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Incorporation Into Select Uranyl Phases and Thermal Np5+ INCORPORATION INTO SELECT URANYL PHASES AND THERMAL ANALYSIS OF SELECT URANYL PHASES A Dissertation Submitted to the Graduate School of the University of Notre Dame in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Amanda Leigh Klingensmith Peter C. Burns, Director Graduate Program in Civil Engineering and Geological Sciences Notre Dame, Indiana June 2008 © Copyright 2008 Amanda Leigh Klingensmith ii Np5+ INCORPORATION INTO SELECT URANYL PHASES AND THERMAL ANALYSIS OF SELECT URANYL PHASES Abstract by Amanda Leigh Klingensmith Alteration of spent nuclear fuel in a geological repository under oxidizing conditions is likely to result in abundant uranyl compounds. The proposed repository at Yucca Mountain, Nevada is intended to store about 70,000 metric tons of spent nuclear fuel in the unsaturated zone of a welded tuff sequence. Following failure of canisters that encapsulate the waste, contents may be exposed both to air and water and undergo repetitive wetting and drying events. Incorporation of radionuclides into the uranyl alteration phases may significantly reduce their mobility, thereby impacting repository performance. Of particular interest is 237Np owing to its long half-life (2.14 x 106 years) and potential mobility in groundwater. Powders of the synthetic uranyl phase soddyite, (UO2)2(SiO4)(H2O)2, a framework type structure, and uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5, kasolite, Pb[(UO2)(SiO4)]H2O, Na compreignacite, Na2[(UO2)3O2(OH)3]2(H2O)7, and becquerelite, Ca[(UO2)3O2(OH)3]2(H2O)8, all of which are sheet type structures, were synthesized in the presence of Np5+ under varying temperature and pH conditions. Uranophane, kasolite, boltwoodite K[(UO2)(SiO3OH)](H2O)1.5, and Na boltwoodite iii Amanda Leigh Klingensmith K,Na[(UO2)(SiO3OH)](H2O)1.5 were synthesized in the presence of Np as well as P, Ca and/or Mg. Single crystals of Na metaschoepite, Na[(UO2)4O2(OH)5]·5H2O were synthesized in the presence of Np5+ and laser ablation verified that Np can be incorporated within the structure of a uranyl phase. Incorporation of Np5+ into soddyite increased steadily with synthesis temperature. Np incorporation into uranophane, becquerelite, and kasolite was not dependent on synthesis temperature. Np uptake in uranophane and kasolite was found to be dependent on synthesis pH, with an increase in Np uptake with higher pH. Uranophane, boltwoodite and Na boltwoodite showed an increase in Np incorporation in the presence of P. Boltwoodite showed an even higher Np uptake when Mg and P were both present in the synthesis. Thermal analysis was completed for the uranyl phases soddyite, becquerelite, Na compreignacite, uranophane, and kasolite. TGA curves for becquerelite, Na compreignacite and uranophane showed loss of interlayer water groups by 100°C. Soddyite and kasolite showed more gradual TGA curves and retention of water groups up to 400°C for soddyite and 550°C for kasolite, with agreement shown by high temperature powder XRD data. iv CONTENTS FIGURES ........................................................................................................................... vi TABLES .......................................................................................................................... xvi ACKNOWLEDGMENTS .............................................................................................. xvii CHAPTER 1 INTRODUCTION ........................................................................................ 1 1.1 Uranium and the environment ....................................................................................1 1.2 Crystal chemistry of uranyl phases ............................................................................4 1.3 Natural analogues of nuclear waste ............................................................................7 1.4 Uranium mineralogy and nuclear waste .....................................................................9 1.5 Np5+ crystal chemistry ..............................................................................................12 1.6 Thermal stability of U6+ phases ................................................................................17 1.7 Hypotheses ......................................................................................................................... 21 CHAPTER 2 MATERIALS AND METHODS ............................................................... 22 2.1 X-ray powder diffraction ..........................................................................................22 2.2 Single crystal X-ray diffraction ................................................................................23 2.3 Inductively coupled plasma-mass spectrometry: ICP-MS .......................................23 2.4 Inductively coupled plasma atomic emission spectrometry: ICP-AES ...................25 2.5 Thermogravimetric analysis .....................................................................................26 2.6 High temperature stage with powder X-ray diffraction ...........................................27 2.7 pH measurements .....................................................................................................27 2.8 Laser-ablation inductively coupled plasma mass spectrometry ...............................27 2.9 Electron microscopy and microprobe.......................................................................28 2.10 Ultraviolet visible spectroscopy .............................................................................28 ii CHAPTER 3 CRYSTAL STRUCTURES ....................................................................... 30 3.1 Uranophane sheet topology ......................................................................................30 3.1.1 Uranophane ...................................................................................................31 3.1.2 Boltwoodite and Na-boltwoodite ..................................................................32 3.1.3 Kasolite .........................................................................................................33 3.2 Soddyite ....................................................................................................................33 3.3 Becquerelite ..............................................................................................................37 3.4 Na compreignacite ....................................................................................................38 3.5 Na-substituted meta-schoepite .................................................................................38 CHAPTER 4 SYNTHESIS OF URANYL PHASES ....................................................... 40 4.1 Synthesis of uranophane ...........................................................................................41 4.2 Synthesis of soddyite ................................................................................................41 4.3 Synthesis of becquerelite ..........................................................................................42 4.4 Synthesis of Na compreignacite ...............................................................................42 4.5 Synthesis of kasolite .................................................................................................42 4.6 Synthesis of boltwoodite ..........................................................................................43 4.7 Synthesis of Na-substituted boltwoodite ..................................................................43 4.8 Synthesis of Na-substituted meta-schoepite .............................................................43 CHAPTER 5 Np5+ INCORPORATION IN Na SUBSTITUTED METASCHOEPITE .. 45 5.1 Crystal synthesis .......................................................................................................45 5.1.1 Synthesis of Na-substituted metaschoepite ...................................................45 5.1.2 Natural specimen ..........................................................................................46 5.2 Chemical analysis .....................................................................................................47 5.3 Structure solution .....................................................................................................50 5.3.1 X-ray data collection .....................................................................................50 5.3.2 Structure refinement......................................................................................51 5.3.3 Structure description .....................................................................................59 5.4 Np5+ incorporation into Na-MS-CRY-Np ................................................................66 5.5 Discussion ................................................................................................................68 iii CHAPTER 6 NEPTUNIUM SUBSTITUTION IN SYNTHETIC URANYL PHASES AS A FUNCTION OF TEMPERATURE AND pH................................................... 70 6.1 Synthesis ...................................................................................................................71 6.2 Analysis of samples at ANL .....................................................................................71 6.3 Analysis of samples synthesized at the University of Notre Dame .........................74 6.4 Results ......................................................................................................................75
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