Influence of composition of nuclear waste glasses on vapor phase hydration

Abstract

For the first time, the influence of glass composition on the vapor hydration kinetics of the French AVM nuclear waste glass simulants was investigated. Three complex borosilicate glasses (>20 oxides) along with three simplified Na/(Ca, Na)/(Mg, Na)-alumino-borosilicate glasses with four or five oxides were altered at 50 °C in water vapor (95% RH) for up to 557 days. The solid characterization of the altered samples (by SEM, TEM, XRD, ToF-SIMS, SAXS) revealed that the rate-controlling vapor hydration mechanism is composition dependent. The vapor hydration rate of the more durable glasses, whose molar ratio of Al2O3/(CaO or MgO) is ≥ 1, seems to be limited by network-hydrolysis, whereas the overall glass alteration rate of the less durable glasses, whose Al2O3/MgO ratio is < 1, seems to be driven by the precipitation of Mg-rich secondary phases. All the vapor hydrated glasses show the presence of a homogeneous gel layer of a few tens of nm thickness. The more durable glasses have a smaller quantity of secondary phases precipitated on their surface. The less durable glasses have a relatively significant quantity of secondary phases precipitated above the gel layer and irregularly shaped, scattered and highly porous altered zones formed beneath the gel layer. The overall alteration rate of the less durable glasses was 10–20 times faster than the more durable glasses. For all glasses, FTIR spectroscopy indicated an inflexion in the vapor hydration rate after 120–200 days of alteration, likely due to a passivating effect of the altered layer formed under unsaturated conditions. The average pore size of the altered layer in certain vapor hydrated glasses measured by SAXS is similar to the pore sizes of the gel layer formed in aqueous medium and varies very slightly with glass composition.