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.

Buffer materials play an important role in preventing the leakage of radionuclides from the residue. The mineralogical properties of these buffer materials are critical in repository design. This study presents the fundamental properties of Na-type MX80 and a novel Ca-type Bentonil- WRK. The CaO to MgO ratio in Bentonil-WRK was approximately 1:1, and the CaO to Na2O ratio was approximately 2.8:1. These results suggest that Bentonil-WRK demonstrates a lower swelling index compared to Gyeongju bentonite due to its CaO-to-MgO ratio’s proximity to 1:1, despite having a higher montmorillonite content than Gyeongju bentonite. The results of this research can provide useful foundational data for the evaluation of the thermal-hydraulic-mechanical-chemical behavior of buffer materials.

The sorption of Eu on MX-80 bentonite in Na–Ca–Cl solutions is investigated at a molal proton concentration (pHm) range of 3 to 10 and an ionic strength (I) range of 0.1 to 6 m (mol·kgw−1). The sorption equilibrium of Eu on MX-80 is achieved within 14 to 21 d at I = 0.1 and 6 m. The sorption distribution coefficient (Kd) values of Eu for MX-80 increase as pHm increases from 3 to 6 for all I values, and they are independent of pHm between 8 and 10 at I ≥ 0.5 m. Meanwhile, at I = 0.1 m, the Kd value at pHm = 10 is slightly lower than those at pHm = 8 and 9. The Kd values are not affected by the I values between 0.5 m and 6 m, whereas the Kd value at I = 0.1 m is greater than those at I ≥ 0.5 m, except at pHm = 10. A two-site protolysis nonelectrostatic surface complexation and cation exchange sorption model is applied to the Eu sorption data for I ≤ 4 m, and the equilibrium constants of the sorption reactions are estimated.