The dissolution of simulant UK Ca/Zn-modified nuclear waste glass: the effect of increased waste loading

The influence of increasing the waste loading of Ca- and Zn-containing MW high-level-waste glass, from 20 to 35 wt%, on the chemical durability was investigated using 112-day PCT-B tests at 90 °C in ultra-high-quality water. During the initial stage of dissolution, from 0 to 3 days, increased waste loadings corresponded to reduced boron dissolution rates. By 112 days, however, there was no difference in the normalised mass loss of boron as a function of waste loading, within experimental uncertainty. In comparison to literature values obtained for traditional MW high-level-waste glasses, containing no Ca and Zn, the boron dissolution rates of CaZn MW glasses in the residual stage of dissolution, > 28 days, were greater by at least an order of magnitude. Analysis of secondary phases, combined with geochemical modelling of the solutions, indicate that the dual formation of Mg- and Zn-smectite phyllosilicate clays, potentially saponite (Ca_0.2Mg₃(Si,Al)₄O₁₀(OH)₂·4H₂O) and sauconite (Na_0.3Zn₃(Si, Al)₄O₁₀(OH)₂·nH₂O), respectively, may be the cause of this enhanced dissolution.