Modeling of solution renewal with the KINDIS code: Example of R7T7 glass dissolution at 90 C

The deep underground environment that would correspond to a geological repository is a system open to fluid flow. It is therefore necessary to investigate the effects of solution renewal on the long-term behavior of glass in contact with water. These effects can now be simulated using the new version of the geochemical KINDIS model (thermodynamic and kinetic model). The authors tested the model at 90 C with an SA/V ratio of 400 m-1 at 12 renewal rates of pure water ranging from 200 to 0 vol% per day. With renewal rates between 200 and 0.065 vol% per day, steady-state conditions were obtained in the reaction system: i.e. the glass corrosion rate remained constant as did the concentrations of the dissolved species in solution (although at different values depending on the renewal rate). The ionic strength never exceeded 1 (the validity limit for the Debye-Hueckel law) and long term predictions of the dissolved glass mass, the solution composition and the potential secondary mineral sequence are possible. For simulated renewal rates of less than 0.065 vol% per day (27 vol% per year), the ionic strength rose above 1 (as in a closed system) before steady-state conditions were reached, making it critical to calculate long-term rates; a constant and empirical long-term rate, derived from laboratory measurement, have to be extrapolated. These calculations were based on a first order equation to describe the glass dissolution kinetics. The results obtained with the KINDIS code show discrepancies with some major experimental kinetic data (the long term rate must decrease with the glass-water reaction progress, under silica saturation conditions). This clearly indicates that a more refined kinetic relation is needed for the glass matrix