Investigation of Processing Parameters for the Consolidation of Actinide Glass-Ceramic Wasteforms by Hot Isostatic Press
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Investigation of Processing Parameters for the Consolidation of Actinide Glass-Ceramic Wasteforms by Hot Isostatic Pressing Stephanie Thornber1, Paul Heath1, Ewan Maddrell1,2, Martin C. Stennett1 & Neil C. Hyatt1. 1 University of Sheffield, Immobilisation Science Laboratory, Department of Materials Science & Engineering, Sheffield, S1 3JD, UK 2 National Nuclear Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK. ABSTRACT Glass-ceramics were developed initially for the immobilization of miscellaneous Puresidues at the UK’s Sellafield site from which it was uneconomic to recover Pu for reuse. Renewed interest in the immobilization of a portion of the UK PuO2 stockpile has led to glassceramics being evaluated for bulk Pu immobilization. The Nuclear Decommissioning Authority (NDA) in the UK have proposed hot isostatic pressing (HIP) as a potential consolidation technique for the processing of these wasteforms. In this study, zirconolite based glass-ceramics were investigated to determine an optimum formulation. The yield of zirconolite is shown to vary with glass composition and glass fraction, such that a higher Al content favours zirconolite formation. The sample preparation process is discussed to highlight the importance of a high temperature heat-treatment during sample preparation to achieve high quality HIPed wasteforms. INTRODUCTION The UK currently stores a Pu inventory in excess of 100 tons. Significant quantities of this Pu stockpile may not be viable for reuse as fuel and therefore require a suitable processing and disposition route.1–3 Materials such as ceramics and glass-ceramics, are being developed as alternative wasteforms for waste-streams where vitrification is inapplicable. The Nuclear Decommissioning Authority (NDA) identified hot isostatic pressing (HIPing) as a potential technique for processing the Pu-residue stockpile into wasteforms for long-term storage and geological disposal.4 After pretreatment, the calcined waste is mixed with the matrix components and packed into a stainless steel canister. The canister undergoes evacuation and bake-out (the temperature is elevated whilst connected to the vacuum pump) to remove air and volatiles from inside before sealing with a weld. The HIP applies high temperature and pressure to the hermetically sealed waste to produce a wasteform of significantly lower volume than other methods.5 The applied pressure allows wastes that require high processing temperatures to be consolidated at relatively low temperature and removes microporosity from the material to improve its durability, and achieves a finer grain structure to increase wasteform strength.5 It is important to optimise the processing parameters at all stages of the HIP process to ensure maximum efficiency and waste throughput. When using oxide precursors for the matrix reagents, the evacuation and bake out stages significantly inhibit sample throughput. To optimise the processing time, with the aim of improving the overall efficiency of the HIP processing line, an investigation of the effect of
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