Glass ceramic for the vitrification of high level waste with a high molybdenum content

Abstract : Vitrification has been selected in France as the immobilization process for high-level waste arising from spent fuel reprocessing. Several high-level waste solutions from the reprocessing of legacy UMo spent fuel, used in gas cooled reactors, have been stored in the AREVA La Hague facility in stainless steel tanks since the mid-1960s. A special glass-ceramic formulation has been developed and qualified through lab and pilot testing to meet standard waste acceptance criteria for the final disposal of the UMo waste. The Cold Crucible Induction Melter (CCIM) was selected for the vitrification of this particularly hard-to-process waste stream because it could not be reasonably processed in the standard hot induction melters currently used at the La Hague vitrification facilities the waste has a high molybdenum and phosphorous content which makes it corrosive and also requires a special high temperature glass formulation to obtain sufficiently high waste loading factors, higher than 10 wtpercent in Molybdenum oxide. Molybdenum is known to be sparingly soluble in conventional borosilicate glass. The formulated glass-ceramic matrix comprises a major vitreous phase containing secondary phase particles less than 100 et956;m in diameter. These are formed by phase separation and crystallization as the molten glass cools in the canister. The physical and microstructural properties of the UMo glass in the solid and liquid states are reported here. Evolution of microstructure as a function of the sample position in the canister was investigated, given the sensitivity of the crystallization process to the cooling profile. The chemical durability of the UMo glass-ceramic was also studied. The feasibility of this process has been demonstrated in a full-scale pilot facility with inactive surrogate solutions.