Effects of Thermal Treatment at 105 °C and Conditions of Injected Water Pressure on Swelling and Hydraulic Conductivity of Ca-Bentonite Buffer Materials

Gi-Jun Lee¹, Seok Yoon^2,*, Gyu-Hyun Go³, Seong-Won Lee¹

Received 2024-12-31 ; Revised 2025-01-14 ; Approved 2025-01-30

Abstract

The swelling capacity of bentonite buffers is vital in high-level radioactive waste (HLW) repositories, as it minimizes groundwater infiltration, prevents nuclides from reaching the biosphere, and stabilizes the HLW canisters. As swelling capacity is a function of temperature, understanding bentonite’s behavior at approximately 100 °C (its presumed upper limit) is essential. However, research on this subject has been scarce. Hence, this study explored the effects of thermal treatment of Ca-bentonite at 105 °C under injected water pressures. The results suggest a 19% reduction in “swell index” and a 35%–36% decrease in the total pressure in thermally treated bentonite. The heated samples demonstrated higher hydraulic conductivity than the non-heated ones, indicating potential performance deterioration in controlling the fluid movement. Furthermore, the injected water pressure (base pressure) was not fully transmitted to the sample owing to the difference between the base and back pressures, leading to variations in the total pressure despite maintaining a constant differential pressure. Thus, the results demonstrated a degradation in bentonite’s swelling capacity and its compromised role in safe HLW disposal, when subjected to treatment at 105 °C. The insights from this research can assist in HLW repository design, while highlighting the need for further research into bentonite’s performance.

Key Words : Thermal treatment, bentonite, swelling, hydraulic conductivity, injected water pressure