles after γ-irradiation that have a high surface area and charge to mass ratio. In Chapter 5, the impact of heat and α-emitters (actinides and decay products) released from heat generating wastes after canister failure and breach on the ECB within the GDF environment, were investigated with regards to the safety case. This was achieved through studying the effect of α-irradiation (He2+, 5 MeV) at varying doses (0.15-1.5 dpa) on the chemical structure of a number of untreated clays (SWy-1, SCa-3, SBld-1, NAu-1, NAu-2). A case study on STx-1b investigating the effect of heat (120 °C) and α-irradiation (He2+, 5 MeV, 0.16 dpa), with respect to the interlayer cation (Ca2+, Na+, Mg2+), on crack formation and propagation (pore distribution), and on changes to the chemical structure preceding the treatments were undertaken. The 314 tested hypotheses were that α-irradiation would cause crack formation within the clays and alter the local chemical structure of the clays. The nature of the interlayer cation played a key role in the resistance of the clay barrier to heat and α-irradiation; divalent interlayer cations (Mg2+ and Ca2+) were desirable to maintaining advantageous properties over monovalent cations (Na+). This was confirmed through the comparison of homo-ionised STx-1b (0.16 dpa, Na+ vs. Ca2+/Mg2+) and SWy-1 vs NAu-1 (0.47 dpa, Na+ vs. Ca2+) samples and showed the preservation of the interlayer space (d001 spacing) with the presence of a divalent cation in the interlayer at comparable doses. Long range order of the tetrahedral- octahedral-tetrahedral sheets was maintained in samples after α-irradiation, as demonstrated by the lack of change in the XRD pattern beyond the interlayer spacing and in longer range EXAFS interactions. The critical α-dose was variable across samples where a collapse or partial collapse of the interlayer was linked to the composition of the interlayer cation (divalent vs. monovalent), and structural Fe content of the clay. Clays with higher Fe content showed greater resilience, potentially due to electron transfer pathways mitigating radiolysis effects. The chemical structure of the clays (EXAFS) showed an increase in disorder in both the interlayer (Ca) and structural (Fe) cations of the clays at higher doses, due to local disruption of clay structure caused by α-irradiation. This was not significant at lower doses, and reduced with distance from the main area of damage. Heating showed negligible effects in comparison to the unheated samples before and after α-irradiation, this was a key benefit in the planning of a GDF allowing for the minimisation of storage time above ground and allowing closer packing of wastes, reducing the overall footprint of the planned GDF. The proposed cause of the interlayer collapse in smectites seen in the XRD results, to that of a dehydrated illite- or biotite-like species, at higher α-irradiation doses was due to a number of factors; the local loss of structure through the displacement of atoms, radiolysis reactions, and associated ionisations. Some re-hydration would be expected but further studies into the extent of this should be undertaken with possible parallels to the heat treatment of expandable clays to be drawn. Tomography measurements on STx-1b showed a small increase in average porosity, but more pertinently a visible increase in the cracks present at the surface of the clay (open to α-damage), which if coupled with a self-propagating sorption-desorption cycle of α-emitters could lead to 315 the creation of a possible migration path for radionuclides away from disposal sites over geological timescales. Thus, the hypothesis was proven that α-irradiation caused crack formation and local changes to the clay structure. SRIM modelling showed the penetration of α-particles to be localised at the surface of the clay too (≈26 μm), but the propagation suggested may allow for the transport of contaminants away from the GDF environment. Foundations have been laid for further studies into dose rate effects on the clays and investigation into sorption-desorption processes with radionuclides and contaminants of relevance to radioactive waste disposal. 2+ In the final research paper (Chapter 6), the sorption of cationic (U(VI), UO2 ) and 2- anionic (Cr(VI), CrO4 ) contaminants on a set of γ-irradiated (5 MGy) clays (STx-1b and NAu-1), under conditions relevant to geological disposal at the point of canister failure (γ-irradiated, anaerobic, high and low carbonate systems), was investigated. The hypothesis was that sorption of contaminants (anionic and cationic) would increase after γ-irradiation, due to the previously observed increase in surface potential and partial Fe reduction within clay structures seen with γ-irradiation (Chapter 4). A steady state in sorption of U(VI) was reached across all experiments within 48 hrs, and after this time there were minimal changes in uranium uptake. Sorption was dependent on the concentration of carbonate in solution; higher carbonate concentrations (5 mM), well above that of the uranyl concentration (30.1 ppm), showed minimal sorption (≈20-30%) due to the formation of stable uranyl carbonate complexes in solution (Brookshaw et al., 2015; Barnett et al., 2000). The effects of γ-irradiation (5 MGy) on clay (STx-1b and NAu-1) behaviour under low carbonate conditions (0.5 mM) appeared to be minimal, with high sorption (>80%) across samples throughout due to the high sorption capacity of clays and high relative charge on uranium (+3.3). Variations in the U(VI) sorption after a clear steady state was observed was attributed to small changes in the pH impacting largely on sorption capacity of the clay. Solid phase analysis of the clays using EXAFS showed U(VI) sorbed as an inner-sphere complex, with peaks past those corresponding to the axial and equatorial oxygens relating to U-Fe/Si/C interactions. Formation of U(VI) or U(IV) surface precipitates was possible as peaks were present at longer distances in R-space (≈3.8-4.2 Å). A clear alteration in the bonding environment was shown in one sample, the higher Fe NAu-1, this was attributed to the increased Fe within the mineral and the 316 partial reduction of Fe3+ to Fe2+. LCA of the XANES spectra, carried out with U(VI) schoepite and U(IV) uraninite standards showed possible reduction within the samples, most likely from the interaction with Fe2+ produced as a product of γ-irradiation, but also possible from in-situ reduction from the high flux of the measurement beam used, as the effect was enhanced in samples of lower concentration (lower sorption). Only small changes were apparent overall in the measured clay systems after γ-irradiation for U(VI) and the effect of carbonate dominated, this was due to the stabilising effect it had on the U(VI) in solution. 2- The sorption of the anionic contaminant (CrO4 ) was slower in reaching a steady state (3 weeks), with an increased uptake shown on γ-irradiated clays (60-95%) in comparison to the un-irradiated clay systems (≈10-20%). Again variation in sorption was seen around this steady state was postulated to be associated with small changes in the pH affecting the mineral properties and these effects being enhanced due to lower contaminant concentration in the measured systems. It was apparent that the Fe content of the clay and associated Fe3+ to Fe2+ reduction was key to the effects shown; Fe2+ was involved in reducing mobile Cr(VI) to insoluble Cr(III), as observed by enhanced sorption in the higher Fe clay (NAu-1) over the lower Fe clay (STx-1b). The redox state of the Fe in the clay was shown to be paramount to the sequestration of contaminants with natural Fe(III) in the clays studied showing a low sorption across all samples. Therefore, the formation of Fe(II)-bearing corrosion products in close vicinity to the container may help the retardation of mobile radioactive species, especially - through the reduction of mobile anionic contaminants such as Tc(VII)O4 , which would otherwise be repelled by negatively changed mineral surfaces, to immobile Tc(IV)O2. Larger changes were seen for the interaction of anionic species with clay through Fe redox reactions, assisting in the formation of insoluble species or species that had a higher affinity for sorption to the clay minerals. Further research into the coupled effects of heating and γ-irradiation, as well as subsequent effects of α-irradiation on clay minerals, was required, including implications for sorption of cationic and anionic species in relation to these geological disposal challenges. These fundamental studies give a realistic estimation of ECB behaviour within a GDF environment, which were essential as part of a safety case for radioactive waste disposal. The effects caused by the initial challenges of heat and γ-irradiation within 317 the GDF, were an increase in the surface charge density on the clay. Subsequent studies were related to GDF conditions after canister failure. Sorption of contaminants onto γ-irradiated clay was explored and increased uptake was shown under these conditions for anionic contaminants. Heat and simulated α-irradiation expected from actinides and decay products (U, Pu, Np), showed a loss in the localised structure of the clay, along with a small increase in crack formation. The buffer material chosen needs to withstand the challenges tested (heat, γ-irradiation, α-irradiation) and continue to isolate the waste by minimising groundwater flow and abating corrosion of the canister. After canister breach, the released radionuclides would need to be effectively sequestered to allow the slow decay of radioactivity for 105—106 years. 7.2. Conclusions There are many challenges involved with the safe disposal of radioactive waste, varying in type and composition, at geological disposal facilities (GDF) in the UK and internationally (DECC, 2014). This work was based on part of the safety case required internationally for the nuclear disposal logistics of heat generating higher level waste (HLW) and spent nuclear fuel (SNF). Understanding of key processes at a mechanistic level allow for long-term prediction of GDF evolution, permitting a robust safety case to be built in the UK, focussing on the fundamental materials. This work would further add to the growing literature on disposal procedures internationally and help to build a standardised set of best practices under relevant conditions that can be used in a scientific context to build a stronger safety case and publically to strength stakeholder confidence in the design of concepts for HLW disposal. A number of challenges that were expected to be faced by the buffer material of a planned GDF were studied relating to the initial disposal environment of the wastes (heat and γ-irradiation) and the later disposal environment of wastes (heat and α-irradiation, and the sequestration of contaminants U(VI) and Cr(VI)). The first experimental set up showed that the clay would withstand the challenges of heating (160 °C) and γ-irradiation (5 MGy) under conditions relevant to initial disposal of wastes and continue to perform its safety function effectively. Negligible effects were seen on the macro-scale with minimal changes seen in the bulk XRD and EXAFS after γ-irradiation and with heating up to 160 °C, although the minimal changes that 318 were observed were reduced with a divalent cation present within the interlayer. However, small but measureable changes in surface properties after γ-irradiation were shown through and increase in the electronic defects within the structure, partial reduction of Fe3+ to Fe2+ and increase in colloid stability (increased surface charge potential). This increased charge could be beneficial for the sorption of cationic contaminants and the small reduction of Fe within the clays may support redox activity and therefore reductive potential of the clays. Conversely, these effects may be problematic if colloidal suspensions are present after γ-irradiation, with enhanced stability and a high affinity for radionuclides providing a possible path for radionuclide migration. Secondly, α-irradiation did cause crack formation within the clays and altered the chemical structure of the clays locally. This was not significant at lower doses (~0.15 dpa), and increased resistance was shown by clays containing a higher Fe content and in those with a divalent cation in the interlayer. At higher doses the effects shown were a loss of local ordering, shown in the EXAFS through an increase in the disorder of the systems measured and through a collapse of the interlayer space in XRD results to that of a illite- or biotite-like species. The collapse of the interlayer was postulated to be from local loss of structure from atomic displacements and from radiolysis reactions within the interlayer, with the rehydration of the layers assumed to be relatable to that of the heat treatment effects within the clays but with further research into this area required. These α-damage effects reduced with distance from the source and were confined locally with long range XRD and EXAFS results remaining largely unchanged after α-irradiation. Heating of the samples prior to α-irradiation showed negligible effects, showing a resistance to higher temperatures than are proposed for GDF wastes and allowing for the possible minimisation of storage time and a favourable reduction in the GDF footprint. The XCT results displayed changes to be limited to the surface of the clays after α-irradiation, showing only a small increase in the porosity of the clays, with visible cracks at the surface enhance but localised. This could be problematic if sorption-desorption cycles occurred with α-emitting wastes as it could allow for the migration of radionuclides away from the GDF environment. The final hypothesis was proven to some extent, with the maintenance of cationic sorption properties and an increase in the sorption of anionic contaminants after 319 γ-irradiation. These beneficial effects were thought to be related to the previously shown increase in surface potential of the clays after γ-irradiation and associated reduction of Fe3+ to Fe2+ within the clay structure. This was highlighted through the large increase in uptake of Cr(VI) following γ-irradiation which was enhanced in the higher Fe clay systems studied. Sorption of U(VI) was dominated by carbonate presence, regardless of γ-irradiation, with stable aqueous complexes formed a higher carbonate concentrations and high sorption seen onto the smectites at a lower carbonate concentration. This study showed that variations in sorption to the minerals were possibly relatable to small changes in the pH of the systems. The experiments were undertaken in an artificial groundwater relating to an early surface release, further work on in a brine system expected within a GDF environment should also be carried out. These initial studies give a positive outlook on the use of a clay buffer material and the key findings of the research for academia and industrially are concluded below with the impacts on relevant bodies highlighted. Divalent interlayer cations were shown to be preferable, and more resilient to damage from heating and both γ- and α- irradiation. This would infer that a natural bentonite with a divalent interlayer cation would be favoured over that of mono-valent cation such as SWy-1. The effect of heating on the clay throughout the experiments appeared to be minimal below temperatures of 160 °C, with heating to 500 °C showing the formation of new phases within the clays. This would permit the disposal of wastes safely at higher temperatures, reducing the time required for radioactive waste cooling at surface sites, allowing the implementation and closure of disposal sites earlier, as well as enabling the closer packing of canisters and therefore the giving a smaller overall footprint of the complete GDF. The reduction of surface storage times and earlier isolation of the wastes would be advantageous for the safety and security of nuclear sites within the UK. The effects of γ-irradiation on the clays were molecular-scale effects showing an increase in surface charge on the clays and partial reduction of Fe3+ in the clays with the effects not seen on a macro-scale structurally. These effects enhanced colloidal stability and this may permit the transport of radionuclides away from the disposal site if irreversibly bound. However the activation of redox active species in the clay (i.e. Fe) was shown to be beneficial to the sorption of anionic 320 contaminants (Cr(VI)), with minimal impact on the cationic sorption studied. This would be advantageous to the building of a safety case for a GDF as anionic contaminants ordinarily show poor affinity for natural clays and often require redox reactions to be removed to an immobile state (i.e. Tc(VIII) to Tc(IV)). An increase in the Fe content within the structure of the clay appeared to be valuable to sorption properties observed and with respect to resistance to irradiation damage. This could be advantageous as a cheaper Fe based canister could be used, over a more resistant expensive copper based canister, with alteration products formed through incorporation of Fe into the clay structure at the canister/clay interface shown to be favourable. This would largely reduce associated cost of canister manufacture as well as reducing the likelihood of a human intrusion event because the less valuable Fe would not be as attractive to trespassers as a high purity copper canister, proposed for use in concepts that plan for a very low canister decay rate such as the SKB KBS-3 concept. The effects of α-irradiation on the clay were shown to be localised and constrained to the surface of the clay (26 μm), showing a small increase in average porosity in these areas and a collapse of the interlayer completely or to that of a dehydrated smectite dependent on dose. The clays showed resilience to the irradiation at lower doses with only higher doses effecting change. This provided a clearly preferable outcome if α-emitters are permanently sorbed by clay minerals or exhibit slow desorption kinetics. Although if the progressive sorption-desorption cycles of α- emitters were to occur then this could open up advection flow pathways within the GDF allowing for the migration of radionuclides, so the kinetics of relevant radionuclide studies should be initiated. Research onto the stability of the α-irradiation damage in the interlayer of the clays should be examined further with behaviour exhibited expected to be similar to that of heat treated clays. These worse case α- emitter studies show that the safety case for the buffer material can be upheld if kinetics were favourable as the damage would be constrained locally within a small fraction of the buffer and radionuclide migration would be inhibited. Therefore the main component of the engineered clay barrier (ECB), montmorillonite, would be able to mitigate potential detrimental effects corresponding to heat and radiation (α, γ), whilst maintaining the bulk advantageous properties associated with clay buffer material such as high swelling capacity, low hydraulic conductivity (small 321 pore size) and high sorption capacity for radionuclides. Further research into a number of key areas such as impact of canister corrosion (phase formation), sorption of other relevant radionuclides, microbial effects and the production of hydrogen (radiolysis) within the buffer material are needed for a GDF safety case and are subsequently discussed below. 7.3. Future Directions Future work can be linked to other relevant processes expected in a GDF environment, summarised in Figure 7.1. Scientific Question: What are the effects of GDF conditions on the ECB? WP1.1 Heat WP1.2 Radiation WP1.3 Waste package WP1.4 Corrosion products Groundwater • Dehydration • Radiolytic H2 • Dehydroxylation • Fe reduction by • Reaction with Fe/Cu- • Ion exchange • Collapse radicals based corrosion • Swelling • Cracking • Radiolysis of products • Sorption structural water • Radionuclide properties • Amorphisation sorption and reactivity Samples carried forward to probe coupled effects Figure 7.1. Key research links and further research questions to be answered (Pearce, 2014). A large part of a safety case for a GDF is the possible formation of Fe corrosion products in the ECB around the planned secondary Fe containment (stainless steel/ steel). Fe corrosion products present at the canister/clay interface will affect clay mineralogy, and understanding the behaviour of this altered clay will be vital to predicting behaviour over geological timescales. A large amount of work has been undertaken into the associated corrosion expected within (some spent fuel concepts) and around canisters for planned use in the long-term disposal of heat-generating radioactive wastes. The formation iron corrosion products has been studied with regards to the canisters themselves (Agrenius and Spahiu, 2016) showing the formation of magnetite and hydrogen within canisters under anaerobic conditions. Further to this the British geological society and Serco (through SKB and the EU Framework 6 NF-PRO project) undertook a scheme of work to characterise mineralogical changes within bentonites (Milodowski et al., 2009; Milodowski et al., 2009). Bentonite samples with Fe wires and Fe coupons in situ were heated at 30 and 322 50 °C, over variable time scales (571-618 days and 100-200 days respectively); under anoxic conditions at pH 8.4 and 11 in both cases. Alteration was seen in both cases with visible colouration changes around the iron objects. XRD results confirmed some alteration within both sample sets; this was assumed to be a Fe-rich dioctahedral smectite (i.e. nontronite), with the maintenance of the d060 associated with smectites and a reduction of the d001 interlayer peak, for example from 14.9 to 12.9 Å in the 30 °C sample set. In the higher temperature sample set (50 °C) an number of processes were shown to occur: the formation of a mixed layer smectite with non-swelling layers (i.e. chlorite-smectite) was postulated, the uptake of Fe into the interlayer with movement of Ca out (exchange) was associated with the formation of aragonite (Ca- rich), and the formation of Fe oxides (magnetite) and Fe-hydroxides (åkaganeite) were suggested through XRD analysis. The lower temperature samples (30 °C) showed reactions to have occurred to a lesser extent but similar results with regard to the types of changes shown through chemical extractions and XRD. Ferrous hydroxides were found to form within bentonite studied at low temperatures (30 °C), with magnetite formation present at higher temperatures (50 and 80 °C). However, Carlson et al. (2007) showed, under similar conditions (30 and 50 °C), over a longer time scales (500-900 days), that the only change observed in MX-80 samples (Na-rich) was the reduction of Fe3+ to Fe2+ within the octahedral sheets of the structure, with only a thin contact zone of alteration around the iron (Fe coupons and wires) within the clay and no changes eluded to in the XRD (minerology). King and Watson (2010) showed the corrosion rate to decreased with time due to the formation of a protective film with a steady state of corrosion reached after 6 months (0.1-1 μm/y), very similar to the findings of Norris et al. (2013), and a negligible effect was observed with the presence of elevated chloride concentrations. Further Experiments should be undertaken on montmorillonite, the key component of the buffer material, to examine the effect of Fe corrosion under varying conditions relevant to a GDF (anaerobic, aerobic (oxic), and water saturated). This will allow for the identification and confirmation of any new mineral phases formed such as 2+ 3+ magnetite (Fe3O4) or the Fe-rich phyllosilicate berthierine, (Fe (Al, Fe ))3(Si, Al)2O5(OH)4. Further to this, the behavior of the corroded phases in response to γ-irradiation or α-irradiation should be examined, to give a more precise picture of all 323 the materials present and their behavior. This could be achieved using Fe foil sandwiched between clay pellets, forming a pseudo-canisters/clay interaction zone. Initially, a study on the formation of new phases under accelerated corrosion conditions (enhanced chloride concentration in samples) would give an idea of anticipated phases. A long-term study (>10 years) using these replica samples could be conducted, with extraction for measurement and the testing of samples periodically (each year). Samples could be analyzed using: (i) 56Fe Mössbauer to look at Fe redox and bonding; (ii) XCT to look at Fe migration in the clays; and (iii) μXRD and μXAS to investigate structural (bonding) and mineralogical changes. Radiolytic production of hydrogen gas (H2) from clays as a function of acquired dose and Fe content may have large implications for the safety case, due to pressure build- up within the containment area around the canisters. Again, a lot of previous work has been undertaken in the production, migration and effects of gas formation, such as hydrogen in repositories, largely carried out under the European FORGE project (Shaw, 2015). The formation of hydrogen under anoxic conditions either in the waste forms themselves or within the buffer material has been shown to be undesirable but inevitable in most European concepts for storage for radioactive wastes; generation of hydrogen within these disposal concepts (relating to a GDF) were revealed to be predominantly from canister corrosion and radiolysis of water (Sellin, 2014; Lemy et al., 2010). The formation of a discrete gas phase and the migration of this phase have been largely studied within the FORGE project. The extent of migration was shown to be linked to the density of the material and occurred through consolidation of bentonite and from the formation of dilatant pathways, only possible when the applied pressure from the gas was equal to or higher than the total stress on the bentonite (Sellin, 2014; Norris et al., 2013). Ionizing radiation was shown to increase corrosion processes under repository conditions (King and Watson, 2010) as well as increase water radiolysis (Norris et al., 2013). Eriksson and Jacobssson (1983) examined the effect of γ-irradiation on hydrogen production in bentonite from radiolytic reactions; the effective production was related to γ-irradiation dose and OH scavengers present (Fe2+ and HCO-) with the production decreasing sharply at lower doses. A comprehensive study into the effect of dose and composition on hydrogen 324 formation would greatly strengthen the knowledge underpinning the safety case for gas formation and migration within a planned GDF repository. A hypothesis to investigate would be, a higher Fe content within clays would reduce the evolution of hydrogen gas as a function of acquired γ-irradiation dose. The response of a range of clays (montmorillonite and nontronites) with variable Fe contents over a range of total γ-irradiation doses (up to 2 MGy), could be assessed by measuring the H2 production using gas chromatography. Additionally, the effect of structural Fe and Fe within interlayer could explored through substitution of Fe for interlayer cations in a low Fe clay sample (i.e. STx-1b); this would determine if Fe must be structurally incorporated to affect H2 production. After an initial probing study on γ-irradiation dose and compositional effects, a maximal amount of H2 that can be produced per gram of clay could be found, where additional dose no longer afford any further H2 production. The effect of α-irradiation on the production of H2 could also be investigated; although, an in-situ measurement method of H2 would be required due to the complexity of simulated α-irradiation damage. The ‘depressurising’ ability of the clay has been shown to allow H2 release through the micro-pore structure (Sellin and Leupin, 2013), but further investigation of the behaviour of the clay and the Fe canister in response to elevated H2 production should be carried out. The effect of α-irradiation on the clay caused local amorphisation and induced widening and formation of cracks at the irradiated surface of clays. This was localised to a small region of damage due to the limited penetration distance of α-particles. However, prorogation of crack formation and sorption–desorption cycles of α-emitting radionuclides (i.e. U, Th, Am, Pu) could generate an advective flow path. Additional studies could investigate the effect of α-irradiation on the sorption of relevant contaminants onto clay. The potential for migration of radionuclides away from the canister (after canister breach) after α-damage of the clays would be assessed. The high sorption capacity for radionuclides must be maintained or increased through these damage processes for an effective safety case to be built. This study would build on research presented in Chapter 6 on γ-irradiation, allowing a comparison between the two irradiation types on the clays, with the possibility for mixed irradiation studies using both γ- and α-irradiation on clays prior to sorption experiments. 325 Each of the sorption processes inspected could be expanded on using other radionuclides (Am, Tc, Np, Pu, Cs) under similar conditions for comparison. These studies could look at the effect of redox cycling on the mobility of multiple oxidation state radionuclides, as well as the ability of the clay buffer to retard each contaminant. Other mineral phases could also be examined in regards to sorption; the use of modified clays (insertion of an organic ligand) as enhanced ‘getters’ for radionuclides - such as anionic TcO4 could be studied. This work would give information on the stability and efficiency of these phases under challenges relevant to geological disposal. Research into the behaviour of clay colloidal suspensions is also needed for the safety case of a GDF. The enhanced colloidal stability after γ-irradiation, increased surface potential and high charge density giving a high affinity for radionuclides, may lead to the transport of radionuclides away from the containment areas if irreversibly bound (Möri et al., 2003). The sorption of radionuclides relevant to geological disposal, and the kinetics between bulk and colloidal clay processes, must be assessed to prove that natural clay colloids, and waste or repository-derived colloids, do not enhance radionuclide transport within a GDF environment. Finally the effect of microbial processes within the clays should be investigated; sulfur- and iron-reducing microbes may be beneficial or hinder the passage of radionuclides. Microbial-induced Fe reduction activates clays for sequestration of anionic and cationic contaminants through reductive precipitation (Bishop et al., 2014; Brookshaw et al., 2015). The effects of γ- and α-irradiation on microbial cultures within the clays over a range of dose rates should be studied. The proposed clay buffer is a common theme across many international nuclear programmes. The buffer material is a very important barrier within the planned GDF, linking the host geology and waste canister, and minimising the detrimental effects that ingress from each. The evolution of the nuclear industry in the UK and the need for ‘renewable’ fuels will result in the increased use of nuclear reactors (and fuel). The current radioactive waste accumulated and stored as well as future wastes will require a permanent solution for disposal (DECC, 2014). The expansion of research into the 326 clay barrier performance at a fundamental level is paramount for the safe disposal of wastes over geological timescales. 7.4. References Agrenius, L. and Spahiu, K., 2016. Criticality effects of long-term changes in material compositions and geometry in disposal canisters. SKB, TR16-06. Barnett, M.O., Jardine, P., Brooks, S. and Selim, H.M. (2000) Adsorption and Transport of Uranium(VI) in Subsurface Media. Soil Sci. Soc. Am. J. 64, 908–917. Bishop, M.E. Glasser, P. Dong, H. Arey, B. and Kovarik, L. (2014) Reduction and immobilization of hexavalent chromium by microbially reduced Fe-bearing clay minerals. Geochimica et Cosmochimica Acta 133, 186-203. Brookshaw, D.R. Pattrick, R.A.D. Bots, P. Law, G.T.W. Lloyd, J.R. Mosselmans, J.F.W. Vaughan, D.J. Dardenne, K. and Morris, K. (2015) Redox Interactions of Tc(VII), U(VI), and Np(V) with Microbially Reduced Biotite and Chlorite. Environmental Science & Technology 49, 13139-13148. Carlson, L. Karnland, O. Oversby, V.M. Rance, A.P. Smart, N.R. Snellman, M. Vähänen, M. and Werme, L.O. 2007. Experimental studies of the interactions between anaerobically corroding iron and bentonite. Physics and Chemistry of the Earth, Parts A/B/C, 32(1-7), 334-345. Department of Energy and Climate Change (DECC) (2014) Implementing Geological Disposal, A Framework for the long-term management of higher activity radioactive waste, H.M. Government, UK, 1-55. Eriksson, T. and Jacobsson, A., 1983. Radiation effects on the chemical environment in a radioactive waste repository (No. SKBF-KBS-TR--83-27). Swedish Nuclear Fuel Supply Co. King, F. and Watson, S., 2010. Review of the performance of selected metals as canister materials for UK spent fuel and/or HLW. In 4th international workshop on long-term prediction of corrosion damage in nuclear waste systems. 16. Lemy, F. Nys, V. Weetjens, E. Yu, L. Koskinen, K. Plas, F. Wendling, J. Caro, F. Laucoin, E. Dymitrowski, M. Pellegrini, D. Justinavicius, D. Poskas, P. Sellin, P. Altorfer, F. Johnson, L. and Norris, S. 2010. Summary of Gas Generation and Migration Current State-of- the-Art. FORGE Milestone M15, 31, 5. Milodowski, A.E. Cave, M.R. Kemp, S.J. Taylor, H. Vickers, B.P. Green, K. Williams, C.L. Shaw, R.A. 2009. Mineralogical investigations of the interaction between iron corrosion products and bentonite from the NF-PRO Experiments (Phase 1). SKB, TR09- 02. Milodowski, A.E. Cave, M.R. Kemp, S.J. Taylor, H. Green, K. Williams, C.L. Shaw, R.A. Gowing, C.J.B. and Eatherington, N.D. 2007. Mineralogical investigations of the interaction between iron corrosion products and bentonite from the NF-PRO Experiments (Phase 2). SKB, TR09-03. 327 Möri, A., Alexander, W., Geckeis, H., Hauser, W., Schäfer, T., Eikenberg, J., Fierz, T., Degueldre, C. and Missana, T. (2003) The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock. Colloids and Surfaces A: Physicochemical and Engineering Aspects 217, 33-47. Norris, S. Lemy, F. Del Honeux, C.A. Volckaert, G. Weetjens, E. Wouters, K. Wendling, J. Dymitrowski, M. Pellegrini, D. Sellin, P. and Johnson, L. 2013. Synthesis Report: Updated Treatment of Gas Generation and Migration in the Safety Case. FORGE Report D, 31, 5. Pearce, C.I. (2014) Engineered clay barrier performance in a geological disposal facility for radioactive waste, PhD studentship proposal. University of Manchester, Manchester, UK, 1-6. Posiva. Oy. (2009) Nuclear Waste Management at Olkiluoto and Loviisa Power Plants Review of Current Status and Future Plans. Posiva, TKS-2009. Sellin, P. and Leupin, O.X. (2013) The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review. Clay and Clay Minerals 61, 477-498. Sellin, P. 2014. Experiments and Modelling on the Behaviour of EBS. FORGE Report D, 3, 38. Shaw, R.P. 2015. The Fate of Repository Gases (FORGE) project. Geological Society, London, Special Publications, 415(1), 1-7. 328 8. Appendices 8.1 Unpublished work in progress The following extracts are from studies undertaken during this PHD project with a scope to further the research into canister decay products and H2 production within a GDF environment. 8.1.1. Notes on: Impact of canister corrosion products on the structure and chemical composition of the engineered clay barrier in a geological disposal facility Authors: A. P. Sims1, J. Entwistle2, R. Fletcher-Wood3, A. Davenport4, J. F. W. Mosselmans5, A. Newman2, K. Morris1, F. R. Livens1, C. I. Pearce6. 1 University of Manchester; 2 Newcastle University; 3 Science Oxford; 4 University of Birmingham; 5Diamond Light Source; 6 Pacific Northwest National Lab. 8.1.1.1. Abstract A crucial part of the safety case for a geological disposal facility (GDF) for radioactive waste is the need to understand how iron corrosion impacts upon clay mineral transformation. Iron would be present in a GDF environment as a primary constituent of the containment canisters, and clays act as the principal buffer material between the surface of a corroding waste canister and the host geology. Fe corrosion in anaerobic, water saturated, smectitic clay can yield new mineral phases including 2+ 3+ magnetite (Fe3O4) and the Fe-rich phyllosilicate berthierine, (Fe (Al, Fe ))3(Si, Al)2O5(OH)4. In order to simulate the effect of canister corrosion on clay minerals, a model montmorillonite (STx-1b), the predominant (>75%) and most advantageous constituent of the chosen buffer material, bentonite, was heat treated under expected GDF conditions in the presence of iron. Specifically, a series of clay pellets with thin iron sheets sandwiched within were heated at 90 °C for 21 months under 3 discrete - conditions (aerobic Cl enhanced, anaerobic MgCl2 controlled, anaerobic untreated). The conditions simulated an aerobic accelerated corrosion environment; an anaerobic, humidity controlled accelerated corrosion environment; and an anaerobic environment (for comparison) respectively. 329 The structure and properties of the natural dioctahedral smectite (STx-1b) were followed using 56Fe Mössbauer spectroscopy and X-ray tomography (μXCT), shown in Figure 1A for an initial sample. The use of micro X-ray diffraction (μXRD) and X-ray adsorption spectroscopy (XAS) was used to examine changes in mineralogy and bonding. Changes in the Mössbauer spectroscopy, μXCT, and μXRD and XAS after heating at 90 °C for 21 months would allow the possible formation of a new oxide layers within the clay to be observed and identified. Figure 8.1A. Set up for controlled atmosphere corrosion experiments (A); Close up of Fe foil pressed into a clay pellet (B); XCT image of intact Fe foil and pellet before corrosion (C); Mössbauer spectra of low Fe clay (D); and of Fe foil in clay (E). 8.1.1.2. Experimental Details Fe foil containing clay pellets were created through measuring half a pellet mass (≈0.15 g) and pouring into the pellet press, flattening as required; a thin sheet of Fe foil cut to 9 mm was laid on top of the sample in the pellet press, and the remaining half pellet mass (≈0.15 g) was added on top. Samples were compacted using a hydraulic press (Specac) with stainless steel die (Lightpath Optical (UK) Ltd) to produce 13 mm (diameter) pellets (9 tonnes, 300 mg, 30 secs) with a compacted density of 2 g cm-3. Following heat treatment (90 °C, 21 months), 56Fe Mössbauer analysis and XCT was conducted on whole pellets and then samples were sectioned in half and sealed in kapton for μXAS. Samples for μXRD were cut from the sectioned edge of the clay pellet 330 and sealed in resin, before samples were thin sectioned on a glass slide to give a sample of ≈100 μm thickness. 8.1.1.3. Preliminary Results After heating at 90 °C for 21 months complete samples were extracted from each environment and stored under an anaerobic atmosphere. Samples were initially measured as complete samples using 56Fe Mössbauer and XCT. X-ray computed tomography (XCT) Tomography was used to examine visual changes to the clay pellets and area around the foil inserts before sectioning. (a) (b) Figure 8.2A. Showing (a) Fe foil within the heat treated STx-1b pellet and (b) Fe foil damage. It could be seen, in the exemplified Cl treated environment in Figure 8.2A, that crack formation has occurred in the sample with the brighter regions around the edge of the foil suggesting Fe leaching. It was also apparent by looking at the Fe Foil without the clay that areas of higher iron are present in the clay and there was pitting visible on the Fe surface not present before the heating, again suggesting corrosion of the foil over time with heat. 56Fe Mössbauer Mössbauer was used to look at the bulk iron, within the low Fe clay, before and after the heating of the clay. It was shown prior that that clay had a the majority of Fe in an octahedral Fe(III) form (91%), giving a doublet in the centre field, with the rest made up from octahedral Fe(II) (or varied environment Fe(III) in octahedral and tetrahedral positions). The Fe foil gave a distinct Fe(0) sextet shown in Figure 8.1A. 331 Figure 8.3A. Showing 56Fe Mössbauer of Cl-treated STx-1b after heating at 90 °C for 21 months; the formation of a new Fe(III) sextet was present. It could be seen, from the Cl treated environment example, that there was the formation of a new Fe(III) sextet in the clay (Figure 8.3A). This implied the formation of a new phase in the clay from corrosion of the foil into the surrounding clay. XAS studies From these results the further probing of samples was completed through μXAS and μXRD, on I18 at Diamond light source, to determine the changes in Fe bonding environment and Fe oxidation state from bulk pellet into the alteration zone around the Fe foil in the center of the clay (μXAS) and changes in the mineralogy across the sectioned samples (μXRD). For ease of analysis the samples will be denoted as Cl1, - MgCl1, ANO1, Un1, referring to the aerobic Cl enhanced, anaerobic MgCl2 controlled, anaerobic untreated, and aerobic untreated (control) respectively (all with foil). A traverse of discrete Fe K-edge μXAS points using a focused (≈3 micron) beam spot across a sample from each environment was carried out. Initially, samples are overlaid to show the difference in the general Fe content in each sample, obviously each dependent on the parameters and attenuation used but comparable within samples (Figure 8.4A). It can be seen that the samples are very similar in the aerobic and Un1 samples, as expected. There seems to be a variation of adsorption shown in each of MgCl1, Cl1, and ANO1 samples. MgCl1 shows the highest adsorption in the middle point of the map suggesting the foil to be in the middle. The other plots show the highest adsorption at the edge of the sample (opposite in each case), possibly 332 suggesting the Fe was close to the edge of the sample, although attenuation and detector distance could have played a role in these results. Figure 8.4A. Showing the un-normalised XAS (XANES) plots of the Un1, MgCl1, Cl1, ANO1 STx-1b samples and an aerobic blank; with an image of the cross-section mapped. 333 The STx-1b samples were examined in Athena (Ravel and Newville, 2005) to make comparisons of the three environments studied in comparison to the starting materials. The aerobic blank (AER) gave a pre-edge region characteristic of a Fe3+ in a 6-co-odrinated system (Wilke et al., 2001) and the Fourier transform in R-space showed Fe-O, Fe-Al/ Fe-Fe, and Fe-Si bonds, similar for those previously shown for structural Fe in STx-1b; Fe-O at 2.00 ± 0.02 Å, Fe-Al at 2.99 ± 0.05 Å, and Fe-Si at 3.20 ± 0.03 Å, characteristic for octahedrally located Fe cations (Soltermann et al., 2014). These peaks are seen throughout the EXAFS of all the samples with the intensity of the Fe-Al/ Fe-Si peak varying in relation to the initial Fe-O peak dependent on additional Fe-Fe interactions within the clay. The clay foil presence could be seen from a shifted XANES Fe K-edge peak and a large Fe-Fe interaction in R-space (Figure 8.5A). Figure 8.5A. Showing the normalised XANES and EXAFS plots for the STx-1b aerobic sample run and Un1 STx-1b sample. XRD Micro-focus transmission X-ray diffraction measurements across a sample from each environment were taken in order to assess changes in mineralogy, basal spacing (d001), and higher energy clay peaks (lower d-spacing). Peaks seen at some scans points between 7—8.5 Å (6.7 KeV) could allude to a partially collapsed interlayer, similar to that of berthierine or biotite, with the normally visible clay interlayer peak was not visible due to the scan parameters and detector position. Most information was collected from the higher energy scans (12 KeV), giving a diffraction pattern similar to that expected for the a normal montmorillonite clay with 334 peaks for the d110/d020 clay peak (4.45 Å), d100 quartz peak (4.1 Å, seen in STx-1b), a d130/d004 clay peak (≈3.20 Å), a d005 clay peak (2.56 Å) and a d110 quartz peak (2.48 Å). There are peaks in the diffraction pattern for points near the clay foil interface of 2.82 Å and 3.50 Å, possibly from the Fe corrosion that could be associated with a new phase such as magnate, berthierine, or glauconite or possibly due to the Fe foil itself (Sorieul et al., 2008). There are also a few other diffraction peaks present the 3.5-2.8 Å zone that could be due to new phases. The diffraction pattern for the higher energy scan and key peaks are shown in Figure 8.6A. New phases Figure 8.6A. Showing diffraction pattern of STx-1b MgCl1 at 12 KeV; main peaks from clay indicated by the red arrows and the important area of new phase formation. 8.1.1.4. Initial Findings From the study undertaken it was clear that the simulated canister (foil) interacts with the clay barrier under accelerated corrosion conditions (high Cl content) with heating to 90 °C. The structure of montmorillonite and Fe bonding in the affected areas appeared to alter with the formation of new phases shown through XRD and 56Fe Mössbauer measurements, and increased Fe content and bonding interaction changes shown through XAS. Further processing and modelling of bonding interaction in EXAFS 335 can confirm the atomic distances and interactions shown. A full inspection of the XRD and complete assignment may allow the determination of the phases seen. The conclusion drawn was that the Fe corrosion appeared to form a Fe oxide phase, shown from the XRD, XAS (R-space) and Mössbauer data. These oxides appeared to be concentrated around the centre of corrosion (foil) with the effect reducing with distance. The effect appears to be on a small scale here but may not be minimal on a geological time scale. Although, previous work showed an increased Fe content and a divalent cation within the interlayer was advantageous, increasing affinity for negatively and positively charge radionuclides. 8.1.1.5. References Soltermann, D. Fernandes, M.M. Baeyens, B. Dähn, R. Joshi, P.A. Scheinost, A.C. and Gorski, C.A. (2014) Fe(II) Uptake on Natural Montmorillonites. I. Macroscopic and Spectroscopic Characterization. Environmental Science & Technology. 48, 8688-8697. Wilke, M. Farges, F. Petit, P. Brown, G.E. Martin, F. (2001). Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study. American Mineralogist, 86, 714-730. B. Ravel and M. Newville (2005) ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X- ray absorption spectroscopy using IFEFFIT. Journal of Synchrotron Radiation, 12, 537– 541. Sorieul, S. Allard, T. Wang, L.M. Grambin-Lapeyre, C. Lian, J. Calas, G. and Ewing R.C. (2008) Radiation-Stability of Smectite. Environmental Science & Technology. 42, 8407- 8411. 8.1.2. Notes on: The production of H2 gas as a factor of γ-radiation dose and Fe content on a series of montmorillonites and nontronites. The effect of hydrogen (H2) production from the radiolysis products of water and corrosion of metals (canisters) within the buffer material in a GDF environment from γ- and α-irradiation must be released efficiently to alleviate the build-up of pressure, preventing containment failure. Clays have been shown to release H2 through the clay pore structure (Norris et al., 2017; Sellin and Leupin, 2013). The generation of H2 as a factor of the γ-irradiation dose and Fe content (μM) may play a key role in the behaviour observed within the initial stages of disposal at a GDF site. An initial study into these factors was undertaken at the University of Notre Dame (USA). 336 8.1.2.1. Experimental This was carried out on a number of montmorillonites (STx-1b, SWy-1, SCa-3, SAz-2), a beidellite (SBld-1), and two higher Fe nontronites (NAu-1 and NAu-2). Each clay sample (0.3 g) was weighed out into a custom made glass tube and flame sealed under vacuum prior to irradiation (0—2.35 MGy) to allow the amount of H2 produced to be measured through gas chromatography (GC). The samples tubes were crushed within sealed tubing connected to the GC for measuring and results were obtained using a calibration from the direct measurement of H2 standards (μL) prior to measurement. The samples used are shown in Table 8.1A. Table 8.1A. Showing the each of the clays used and calculation of Fe concentration in each sample from previous XRF results (Chapter 4). Clay Fe % Fe mole/ 100 g Fe mole/ g Fe mM/ g STx-1b 0.674 0.012 0.00012 0.121 NAu-2 22.327 0.400 0.00400 3.998 NAu-1 20.871 0.374 0.00374 3.737 SCa-3 1.906 0.034 0.00034 0.341 SBld-1 0.976 0.017 0.00017 0.175 SAz-2 0.944 0.017 0.00017 0.169 SWy-2 2.615 0.047 0.00047 0.468 8.1.2.2. Results Initial results showed that over a range of doses (0, 100, 162, 500 and 1000 kGy), clays with a higher Fe content sequestered of the production of H2, with an increased production seen across all samples as a function of acquired dose (Figure 8.7A). The Si content of the clay also appeared to play a role in the amount of H2 produced showing an increased production in the clays as a function of the total Si content. The effect of the Fe content was shown to be a structural one; Fe substitution into the interlayer of both a low Fe clay (STx-1b) and a higher Fe clay (NAu-1, as a control) was shown to have little effect in comparison with the natural samples. 337 Figure 8.7A. The production of H2 in clays as a function of dose and as a function Fe content (μM). 338 8.1.2.3. Summary This study showed that H2 production within clays was variable and dependent on the dose and structural Fe content of the clay predominantly. A steady state of maximal H2 production could be researched further, although for doses up to half of the expected GDF lifetime does (2.35 MGy) show a linear trend to be maintained. This could impact the GDF environment favourably if corrosion product alteration occurs around the canister, forming regions of higher Fe phases, and minimising the H2 production near the wastes that may speed up corrosion processes or increase pressure within the GDF environment. The study could be furthered by looking into the generation of H2 in expected Fe phases and through probing the effect of dose rate on the generation of H2. 8.1.2.4 References Norris, S., Bruno, J., Cathelineau, M., Delage, P., Fairhurst, C., Gaucher, E., Höhn, E., Kalinichev, A., Lalieux, P. and Sellin, P. (2014) Clays in natural and engineered barriers for radioactive waste confinement. Geological Society of London. Sellin, P. and Leupin, O.X. (2013) The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review. Clays and Clay Minerals 61, 477-498. 8.2. Paper contributions Paper was submitted in The Clay Minerals Society Workshop Lectures Series, Vol. 21 (2016), Chapter 6, 65–77 and is presented directly from the journal. Contribution involved was collection and work up of data on montmorillonite (XRD). 339 340 341 342 343 344 345 346 347 348 349 350 351 8.3. Other relevant courses and volunteering 8.3.1. Roles of responsibility Graduate teaching assistant, University of Manchester (2014 – 2018) Part-time demonstrating; 3rd year MSc experiment (1 year), 1st year measurement labs (2 years), and marking 1st year measurements lab reports—adapting verbal and written feedback to an audience (present). MChem student supervision (2014 – 2015) Help co-supervise a MChem student in first year of PhD; involved scheduling work and equipment time, regular progress meetings, and helping with writing and analysis. 8.3.2. Additional technical training PHREEQC basic modelling course; November 2014, UoM (UK) BELBaR Training course: October 2015, KIT (Germany) Accelerator Theory Training, September 2015, DCF (UK) XAS Processing Workshop: November 2015, Diamond (UK) Avizo quantification basics: April 2015, Diamond (UK) Avizo Advanced Course: March 2016, MXIF (UK) 352