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.

Addition of ZnO and CaO to current United Kingdom high level nuclear waste (HLW) glass frits is currently under consideration with the aim of improving processing behavior and aqueous durability. This study was performed in order to better understand the structural role of Zn in model U.K. HLW glasses and, in particular, the role of Zn as a network modifier, intermediate, or former in such materials. Zinc K-edge X-ray absorption spectroscopy was applied to simple soda-lime-silicate glasses doped with ZnO and on model inactive U.K. HLW glasses consisting of simulated Magnox and Blend (25 wt% Magnox + 75 wt% ThORP reprocessing waste) wastes. X-ray absorption near edge and extended X-ray absorption fine structure spectroscopy provided conclusive evidence that Zn occurs in four-fold coordination with Zn-O contact distances of 1.95 ± 0.01 Å in all glasses studied. Analysis of the extended X-ray absorption fine structure data also indicates the presence of short range order with Si next nearest neighbor atoms present at a Zn-Si contact distance of 3.58 ± 0.03 Å . *n.c.hyatt@sheffield.ac.uk

The first comprehensive assessment of the dissolution kinetics of simulant Magnox–ThORP blended UK high-level waste glass, obtained by performing a range of single-pass flow-through experiments, is reported here. Inherent forward rates of glass dissolution were determined over a temperature range of 23 to 70°C and an alkaline pH range of 8.0 to 12.0. Linear regression techniques were applied to the TST kinetic rate law to obtain fundamental parameters necessary to model the dissolution kinetics of UK high-level waste glass (the activation energy (Ea), pH power law coefficient (η) and the intrinsic rate constant (k0)), which is of importance to the post-closure safety case for the geological disposal of vitreous products. The activation energies based on B release ranged from 55 ± 3 to 83 ± 9 kJ mol–1, indicating that Magnox–THORP blend glass dissolution has a surface-controlled mechanism, similar to that of other high-level waste simulant glass compositions such as the French SON68 and LAW in the US. Forward dissolution rates, based on Si, B and Na release, suggested that the dissolution mechanism under dilute conditions, and pH and temperature ranges of this study, was not sensitive to composition as defined by HLW-incorporation rate.

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