Obsidians and Tektites: Natural Analogues for Water Diffusion in Nuclear Waste Glasses

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Abstract
Molecular water diffusion in natural obsidians and tektite was investigated in vapor hydration tests performed between 75 and 230°C for up to 400 days. Reaction progress was monitored using measurements of the birefringent hydration layer, an alteration feature associated with strain caused by molecular water diffusion in obsidians. The hydration rate constants and temperature dependence of the reaction are strongly correlated with the logarithm of the initial total water content of the glass. These values have been quantified for conditions relevant to the potential Yucca Mountain repository. The low initial total water concentrations of Savannah River Lab nuclear waste glasses produced at the bench-top scale help to minimze the effects of molecular water diffusion in waste glasses. The results of this study indicate that molecular water diffusion does not dominate waste glass reactions under conditions considered in this study. However, it is unknown whether molecular water diffusion will be important under other reaction conditions, especially longer time periods.