Current state of knowledge of nuclear waste glass corrosion mechanisms: the case of R7T7 glass

The mechanisms of aqueous corrosion of nuclear waste glasses are reviewed. After an initial glass hydration step, a thin reaction zone is created within which a H + ⇄ alkali metal interdiffusion profile is formed. The hydrated silicate network then dissolves congruently, hydrating all the glass elements. The dissolution kinetics are controlled principally by the H 4SiO 4 activity in solution, and can be described by a first-order relation. Many glass components, however, including the oxides and hydroxides, are highly insoluble in alkaline media (pH 7–10): the actinides, the rare earths, Fe, Zr, Al and other elements do not enter solution but recombine in situ to form an amorphous surface ‘gel’ layer. Thus dissolution remains nonselective (all the glass elements are hydrated at the same time) but becomes incongruent (not all the elements enter solution). The formation and structural properties of the gel are discussed. The composition of the gel changes as the reaction progresses, approaching the hydrated glass composition as the leaching solution nears saturation. The gel must be taken into account as a diffusion barrier and trap for the species released from the glass. The role of the secondary mineral phases formed during glass alteration is then discussed. Geochemical models show that none of the minerals that have been experimentally identified can control the long-term H 4SiO 4 activity in the French ‘R7T7’ reference glass, and can thus account for the low residual dissolution rates observed. Finally, the current state of knowledge in the area of glass source term models is assessed for the four repository media under consideration in France.