Aqueous corrosion of natural and nuclear waste glasses II. Mechanisms of vapor hydration of nuclear waste glasses

Results of recent vapor phase hydration experiments performed on nuclear waste glasses at various temperatures and relative humidities are presented. Hydration rates were determined using a variety of techniques, including specimen weight gain measurement, alteration layer thickness measured using optical microscopy or scanning electron microscopy (SEM), and near-surface concentration profiles obtained using secondary ion mass spectrometry (SIMS). For the three nuclear waste glass formulations studied, previously observed differences in reaction products and rates of hydration in liquid and vapor environments were confirmed. At 100% relative humidity (RH) and at temperatures between 75° and 240°C, the rate of hydration of SRL 131 glass followed and Arrhenius-type rate law with an activation energy of 17.9 kcal/mol. A significantly higher apparent activation energy was estimated for SRL 131 glass hydrated at 95% RH. The vapor hydration rate of SRL 131 glass decreased with RH and was negligible below humidity of 70% RH at 202° C. Potential mechanisms that may govern the vapor hydration of glass are reviewed and several lines of evidence including the parametric dependence of vapor hydration rate observed in the present work are consistent with a molecular water diffusion model. In the context of such a model, the dependence of hydration rates on RH is explained through a relationship between p_H2O and concentration of absorbed molecular water at the outermost surface of hydrated glasses.