Subsurface manipulations such as those expected from the disposal of heat-emanating radioactive waste in deep repositories can induce strongly coupled Thermal (T), hydrological (H), mechanical (M) and chemical (C) processes. Adequate coupled THMC models are highly desirable or even indispensable for performance assessment of such repositories, for examples for the analysis of bentonite or clay barriers around surrounding the emplaced waste. In this study, we present coupled THMC model simulations of a generic nuclear waste repository in a clay formation with a bentonite-based buffer. The objective is to evaluate the chemical changes in the EBS bentonite and their effects on mechanical behaviors under high temperature, attempting to shed light on whether EBS bentonite can sustain temperatures higher than 100 °C without significant impact on barrier performance.

Two scenarios were simulated for comparison: a case in which the temperature in the bentonite near the waste canister can reach about 200 °C and a case in which the temperature in the bentonite near the waste canister peaks at about 100 °C. Simulations have been done for two types of bentonite: Kunigel-VI and FEBEX bentonite. This enables us to evaluate how different types of bentonite behave in terms of the illitization and subsequent swelling stress change and whether we can generalize these changes to support decision making. The simulations show the occurrence of illitization in the bentonite buffer and the enhancement of illitization under high temperature; the degree of illitization is affected by many chemical factors and subsequently varies a great deal. Our models show that the dissolution of K-feldspar strongly affects illitization in bentonite and the interaction between EBS bentonite and host rock is particularly important for illitization in the long run. Swelling stress reduction in bentonite due to illitization ranges from ~1.5% to ~18% after 1,000 years depending degree of illitization, initial conditions and type of bentonite. FEBEX bentonite undergoes less illitization mainly due to the higher ion concentration in pore water and the lower content of K-feldspar in the bentonite mineral composition. Moreover, the reduction of swelling stress by chemical changes is more pronounced for Kunigel-VI bentonite than for FEBEX bentonite. Overall, the results of our model simulations suggest that an argillite repository with a bentonite-based EBS that is similar to FEBEX bentonite could sustain temperatures much higher than 100°C as far as illitization concerns.