Quantification of the Kinetics and Mechanisms of Dissolution of Full-Scale Simulant UK HLW Glasses Incorporating Post Operational Clean Out (POCO) Waste

As the UK’s spent nuclear fuel reprocessing capability draws to a close, a period of Post Operational Clean Out (POCO) of the highly active liquor storage tanks (HASTs) will begin, with the resulting waste vitrified to form a stable disposal product. Since the waste removed from the HASTs is expected to be compositionally different from the HA liquor generated in the last several decades of reprocessing, most notably enriched in molybdenum, an alternative glass composition, containing Ca to promote formation of insoluble CaMoO₄ (powellite) and Zn to improve viscosity, has been developed.<br/><br/>This work focuses on quantifying the dissolution behaviour of full-scale simulant UK HLW CaZn glass, produced at the Waste Vitrification plant, as a function of waste type and waste loading. Glasses containing different waste loadings of Magnox and Thorp calcine blend are compared with glasses containing a blend of simulant calcine derived from high active liquor, HAST and HAST heel waste. Two additional glasses, the International Simple Glass (ISG) and SON68, were also included for comparison.<br/><br/>Dissolution tests were performed for 730 days at 40°C in highly alkaline solution (saturated Ca(OH)₂) on glass monoliths using the ASTM MCC-1 methodology and the release of elements was followed using aqueous analytical techniques. Surface analysis was used to determine the thickness of gel layer formation and to identify secondary phases.<br/><br/>The CaZn glasses displayed similar chemical durability despite differences in composition, microstructure and waste-type, and the dissolution rates consistently lay below SON68, and above the International Simple Glass. The CaZn-Magnox glasses, which were contained a greater proportion of Mg, displayed a higher dissolution rate compared to the CaZn-POCO glasses. These CaZn-POCO glasses showed the lowest rates of Mo dissolution despite having the highest proportion of Mo. This is credited to the durability of the CaMoO₄ crystalline phase even under hyper-alkaline conditions.