Modelling the long term alteration of the engineered bentonite barrier in an underground radioactive waste repository

In the French design for a High Level Waste (HLW) repository, compacted bentonite may be the main component for the engineered barrier system (EBS) in the spent fuel disposal cell. In such a barrier, the interactions between groundwater and bentonite, as well as between the corrosion products of steel overpacks and bentonite, may modify the chemical and physical properties of the selected swelling clay buffer. Bentonite material has a very low permeability, and consequently molecular diffusion is the main mechanism of mass transport. This study is focused on the possible feedback effects of geochemical reactions on the transport properties (porosity and diffusion) of a compacted bentonite. 100,000 years of simulated mass transport-reaction, the model predicts mineralogical modifications of the EBS in contact with the geological interacting fluid, and with Fe2+ ions provided by the corrosion of the steel overpacks. This corresponds to a transformation of the initial montmorillonite by partial illitization, saponification and vermiculitization due to chemical diffusion from geological groundwater through the bentonite barrier. The aqueous corrosion of steel overpacks generates a chemical perturbation inside the EBS (low redox potential and high values of pH) which could possibly create locally a destabilization of the montmorillonite, while part of the released Fe2+ ions is incorporated into precipitated chlorites and saponites. Formations of magnetite, laumontite, greenalite, chabazite, phillipsite, and chrysotile are also identified in the numerical simulations. Despite these modifications, the predicted evolution of porosity display decreasing values and are limited to the outer parts of the EBS. A mass transport law applied to this study predicts a decrease of the molecular diffusion correlated with the porosity clogging.