Solid state diffusion during nuclear glass residual alteration in solution

The long term alteration rate of the French R7T7 nuclear glass has been investigated since many years because this glass has been selected as radionuclide containment matrix in France. Recent studies have shown that this rate remains constant or is slightly decreasing with time. It never reaches zero. Although this long term rate is very low, only 5 nm per year at 323 K in initially pure water, it would be the dominant alteration phenomenon in a geological repository. Two mechanisms are suggested for explaining this long term behaviour: diffusion towards solution of elements from glass through an amorphous altered layer and precipitation of neoformed phases. Former studies showed that diffusion rates are in agreement with a solid state diffusion mechanism. Additional experiments in initially saturated silica water were performed in order to study the impact of various parameters on the values of the diffusion coefficient for mobile chemical elements: pH, temperature, ionic strength and S/V ratio (glass surface area S to the solution volume V). The solutions and element profiles in altered glass were analysed by ICP-AES and TOF-SIMS, respectively. The results point out that S/V and ionic strength are not key-parameters. pH leads to significant diffusion coefficient decrease at 323 K when it rises from 8 to 10. Variations with temperature can be modelled with an Arrhenius law. The calculated value of activation energy is discussed in comparison with literature data.