In 1996 the first plant built in the US for vitrification of defence-related HLW, the Defence Waste Processing Facility (DWPF) at the Savannah River Laboratory (SRL), commenced operation. Unlike the British and French civil HLW vitrification plants, which operate using Inconel 601-lined induction furnaces, the DWPF operates a Joule-heated ceramic-lined furnace, as will the new facility, the Waste Vitrification Plant (WVP) being built at Hanford. In the DWPF, waste is continually fed into the melter as a wet slurry to minimize dusting of radionuclides along with glass frit and

heated up to 1150°C. The resulting glass is poured into the metal waste canisters at intervals. Borosilicate glasses are used in all of these facilities, but each has its composition tailored to meet the specific requirements of the wastes being processed. Comprehensive tables of glass compositions for both defence and civilian use have been given by Donald (2010) and Jantzen (2011).

Experience gained during the operation of DWPF has led to new glass compositions being developed (Table 25.5) which allow waste loadings to be increased from a nominal 28 mass% to 38 mass% (Marra et al, 2008). These compositions are aimed at the high alumina content wastes to be processed at SRS and Hanford, which are especially problematic due to the refractory nature of alumina which reduces throughput and increases the formation of nepheline (NaAlSiO4) crystals which can be detrimental to the durability of vitreous waste forms by reducing the alumina and silica content of the residual glass. Table 25.5 highlights the variations in composi­tion as glasses are developed for specific waste streams with HAL-17 being developed for Hanford tank sludges containing approximately 53 mass% alumina.

The use of a different furnace technology, the Cold Crucible Induction Melter (CCIM), or skull melter, allows higher vitrification temperatures to be achieved without enhanced corrosion of the refractory liner. Using a small-scale CCIM furnace SIA Radon were able to vitrify SRL Sludge Batch 2 simulated waste at a 50 mass% loading at 1320-1440°C using Frit 320 (Table 25.5) (Stefanovsky et al., 2008). The actual Batch 2 sludge con­sists primarily of the oxides of Al, Fe, Na and U (Elder et al., 2000).