Evaluation of the kinetics of cement-bentonite interaction in a HLW repository using the reactive solute transport simulator

Bentonite is not thermodynamically compatible with the cementitious materials that will be present in the repository. However, the evolution of the bentonite buffer, induced by high-pH plumes originating from the cement-water reactions, occurs over very long time-scales. The kinetics of bentonite dissolution is a key feature to determine if the use of cement results in significant alternation of the bentonite buffer. To address this issue, a series of simulations were carried out using a reactive solute transport model. The code used in this study fully couples geochemical reactions, flow and transport, without splitting these processes, which can result in misleading solution timescales. Mineral precipitation and dissolution reactions can be modelled using user-defined kinetics while instantaneous equilibrium assumptions can be made when the reactions of interest are fast enough compared with mass transport or the data supporting the specification of a kinetic rate is insufficient. Results of the simulations suggest that provided the initial hydraulic conductivity of the bentonite buffer is as low as designed, the dissolution kinetics of the bentonite are sufficiently slow to ensure its alteration is not significant even in the case of a combination of pessimistic assumptions on the dissolution kinetics and mass transport at the cement-bentonite interface. (author)