Investigation of Ce incorporation in zirconolite glass-ceramics for UK plutonium disposition
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Investigation of Ce incorporation in zirconolite glass-ceramics for UK plutonium disposition Stephanie M. Thornber1, Martin C. Stennett1, Neil C. Hyatt1 1 Immobilisation Science Laboratory, Materials Science and Engineering Dept., The University of Sheffield, Mapping Street, Sheffield, S1 3JD, UK ABSTRACT Cerium incorporation in zirconolite glass-ceramic systems, consolidated by hot isostatic pressing, was investigated. Samples were formulated to target Ce incorporation on the Ca and / or Zr sites. Results show successful incorporation of Ce into the ceramic phase although complete waste digestion was not achieved. The formation of a Ce-bearing perovskite phase when targeting Zr substitution, was associated with reduction of Ce4+ to Ce3+. This study is part of preliminary work towards fabricating Pu-bearing glass-ceramic HIP samples. INTRODUCTION The UK stores over 120 tonnes of separated PuO2 derived from the reprocessing of spent nuclear fuel. The current UK policy for dealing with the stockpile is to fabricate all usable material into MOx fuel. Material unsuitable for use as fuel has been identified as higher activity waste and is to be immobilized into suitable wasteforms for storage and eventual geological disposal [1], [2]. Zirconolite (CaZrTi2O7) glass-ceramics are considered a potential wasteform for immobilizing Pu-residue waste-streams. The glass-ceramic matrix acts as a double barrier system against radionuclide release by partitioning actinides into the ceramic phase, simultaneously increasing the proliferation resistance. The glass component adds flexibility to the ceramic to accommodate variations in the waste composition. Zirconolite accepts actinides and rare earth elements into its structure and has excellent radiation tolerance and durability properties, known from studies of natural analogues and from its use in Synroc [3]–[6]. The zirconolite structure is a derivative of the pyrochlore structure (general formula A2B2O7) and consists parallel planes of Ca and Zr ions alternated with planes of Ti octahedra. This is known as the nominal zirconolite-2M monoclinic polytype [7], [8]. Other known polytypes include zirconolite-4M, zirconolite-3T, zirconolite-3O and zirconolite-6T [9]. In this study CeO2 was utilized as a non-active surrogate for PuO2. We investigated the incorporation of Ce into our glass-ceramic system when targeting substitution onto the Ca and / or Zr sites in zirconolite. Our starting formulation was determined from previous work where glass fraction and composition was shown to affect zirconolite formation [10]. An optimized formulation with a low glass fraction (30 wt%) of composition Na2Al2Si6O16, yielded maximum zirconolite formation and was selected for these Ce incorporation experiments. The matrix of samples discussed below investigates the success of Ce incorporation in our optimised glassceramic formulation as a non-active preliminary study before active Pu incorporation experiments are conducted.
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