The UK is currently in the process of adopting a modified seven-oxide base glass (ZnCa MW), containing ZnO and CaO for the vitrification of high level waste at a molar ratio of ZnO:CaO 40:60 in the nominal Mixed Windscale (MW) formulation. Previous studies have given an insight to how Zn and Ca may influence glass dissolution, however the dual role of Zn and Ca remains poorly constrained.
This investigation studied a series of six ZnCa MW compositions with different ZnO/CaO ratios, with the aim of understanding how the varying ratios influence the structure of the glass and how this relates to the aqueous durability in all stages of glass corrosion. Structural analysis using 29Si and 11B NMR has been utilised to determine the relative influence of Zn and Ca on the borosilicate glass network and on the altered glass post-dissolution. Dynamic flow-through (SPFT) and static (MCC-1 and PCT-B) batch dissolution experiments give insight to the behaviour of the various ZnO/CaO ratios, whereby dissolution rates and formed altered layers were sensitive to small compositional changes.
Waste-loaded ZnCa MW formulations with varying ZnO/CaO ratios and varying waste loads (20, 28 and 35 wt.%) were developed and subjected to static dissolution (MCC-1 and PCT-B) in clay, granite and saline groundwaters under conditions relevant to geological disposal. From these studies, optimised glass formulations have been suggested in terms of their long-term aqueous durability.