Influence of Transition Metal Charge Compensation Species on Phase Assemblage in Zirconolite Ceramics for Pu Immobilisat
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.100
Influence of Transition Metal Charge Compensation Species on Phase Assemblage in Zirconolite Ceramics for Pu Immobilisation L.R. Blackburn1, S.K. Sun1, L.J. Gardner1, E.R. Maddrell2, M.C. Stennett1, N.C. Hyatt1 1 Immobilisation Science Laboratory, University of Sheffield, Department of Materials Science and Engineering, Sir Robert Hadfield Building, Mappin Street, S13JD, UK
2
National Nuclear Laboratory, Workington, Cumbria, CA14 3YQ, UK
ABSTRACT
Immobilisation of Pu in a zirconolite matrix (CaZrTi2O7) is a viable pathway to disposition. A-site substitution, in which Pu4+ is accommodated into the Ca2+ site in zirconolite, coupled with sufficient trivalent M3+/Ti4+ substitution (where M3+ = Fe, Al, Cr), has been systematically evaluated using Ce4+ as a structural analogue for Pu4+. A broadly similar phase assemblage of zirconolite-2M and minor perovskite was observed when targeting low levels of Ce incorporation. As the targeted Ce fraction was elevated, secondary phase formation was influenced by choice of M3+ species. Co-incorporation of Ce/Fe resulted in the stabilisation of a minor Ce-containing perovskite phase at high wasteloading, whereas considerable phase segregation was observed for Cr3+ incorporation. The most favourable substitution approach appeared to be achieved with the use of Al 3+, as no perovskite or free CeO2 was observed. However, high temperature treatments of Al containing specimens resulted in the formation of a secondary Ce-containing hibonite phase.
INTRODUCTION The United Kingdom Nuclear Decommissioning Authority (NDA) is responsible for the safe and long term management of waste derived from the reprocessing of spent fuel from civil power generation 1. Reprocessing of nuclear fuels in the UK has resulted in a significant stockpile of civil separated Pu, forecast to reach 140 teHM 2. The default strategy for this material is continuous interim storage (subject to periodic repackaging) at the Sellafield site. However, NDA is currently in the process of refining the ‘credible options’ framework, exploring potential reuse and disposal options for UK stockpiled Pu. The UK Government has
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placed preference on the fabrication of (U/Pu)O2 mixed oxide fuels (MOX) for use in a fleet of civil light water reactors 3. Yet, some portion of the stockpile may not meet the requirements for the manufacture of MOX fuel due to degradation and contamination, hence, prompt immobilisation and disposal in a suitable glass or ceramic matrix is considered a viable and proliferation resistant management strategy for this material. Alkali borosilicate glass wasteforms are currently operational for the immobilisation of chemically heterogeneous calcines derived from aqueous reprocessing operations, howe
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