Long-term Evolution of Bentonite-Based Seals for Closure of a Radioactive Waste Repository in Claystone: A Hydro-Chemo-Mechanical Modelling Assessment

Prior to the implementation of a deep geological repository for radioactive waste in claystone, a sound system understanding of the repository closure structures is needed. One of the main tools to quantify the long-term evolution of these systems is performance assessment by means of numerical modelling of the governing physical and chemical processes. Within this framework, a hydro-chemo-mechanical (HCM) modelling study of a bentonite-based seal for the closure system of the Cigéo project is presented. The model encompasses reactive transport processes, fluid flow under partially saturated conditions, and nonlinear solid mechanics, with special emphasis on their coupling. The closure of the galleries relies upon the performance of several bentonite–sand seals, surrounded by a concrete liner and a clay-based backfill material. The HCM interactions between these engineered barriers are simulated to predict the evolution of the swelling pressure of the seal. In this regard, the impact on bentonite swelling of cation exchange reactions between bentonite interlayer and bulk water changes in salinity, and montmorillonite dissolution in highly alkaline environments is explicitly accounted for. The results show the potential of the model to quantify the governing processes of swelling pressure evolution and to increase system understanding. The influence of considering a recently developed liner design with a compressible layer material in between the host-rock and concrete is analysed as an alternative scenario. Overall, the models predict a rather limited impact of geochemical alteration on the swelling pressure of the bentonite–sand seal. Results suggest that the key alteration processes are the potassium release from concrete porewater and montmorillonite dissolution to a lesser extent.