Vitrification of simulated highly active calcines containing high concentrations of sodium and molybdenum
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Vitrification of simulated highly active calcines containing high concentrations of sodium and molybdenum Mike T. Harrison1 and Carl J. Steele2 1 National Nuclear Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK. 2 Sellafield Ltd, Sellafield, Seascale, Cumbria, CA20 1PG, UK. ABSTRACT Sodium carbonate is currently being considered as a wash-out reagent for the removal of the settled solids in the unagitated Highly Active Liquor (HAL) storage tanks at Sellafield. As the settled solids are expected to comprise mainly zirconium molybdate (ZM), this will result in a challenging feed to the Waste Vitrification Plant (WVP) containing high concentrations of both molybdenum and sodium. In previous studies, it was shown that at high wash-out waste loadings, i.e. 10 – 12 wt% MoO3 incorporation, there was very little tolerance in ‘Ca/Zn’ base glass for extra sodium before the formation of significant separated sodium molybdate salt phase. However, higher amounts of sodium can be accommodated in borosilicate glasses if the wash-out waste loading is reduced. Further studies have now been carried out to investigate the vitrification of more representative calcined waste feeds. Both pure zirconium molybdate (ZM) and blended ZM-reprocessing waste calcines were produced from the appropriate liquor feeds. The maximum waste incorporations of these two calcines in ‘Ca/Zn’ base glass have been determined, along with a complete product quality assessment. This assessment included measuring the bulk density, degree of crystallinity, heat treatment, durability (Soxhlet and PCT), glass transition temperature, and viscosity. INTRODUCTION The Waste Vitrification Plant (WVP) at Sellafield immobilizes the HAL arising from the reprocessing of civil spent nuclear fuel by converting it to a vitrified waste form suitable for long term disposal [1]. Pending vitrification, the HAL is kept in highly active storage tanks (HASTs) where solids have accumulated over a number of years. Following the cessation of reprocessing Sellafield, these solids need to be removed and immobilized [2]. A number of the Sellafield HASTs have no engineered agitation mechanism and, hence, their final emptying will potentially involve a chemical wash-out procedure to dissolve any settled, compacted solids. These solids are expected to contain significant amounts of zirconium molybdate hydrate (ZMH), i.e. ZrMo2O7(OH)2.(H2O)2 [3], with sodium carbonate being considered as a potential reagent. This wash-out feed will potentially be problematic in that it will contain both high Na and Mo concentrations that could restrict the waste incorporation in the glass. For example, Mo has a limited solubility in alkali borosilicate glass [4 - 6] and excessive alkali above that already present in the base glass is expected to have implications for the thermal stability and aqueous durability of the vitrified product. Although a new ‘Ca/Zn’ base glass formulation has been developed specifically for high-Mo HAST emptying waste [7], its tolerance to increased levels of sodium is current
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