Evolution of bentonite under high temperature heating and hydration: bench-scale laboratory experiments and coupled thermo-hydro-mechanical modeling

Bentonite buffer in the geological repository for high-level radioactive waste undergoes the heating from the waste package and hydration from the geological formation and goes through coupled thermo-hydro-mechanical-chemical (THMC) changes over the life span of a repository. For a better understanding of such process under higher temperatures, we report bench-scale laboratory experiments with heating up to 200◦C and the corresponding THM model. The bench-scale laboratory experiments included two test columns, with the non-heated control column undergoing only hydration, and a heated column experiencing both heating in the center up to 200◦C and hydration from a sand-clay boundary surrounding the column. During the experiment, we took frequent X-ray CT images to provide insight into the spatio-temporal evolution of THMC due to heating, hydration, bentonite swelling/compression. Based on the experiment setup, 2-D axisymmetric simulations were performed for the heated column and the mechanical changes were investigated in 3-D. The model first matched the temperature evolution with step-wise temperature boundary conditions at the heater and calibrated the thermal conductivity and specific heat of the materials. Then model interpreted the spatio-temporal distribution of bulk density by considering the combined effect of hydration, fluid pressure, and porosity change due to swelling/compression.