The Impact of Increased Waste Loading on Vitrified HLW Quality and Durability
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The Impact of Increased Waste Loading on Vitrified HLW Quality and Durability Nick R Gribble1, Rick Short1, Edward Turner1, Andrew D Riley2 1 National Nuclear Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, Cumbria 2 Sellafield Ltd, Sellafield, Seascale, Cumbria, CA20 1PG, Cumbria ABSTRACT The Sellafield Waste Vitrification Plant (WVP) immobilises highly active liquid waste (HAL) arising from the reprocessing of spent nuclear fuel in the UK. In order to optimise WVP operations a full scale working replica of a WVP processing line, the Vitrification Test Rig (VTR), was constructed to processes non-active HAL simulants. Recently the VTR has been used to determine an operational envelope for the vitrification of HAL from Magnox reprocessing at a waste oxide incorporation rate in glass of up to 35wt% (compared to a “standard” incorporation rate of 25wt%). This paper discusses the differences in operating conditions necessary to achieve acceptable waste throughput at the increased incorporation rate. The chemical durability of the resulting vitrified product is also discussed, along with the formation of secondary phases, and a comparison is drawn between 35wt% incorporation glasses and products made at the standard 25wt% incorporation. INTRODUCTION Reprocessing of spent Magnox nuclear fuel has been carried out at Sellafield in the UK for over 4 decades, and Oxide fuel has been reprocessed in the Thermal Oxide Reprocessing Plant (THORP) for 15 years. The resulting High Level radioactive Waste (HLW) streams from these processes are concentrated by evaporation and stored as a nitric acid solution of fission product and other metal nitrates prior to being treated in the Sellafield WVP. WVP immobilises HLW by bonding the fission products as metal oxides into a borosilicate glass matrix. This provides a stable and durable waste form suitable for safe long term storage and ultimate disposal. The vitrification plants in the UK use a two-stage vitrification process to incorporate the Highly Active Liquor (HAL) into glass. An overview of the key vitrification equipment is given in [3]. The HAL, along with other calcination additives, is fed into a rotating kiln furnace known as a calciner to evaporate the liquor and de-nitrate the resulting solid thus producing an intermediate calcine product. The calcine product is a granular material consisting of mainly metal oxides with a residual nitrate content. The calcination process generates an off-gas stream and this requires abatement. The primary off-gas (POG) system includes a dust scrubber to remove entrained calcine and to recycle this material back into the process. A condenser removes condensable gases such as water vapour, nitric acid vapour and absorbs some nitrous oxide from the process. The third main vessel for off-gas abatement is the NOx absorber, present to remove NOx from the gaseous effluent by absorption of the soluble gas in water / nitric acid. Secondary off-gas cleanup is achieved using electrostatic precipitators and HEPA filters. The intermediate calcine pro
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