In situ investigation of the transient drying and hydration behavior of bentonite buffer material at elevated temperatures

Understanding the transient behavior of bentonite buffer material during heating, before substantial hydraulic pressurization develops, is important for evaluating the early stage thermal–hydrological–chemical evolution of buffer material in geological disposal systems. We present the results of an in situ heating experiment conducted under hydraulically open conditions without substantial hydraulic pressurization at the Horonobe Underground Research Laboratory, Japan. Water content measurements, CT observations, and visual inspection of recovered blocks indicate localized drying and crack formation near the heater. Radial expansion of the buffer blocks was also observed. Chemical analyses indicate solute redistribution, but mineralogical analyses show no clear evidence of smectite alteration or illitization under the experimental conditions. Laboratory infiltration experiments suggest that thermally induced desiccation cracks may become sealed during subsequent hydration due to the swelling of bentonite. These results provide in situ insights into transient drying, hydration, water and solute redistribution, and the mineralogical stability of bentonite-based buffer materials during heating under hydraulically open conditions without substantial hydraulic pressurization.