THE PREPARATION AND APPLICATION OF THORIUM- BASED NUCLEAR FUELS A thesis submitted to the Department of Materials Science and Engineering, The University of Sheffield by Ross Peel, MEng In support of the application for the degree of Doctor of Philosophy (PhD) Originally submitted December 2016 Final version submitted with minor amendments September 2017 SUMMARY Thorium is currently produced primarily as an impure by-product of the mining and processing of the rare earth phosphate mineral monazite. Thorium concentrates are currently purified industrially by solvent extraction with PC-88a, but this extractant cannot separate uranium and iron from thorium. In this work mixtures of PC-88a and HDEHP were investigated for the mutual separation of uranium, thorium and iron. The extracted complexes were identified. U and Fe were extracted by cation exchange, while Th was extracted by a mixed cation exchange/solvation mechanism. It was found that three contact stages could extract > 99% of the thorium. A flowsheet was proposed. The first modern use of thorium as a nuclear fuel is most likely to be as an oxide fuel within Generation III+ nuclear reactors. In this work a uranium- plutonium mixed oxide was investigated as a fissile driver for thorium in the Enhanced CANDU 6 reactor, as an alternative to the proposed UK CANMOX fuel for irradiation of the UK plutonium inventory. A large number of fuel concepts were considered, and several were analysed by Monte Carlo simulation. It was found that U-Pu-Th fuels could offer transmutation of the plutonium, irradiate UK reprocessed uranium and give improved coolant void reactivities, while irradiating thorium and converting it to fissile 233U. Thorium and uranium may be recovered from spent nuclear fuel by the Acid THOREX process, which uses TBP solvent extraction. However, TBP has a number of disadvantages. In this work several alternative solvent extraction systems were investigated for the separation of Th, U, Fe and Zr. PC-88a was mixed with ten other extractants as potential synergists, extracting from hydrochloric, nitric and sulfuric acids. Several promising systems were identified based on distribution ratios and separation factors. i ACKNOWLEDGEMENTS The work presented in this thesis would not have been possible without the contribution of a great number of people, who have helped to shape my ideas and provide me with those opportunities and experiences which have been vital to me in reaching the point of submitting a doctoral thesis and successfully completing a viva voce examination. At the University of Sheffield I must begin by acknowledging the contribution of my academic supervisory team, which has undergone some significant changes over the three years (and twenty seven months) of my Ph.D. programme. Professor Karl Whittle provided the impetus for this work and encouragement and support through the process, continuing to do so even since taking up a new post at the University of Liverpool. Professor John Provis has provided much needed continuity through the process. Dr. Mark Ogden has been instrumental in providing day-to-day advice and guidance in the technical aspects of the work, particularly regarding the thorium separations work presented in Chapters 3 and 6, in addition to the necessary counselling when the scope of the work seemed overwhelming. I would also like to thank Dr. Iain Hannah for his assistance in the role of a thesis writing mentor for providing the necessary kick up the bottom required for me to get into the process of writing. The elemental concentration analyses by ICP-MS and ICP-OES presented in this work were performed by external services. ICP-MS was carried about by Dr. Gabriella Kakonyi and Dr. Andrew Fairburn in the Groundwater Protection and Restoration Group of the Kroto Research Institute, University of Sheffield. ICP-OES was carried out by Mr. Martin Jennings of the University of Manchester and Dr. Sarah Pepper, with much advice on the technique to the author from both parties. Due to this work being completed through the Nuclear FiRST Doctoral Training Centre, many people at the University of Manchester have also been very helpful at various stages of the work. In particular, Dr. Clint ii Sharrad and Dr. Richard Foster provided technical insights and help in accessing analytical facilities at the institution. The mineral samples analysed in Appendix A, and a good number of other samples besides, were provided by Dr. David Gelsthorpe at the Manchester Museum on a highly extended loan. For assistance with the work presented on neutronic simulation in Chapter 5 I would like to thank Professor Tim Abram at the University of Manchester for providing access to the Redqueen and CFS2 high power computing infrastructures, and also Seddon Atkinson at the University of Sheffield for his help in working with Redhat Scientific Linux. Also related to the simulation work presented in Chapter 5, I would like to thank Dr. Jim Kuijper of NUCLIC for his expert input and feedback on the work done in this chapter, leading towards to publication of this work. The work described in Chapter 4 was originally carried out during a 12 week industrial placement with AREVA S.A. in Paris, France, under the direction of Dr. Luc Van Den Durpel. This contact was provided by Dr. Stephanie Cornet, now at the OECD Nuclear Energy Agency, for which the author is extremely grateful. All those in the AREVA Research, Development and Innovation management team are thanked for their patience and understanding as I rediscovered my French-speaking abilities after over four years of letting them go to rust. In particular I wish to thank Patrick Chaucheprat for his many inspirational tales of managing research projects on the Superphénix experimental fast reactor, and Isabelle Perraud for her assistance with matters of administration and organisation. There are also a great number of people who have supported me personally during the Ph.D. process. I must begin by thanking my parents, without whose unending love and support over the last 28 years I would never have been able to reach this point. In particular, their provision of free full-board accommodation during the final 12 months of the Ph.D. process are appreciated beyond measure. On that note I must also say thank you to Dr. iii Richard Walker, friend and landlord, for allowing me to continue living in his property even when paying the rent became problematic. I also thank my partner Laura Bird for keeping me sane and happy during the last 2.5 years, making sure I did not forget that there is a life beyond research, and in particular being willing to bear with me during the intensive work period of the last few months. She has provided me with a new outlook on life, and a vision of a happier future than I had heretofore imagined possible. Finally, the work reported in this thesis would also not have been possible without: ~£75,000 in stipends, consumables and travel/conference expenses, plus however much more in on-costs. Thank you EPSRC! Three laptops (two which gradually became too slow to tolerate) 21 GB of online cloud data storage ~250 hours underwater on SCUBA diving holidays ~3000 Tesco finest cookies (various flavours) ~250 litres of cappuccino ~3 litres of espresso at AREVA (because when in Rome….) A truly inestimable quantity of real ale, primarily supplied by the Red Deer (18 Pitt Street, Sheffield), the Cobden View (40 Cobden View Road, Sheffield), and the Hallamshire House (49-51 Commonside, Sheffield). “As for me, I had the time of my life.” – Stephan King, “On Being Nineteen (And a Few Other Things)” iv TABLE OF CONTENTS Summary ........................................................................................................................ i Acknowledgements ................................................................................................... ii Table of Contents ........................................................................................................ v List of Tables ............................................................................................................ xiii List of Figures .......................................................................................................... xvi List of Nomenclature ........................................................................................... xxvi 1 Introduction ................................................................................................. 1 1.1 Aims ................................................................................................................. 1 1.2 The Structure of This Thesis ................................................................... 2 1.3 Global Context – The Need for Nuclear Energy ................................. 3 1.4 The Modern Nuclear Power Industry .................................................. 4 1.4.1 Uranium-based Nuclear Fuel – Supply and Demand ...................... 4 1.4.2 Reducing Fuel Resource Utilisation ...................................................... 5 1.4.3 Options for Sustainable Nuclear Fuel Management ........................ 6 1.5 Thorium as an Alternative Fuel to Uranium ..................................... 6 1.5.1 The Limitations of Thorium ..................................................................... 7 1.5.2 The Use of Thorium as a Nuclear Fuel .................................................. 7 1.6 Nuclear Power in the United Kingdom ................................................ 9 1.6.1 Historical