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 glass degraded by a four-month water immersion 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.
CEA: "Long-term behavior of vitrified waste packages-", I. Ribet et al. (Global 2009)