IENCE C • •S T S E Investigation of ThO2 as a structural analogue for N C O H I N spent nuclear fuel dissolution under repository S O I V Dissertation L • conditions O S 156 G T Y H • R G I E L VTT SCIEN CE S H E G A Currently, the preferred option for the long-term disposal of spent I R H C nuclear fuel (SNF) in Finland and Sweden is disposal in a H geological repository. In deep bedrock, the release of Th, U and 1 5 6 other radionuclides through man-made barriers and the geo- and biosphere will be controlled by the dissolution of the fuel by groundwater. Thorium dioxide is isostructural to uranium dioxide, sharing the same fluorite structure (space group Fm3m) and making it a useful analogue material for nuclear fuel, which mainly Investigation of ThO consists of UO2 (>95%). This thesis aimed to investigate the dissolution of ThO2, which was synthesised to approximate as closely as possible the microstructure of UO2 in a nuclear fuel matrix. The investigation consists of dissolution studies conducted using pellets, fragments 232 2 (2 to 4 mm) and particles (80 to 160 µm) of ThO2. The evolution of dissolving surfaces and grain boundaries were examined by as a structural analogue for... combining different microscope imaging techniques (scanning electron microscopy (SEM), atomic force microscopy (AFM), SEM with electron backscattering diffraction detector (SEM-EBSD) and profilometer imaging). High resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) was used for analyses of low thorium concentrations in leaching solutions. Part of the dissolution experiments were conducted in the presence of a 229Th tracer to gain additional data on the dissolution and precipitation Investigation of ThO2 as a by following the change in isotopic ratio 229Th/232Th. Furthermore, the pellets from these experiments were measured with direct alpha structural analogue for spent spectrometry to estimate the contents and thickness of the 229Th- nuclear fuel dissolution rich layer formed on the pellet surface. under repository conditions ISBN 978-951-38-8547-2 (Soft back ed.) ISBN 978-951-38-8546-5 (URL: http://www.vttresearch.com/impact/publications) ISSN-L 2242-119X ISSN 2242-119X (Print) ISSN 2242-1203 (Online) http://urn.fi/URN:ISBN:978-951-38-8546-5 Emmi Myllykylä VTT SCIENCE 156 Investigation of ThO2 as a structural analogue for spent nuclear fuel dissolution under repository conditions Emmi Myllykylä ACADEMIC DISSERTATION To be presented, with permission of the Faculty of Science of the University of Helsinki, for public examination in lecture hall A110, Deparment of Chemistry, on 16th of June 2017, at noon. ISBN 978-951-38-8547-2 (Soft back ed.) ISBN 978-951-38-8546-5 (URL: http://www.vttresearch.com/impact/publications) VTT Science 156 ISSN-L 2242-119X ISSN 2242-119X (Print) ISSN 2242-1203 (Online) http://urn.fi/URN:ISBN:978-951-38-8546-5 Copyright © VTT 2017 JULKAISIJA – UTGIVARE – PUBLISHER Teknologian tutkimuskeskus VTT Oy PL 1000 (Tekniikantie 4 A, Espoo) 02044 VTT Puh. 020 722 111, faksi 020 722 7001 Teknologiska forskningscentralen VTT Ab PB 1000 (Teknikvägen 4 A, Esbo) FI-02044 VTT Tfn +358 20 722 111, telefax +358 20 722 7001 VTT Technical Research Centre of Finland Ltd P.O. Box 1000 (Tekniikantie 4 A, Espoo) FI-02044 VTT, Finland Tel. +358 20 722 111, fax +358 20 722 7001 Juvenes Print, Tampere 2017 CONTENTS 1. INTRODUCTION ........................................................................................... 12 2. BACKGROUND ............................................................................................ 14 2.1 History and utilization of thorium ............................................................. 14 2.2 Thorium in nature ................................................................................... 14 2.3 Chemistry of thorium .............................................................................. 16 2.4 Characteristics of thorium dioxide ........................................................... 17 2.5 The role of spent nuclear fuel in final disposal ......................................... 20 2.6 Thorium dioxide as analogue material for Uranium dioxide ...................... 21 2.7 Thorium dioxide in future fuel concepts ................................................... 22 3. EXPERIMENTAL........................................................................................... 24 I & II 3.1 Fabrication and characterisation of ThO2 pellets ................................ 24 I 3.2 Production of ThO2 fragments for the dissolution experiments ................ 24 3.3 Dissolution and solubility experiments I ................................................... 25 Pre-experiments........................................................................... 25 Dissolution and solubility experiments with fragments I .................. 25 Dissolution experiments for surface investigations I & II ................... 26 Dissolution experiments with a 229Th tracer III ................................ 27 4. Analysis techniques .................................................................................... 29 4.1 Sector field inductively coupled plasma mass spectrometryI, II & III ............. 29 4.2 Scanning electron microscopy (SEM) and electron backscattering diffraction detector (EBSD) I & II ............................................................................... 31 4.3 Atomic force microscopy (AFM) II ............................................................ 32 4.4 Alpha spectrometry combined with chemical separation procedureIII ........ 33 4.5 Direct alpha spectrometry and advanced alpha-spectrometric simulations (AASI)IV 35 5. RESULTS and DISCUSSION ........................................................................ 37 5.1 Characteristics of sample pellets and fragments ...................................... 37 ThO2 pellets ................................................................................. 37 ThO2 fragments............................................................................ 38 I 5.2 Results of the dissolution and solubility of ThO2 ...................................... 40 Pre-experiments under atmospheric conditions ............................. 40 ThO2 dissolution in anaerobic conditions ....................................... 41 The solubility of ThO2 ................................................................... 44 5.3 Results of the surface investigationsI & II .................................................. 45 I SEM imaging of leached ThO2 surfaces ....................................... 45 Integrated imaging with VSI, AFM and SEM-EBSDII ...................... 47 5.4 Results from dissolution experiments with a 229Th tracerIII ........................ 49 3 Solution results with SF-ICP-MS ................................................... 49 Dissolution results by ICP-MS compared with alpha spectrometry . 53 5.5 Direct alpha measurement of pellets and advanced simulationsVI ............. 54 6. CONCLUSIONS ............................................................................................ 58 4 Supervised by Associated Prof. Marja Siitari-Kauppi Department of Chemistry University of Helsinki Senior Scientist, Kaija Ollila VTT Technical Research Centre of Finland Ltd Reviewed by Prof. Nicolas Dacheux Université Montpellier Marcuole Institute for Separative Chemistry Dr. Ian Farnan Reader in Earth and Nuclear Materials Department of Earth Sciences University of Cambridge Dissertation Opponent Prof. Mats Jonsson Applied Physical Chemistry KTH Royal Institute of Technology 5 I would like to dedicate this thesis for the memory of my beloved grandfather. He was the one, who taught me noble skills of debate and argument at the age of two. 6 ACKNOWLEDGEMENTS “Sometimes you have to go up really high to understand how small you really are.” - Felix Baumgartner The Ph.D. journey has been like a long and challenging hiking path with slippery rocks, steep slopes, strong wind, refreshing rain and sometimes warm sunshine. From such an adventure, you cannot survive without excellent gear, skills and attitude. The bigger the challenge, the more crucial is the company and assistance of others. From this thesis project, I would have not survived without the help of others. First of all, I like to express my warmest appreciation and thanks to my supervisor Marja Siitari-Kauppi, who has encouraged me and guided me through this. I have really enjoyed working with her, and she believed in me even on those very moments I did not. Special thanks go to three unique ladies, Kaija Ollila, Riitta Zilliacus and Maija Lipponen, who have long experience in research and science. I have been privileged to learn from them about dissolution studies and ICP-MS analytics. Thanks also to Professor Jukka Lehto and team leader Kari Rasilainen. They have provided good environment to conduct the research and write the thesis at the University of Helsinki and at VTT Technical Research Centre of Finland. The research of this thesis has been mostly conducted in the frame of the EU project, REDUPP (2011-2014), funded from the European Atomic Energy Community's Seventh Framework Programme (FP7) under grant agreement No. 269903. Posiva, SKB, VTT, are acknowledged as partners of the REDUPP project. The research was continued in the PANCHO project in the research program SAFIR2018, and in the YTERA and the CHEMS doctoral schools. All are acknowledged for their financial support for this thesis. I am grateful to all collaborators. Collaboration is the special spice