Alternative Processing Methods for the Thermal Treatment of Radioactive Wastes THESIS Submitted for the degree of Doctor of Philosophy Paul George Heath Immobilisation Science Laboratory Department of Materials Science and Engineering The University of Sheffield Sponsored by EPSRC as part of the Nuclear First DTC program August 2015 Abstract The UK has large volumes of radioactive materials which are classified as Intermediate Level Waste (ILW). The baseline treatment for these wastes is encapsulation via cementation, however, this method is not ideally suited for numerous wastes, both in the UK and globally. Alternative thermal processing methods for these materials may be capable of producing wasteforms with improved properties. This thesis presents a series of scoping studies on the thermal treatment of a diverse range of ILWs in order to identify the potential benefits and pits falls of such processes. The wastes selected were Tri-Structural Isotropic (TRISO) Fuel Particles, Prototype Fast Reactor (PFR) raffinate, SIXEP sand/clinoptilolite ion exchange materials and SrTreat® Ion exchange material. The scoping studies performed showed promise for the thermal treatment of all selected waste streams. A summary of the main results for each waste stream are provided below; TRISO Fuel Particles: Immobilisation focused on encapsulation of the particles in highly durable glass matrices. Alumino-borosilicates were determined to be the most effective glass composition for the production of composites, in terms of both their physical and chemical properties. The ability of Hot Isostatic Pressing (HIPing) to improve composites was investigated. Unfortunately, this was shown to result in severe fracturing within the composite. This was hypothesised to occur as a result of excessive pressurisation during the cooling cycle. The HIP process did show some benefits in terms of particle coating properties and with small alterations to the HIP cycle utilised it should also be possible to eliminate the detrimental fracturing features. PFR Raffinate: The vitrification of PFR raffinate was investigated using the G73 glass composition, a glass which has been previously proposed for the immobilisation of other ILWs. This glass was proven to be capable of accommodating a waste loading of up to 20 wt% PFR raffinate. The glass produced was homogeneous with good waste retention, had no noted crystal formation, an aqueous durability comparable to currently employed HLW glasses and the thermal characteristics necessary for industrial scale up. Further study should be performed on the ability of this waste to retain Cs during processing and in aqueous solution. SIXEP Sand/Clinoptilolite Waste: SIXEP sand/clinoptilolite was processed at 95 wt% with a 50 % volume reduction by HIPing. The waste produced was a phase separated glass-ceramic. The wasteform was deemed to be suitable for ILW immobilisation and had an exceptional Cs i retention in aqueous solutions. However, the presence of an alkali earth sulphate phase increased the Sr release to solution. Attempts to qualify the suitability of this wasteform for disposal, developed methodologies to investigate the properties of phase separated materials. A combination of vertical scanning interferometry (VSI), dissolution experiments and SEM imaging was shown to be capable of elucidating the dissolution behaviour based upon compositional variation. SrTreat®: SrTreat® was processed at 100% waste loading via HIPing. This aim was to investigate the potential for developing ion exchange columns which could subsequently be HIPed, as such, providing a complete waste treatment solution. The HIP process produced a monolithic, mixed phase sodium titanate ceramic. This ceramic was formed by the sintering of individual grain structures and retained the compositional variations seen in the granular waste stream. The wasteform was porous around the grain edges, determined to occur as a result of carbonate formation prior to HIPing. The carbonation of this material is likely to limit the potential to utilise HIPing as a disposal methodology for these wastes. However the aqueous dissolution behaviour of these wastes was still favourable and the process was shown to create a significant reduction in waste volume. The work performed in this thesis has shown that various methods for thermal treatment can be rapidly investigated to determine the potential benefits and pit falls. The application of thermal treatments was shown to be capable of producing significant improvements in wasteform quality by comparison with the cementitious alternatives. ii Acknowledgements To exaggerate how much the completion of this thesis is owed to those around me would be a difficult task. I have been extremely lucky to be surrounded by fantastic people during my PhD and I count myself blessed for this. To name everyone who has had a part in the competition would likely double my word count, therefore, if you feel you have been unjustly omitted, please come to see me and I’ll buy you a beer. I should start by thanking Professor Neil Hyatt and Professor Russell Hand for their outstanding academic supervision. Their help, support and encouragement; both academic and pastoral, has been extremely beneficial. I am grateful for all that they have brought to both this body of work and my development as a person, scientist and engineer. Neil, I look forward to hearing ‘the story’ soon. Along with my primary supervisors I have been lucky to receive substantial academic support from other sources. I would like to thank Dr. Claire Corkhill and Dr. Martin ‘Stendog’ Stennett who have faced a great deal of pestering. I appreciate how supportive you have both been and that you never throttled me during the course of this research. Dr. Ewan Maddrell from NNL has been equally helpful in providing advice, samples and materials, which have proven invaluable. I would also like to thank Dr. Felix ‘Papa’ Brandt at Jülich Forshungszentrum for helping to arrange my secondment, being so accommodating of a cheeky Englishman and for teaching me the joy of beschӓftigung. I would like to thank EPSRC for their financial support through the Nuclear First DTC programme. I would also like to thank the University of Sheffield for financial support in the form of a KTA grant, which helped advance the work on TRISO particle encapsulation and the Worshipful Society of Armourers and Brasiers for supporting the dissemination of my work on the immobilisation of clinoptilolite. Being part of the Nuclear First DTC course allowed me to be part of a talented group of scientists from whose association and friendship I have benefited. I would like to thank all those from both the University of Sheffield and the University of Manchester who took the time to provide lecture courses and administrative support during my studies. My fellow students from both the Nuclear First DTC and ISL groups have helped to make this a most enjoyable period and are all deserving of my thanks here. I won’t try to thank you all individually here (see paragraph 1) but I would like to single out Sam Walling for his helpful, yet worryingly complete, knowledge of both the cement and NDA literature. iii The skill and knowledge of the technical staff within the Department of Materials Science and Engineering at the University of Sheffield has been the bed rock upon which this research has been built. A special thank you should go to the foundry cabal; ‘Uncle’ Ian Watts, Kyle Armhold, Dr. Amit Khan, Dr. Lisa Holland and Dr. Fatos Derfugi. Not only have you helped me almost daily, but you also made that lab one of my favourite places to be. Another special thanks should go to Andrew ‘Fatman’ Mould for helping with the initial set up of the University’s HIP facility and generally helping to fix things….the viscometer will get there in the end. I can safely say there is not a single technician in this department who hasn’t been subjected to my fluttering eyelashes at some point. Therefore in no particular order I would like to thank Ben Palmer, Dean Haylock, Dr. Nik Reeves, Dr. Oday Hussein, Michael Bell, Richard Kangley, Paul Hawksworth and Beverly Lane. The amount of help you have provided couldn’t possibly be listed but is well remembered. On the same theme I would like to thank Paul Lythgoe and Alastair Bewsher at School of Earth, Atmospheric and Environmental Sciences, the University of Manchester for providing the majority of XRF and ICP-AES/MS data used in this study. I would like to thank John Charnock of the same department for his help in performing the EPMA measurements presented in Chapter 6 and Tristan Lowe at the Henry Mosley Facility for the XCT measurements presented in Chapter 4. One of the highlights of my PhD has been the opportunity afforded to me to travel, especially for the duration of my work in Germany. Thank you to everyone at Forschungszentrum Jülich for being so welcoming and giving me the best placement I could have hoped for. I would like to highlight the help of Martina Klinkenberg and her 200 Mega Pixel eyes on the SEM, Fabi Sadowski who ran almost 1000 ICP-MS samples without a single complaint, Dr Cornelius Fischer of Univeristӓt Bremen for his help in performing the VSI measurements and Andrey Bukaemskiy for accompanying me to Bremen for these experiments. I would like to thank the following people for all their help and for being so welcoming during this time; ‘Onkel’ Philip Kegler, Stefan Neumeirer, Julia Arinacheva, Sarah Finkeldei, Julianna Webber, Giuseppe Modolo, Jakob Dellen, Christian Fischer and Andreas Fichtner. The dubious joy of Mickey Krauser is still with me... Thanks to all those who accompanied me on my research and conference trips for such adventures as; ‘£40 pound cafeteria card’, ‘The Boston Taxi Party’, ‘drei Milano bitte’, ‘The Villigen Spesh - featuring Carly Rae Jepson’, ‘Yakult science day’ and the infamous ‘CIGGARETTU!!!’. iv Naturally an acknowledgements section would not be complete without a mention of family and friends.