Vitrification of high molybdenum feeds in the presence of reprocessing waste liquor
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Vitrification of high molybdenum feeds in the presence of reprocessing waste liquor Rick Short1, Barbara Dunnett1, Nick Gribble1, Hannah Steel2, Carl James Steele2 1 2
National Nuclear Laboratory, Sellafield, Seascale, Cumbria, UK, CA20 1PG Sellafield Ltd, Sellafield, Seascale, Cumbria, UK, CA20 1PG
ABSTRACT 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. INTRODUCTION Highly Active Liquor (HAL) resulting from the reprocessing of used nuclear fuel on the Sellafield site is stored in Highly Active Storage Tanks (HASTs), and then fed to the Waste Vitrification Plant (WVP) to be immobilised in a glass matrix. As vitrification of HAL progresses, the storage tanks will be emptied to a point where they only contain a residual heel of HAL, which is likely to contain a large quantity of solids. The tanks will thus have to be subjected to Post Operational Clean Out (POCO) to reduce their activity, prior to decommissioning. There are several potential routes for undertaking POCO, one of which is to wash the tanks out and direct the resultant waste stream to WVP. Such a waste stream is likely to contain high levels of zirconium and molybdenum; molybdenum is known to be a problematic element during vitrification due to its low solubility in the borosilicate glass matrix used in the UK[1]. At concentrations above the solubility limit molybdenum tends to form compounds with alkali, alkali earth and rare earth elem
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