Solid state diffusion during SON68 glass alterationOne of the key mechanisms explaining residual alteration rate
Résumé
The long term alteration rate of the French R7T7 nuclear glass has been investigated since many years because it will define the overall resistance of this radionuclide containment matrix. Recent studies have shown that this rate remains constant or is slightly decreasing with time. It never reaches zero. Though this long term rate is very low, only 5 nm per year at 50DC in initially pure water, it would be the dominant alteration phenomenon in a geological repository. Two mechanisms are suggested for explaining such 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 SA-V ratio (glass surface area SA to the solution volume V). The solutions and element profiles in altered glass were analyzed by ICP-AES and TOF-SIMS, respectively. The results point out that SA-V and ionic strength are not key-parameters. pH leads to significant diffusion coefficient decrease at 90DC when it rises from 8 to 10. Variations with temperature can be modeled thanks to an Arrhenius law. The determined value of activation energy is discussed in comparison with literature data.