Initial Investigation Into the Vitrification of High Molybdenum Solids in Borosilicate Glass

Abstract

Sellafield Ltd operates a Waste Vitrification Plant (WVP) to immobilise the arisings from the reprocessing of spent nuclear fuel. Washout of solids from the base of waste storage tanks in preparation for decommissioning is likely to produce feeds enriched in molybdenum to the WVP. Vitrification of such feeds in the borosilicate glass formulation currently used by the WVP for vitrification of reprocessing waste has been investigated to determine the maximum achievable loading of MoO3.The vitrification of molybdenum in the absence and presence of reprocessing waste was studied. A number of glasses were manufactured in the laboratory containing various waste loadings. The resultant glasses were examined both visually and under the scanning electron microscope for the presence of any phase separation. Additional aluminium was added to the glasses manufactured in the absence of reprocessing waste to improve the durability of the glass. In borosilicate glass containing 3.5 wt% Al2O3 the onset of a molybdenum phase separation was observed in glasses containing 2.6 wt% MoO3. In the presence of Magnox reprocessing waste, phase separation was observed when the product contained >3.8 wt% MoO3. Soxhlet durability testing of a selection of the glasses manufactured was carried out. The Soxhlet durability of glasses in the absence of phase separation was good.

Crystallization of molybdenum oxide phase from simulated high-level waste glass under slow cooling

“…6). Although several studies have reported the crystallization of powellite from Cs-bearing borosilicate glass, 12), 19) partitioning relationship of sodium and cesium was unclear. The A4-Cs and C4-Cs samples revealed that Cs is certainly distributed to the phase separated Na 2 MoO 4 phase.…”

“…11),26) Precipitation of Ca x Ba y MoO 4 has been reported when the glass is rich in BaO. 19) We investigated the effect of the addition of La in air, and then cooled at a rate of 1 °C/min. This cooling rate is comparable to that expected when the molten HLW glass is loaded in metallic canisters.…”

Composition Changes and Future Challenges for the Sellafield Waste Vitrification Plant

“…Reference [7] shows that borosilicate glasses of the type currently used by WVP can incorporate Mo concentrations of 2.6wt% to 3.8wt%, substantially higher concentrations will require development of new glass formulations. d) There will be a higher proportion of solids phases, left behind in tanks when the liquors were removed during normal operations, and then removed during POCO.…”