Rhyolitic glasses - natural analogues for high-silica nuclear waste glasses

The long-term chemical durability of sintered high-silica waste glasses is expected to be high. This expectation is supported by the observation that natural rhyolitic glasses show only little alteration over geological periods of time. Rhyolitic glasses do not contain appreciable amounts of Zr but they do contain Al. Both Al and Zr enhance the chemical durability of a glass, Zr more than Al. Except for ZrO2, rhyolitic glasses, e.g., obsidian, appear to be suitable natural analogues. The chemical composition and hydration energies of rhyolitic glasses and of sintered high-silica glasses are compared. The author presents a reasoning based on the contribution of silica, Zr and Na to hydration energy to support the hypothesis of analogue long-term behaviour. (A.C.)