At Sellafield, the Post Operational Clean Out (POCO) of solids from the base of the highly active waste storage tanks, in preparation for decommissioning, will result in a high molybdenum stream which will be vitrified using the current Waste Vitrification Plant (WVP). In order to minimise the number of containers required for POCO, the high molybdenum feed could be co-vitrified by addition to reprocessing waste, using the borosilicate glass formulation currently utilised on WVP. Co-vitrification of high molybdenum feeds has been carried out using non-active simulants, both in the laboratory and on the Vitrification Test Rig (VTR) which is a full scale working replica of a WVP processing line.
In addition, a new borosilicate glass formulation containing calcium has been developed by NNL which allows a higher incorporation of molybdenum through the formation of a durable CaMoO4 phase, after the solubility limit of molybdenum in the glass has been reached. Vitrification of the high molybdenum feed in the presence of varying quantities of reprocessing waste liquor using the new glass formulation has been carried out in the laboratory. Up to ∼10 wt% MoO3 could be incorporated without any detrimental phase separation in the product glass, but increasing the fraction of reprocessing waste was found to decrease the MoO3 incorporation. Soxhlet and static powder leach tests have been performed to assess the durability of the glass products. This paper discusses the results of the vitrification of high molybdenum feeds in the presence of reprocessing liquor in both the borosilicate glass formulation currently utilised on WVP and the modified formulation which contain calcium.