Synthesis, characterisation, and chemical durability of UK uranium Butex high-level waste glasses

Two inactive simulant glass wasteforms were developed to incorporate a unique blend of high-level waste calcine from Butex, Magnox, and ThORP fuel reprocessing at a ratio of 50: 37.5: 12.5, with a total waste loading of 32 wt%. The glasses differed compositionally: one used the traditional Mixed Windscale (MW) borosilicate base glass (Butex MW), the other a CaZn-modified borosilicate base glass (Butex CaZn). Glasses were characterised by Energy Dispersive Spectroscopy, X-ray Diffraction, and X-ray Absorption Near Edge Spectroscopy, the latter confirming that U is incorporated as the uranyl (U⁶⁺) moiety in both glasses. Dissolution kinetics were evaluated using the Product Consistency Test-B (PCT-B) in UHQ water at 40 °C for 110 d, under both oxic and anoxic conditions. No significant differences were observed in the dissolution behaviour of the main glass elements (B, Si, Na, Li) between either glass composition or atmosphere. Both glasses followed typical borosilicate dissolution behaviour: a rapid initial rate (∼10⁻³ g m⁻² d⁻¹, 1-35 d), transitioning to a slower residual rate (∼10⁻⁵ g m⁻² d⁻¹, 35-110 d) - up to 16 times lower than Magnox/ThORP-only blends, attributed to reduced Mg content from the Butex component. Uranium release rates were similarly low (∼10⁻⁵ g m⁻² d⁻¹), indicating that Butex calcine addition improves durability in short-term accelerated testing. Minor differences between oxic and anoxic experiments were attributed to CO₂-driven pH changes, which affected Al solubility. Enhanced U release under oxic conditions was linked to U⁶⁺ complexation with bicarbonate ions, offering novel preliminary insight into Butex glass behaviour in (bi)carbonate-rich groundwater.