Dissolution of UK High‐Level Waste Glass Under Simulated Hyperalkaline Conditions of a Colocated Geological Disposal Facility

We report analysis of chemical durability of UK HLW MW +25% simulant glass under model hyperalkaline conditions of a colocated geological disposal facility. Glass powders and monoliths were dissolved for 168 days in saturated Ca(OH) ₂ . Dissolution in the presence of high concentrations of Ca (>200 mg/L) was an order of magnitude lower than dissolution in water. Dissolution of Si did not occur until a Ca : Si ratio of <2 was achieved. The mechanism of dissolution involved the incorporation of Ca into the hydrated surface (initial, incubation regime), the precipitation of C‐S‐H phases, including a range of compositions in the C‐(N)‐(A)‐S‐H and M‐S‐H systems (intermediate regime), and the precipitation of C‐S‐H phases (the residual regime). Thermodynamic analysis and consideration of the CaO‐SiO ₂ ‐H ₂ O phase diagram suggest that the rate‐limiting step of glass dissolution in Ca ‐rich solutions is Ca‐Si equilibrium, involving the precipitation of C‐S‐H phases, which change in chemical composition as a function of solution chemistry. In low SA / V ratio experiments, the dissolution progressed only to the initial incubation regime, resulting from fewer surface sites for Ca incorporation. Overall, these results suggest that Ca and Si in solution play an important role in the long‐term durability of UK HLW in Ca ‐rich solutions.