Water leaching: formation of a protective gel
Degraded vitrified waste packages may be exposed to the effects of water (see inset). The figure shows a scanning electron microscope image of a specimen of R7T7 degraded by a four-month immersion in initially pure water at a temperature of 150 C. An outer layer of "phyllosilicates" (precipitated from the solution) can be seen, together with a porous gel formed by in situ condensation; the underlying glass remains undamaged. The high retaining capacity of this gel impedes dispersion of minor actinides. Degradation stabilises except unless the water flow is very significant, in which case the degradation process continues deeper into the glass.
CEA: "Long-term behavior of vitrified waste packages-", I. Ribet et al. (Global 2009)