Addressing issues related to high level radioactive waste (HLW) from nuclear power industry is an important task for researchers. Deep geological disposal method, which intends to seal HLW deeply underground, was chosen by many countries to deal with HLW. In a Japanese deep geological disposal project, a multi-barrier system, including vitrified waste, stainless over-pack containers, buffer materials, and host rock, is planned to be constructed more than 300 m underground to prevent the leakage of HLW. As indicated by numerous studies, bentonites and their based materials were selected as candidate buffer materials because of their low permeability. Pelleted bentonite has recently been considered as a candidate buffer material. The purpose of using pelleted bentonite as a buffer material is to avoid the challenges identified in the full-scale mock-up study, such as improved workability during barrier installation. Bentonite pellets can be installed using pneumatic projection techniques, which makes backfilling operations easier and potentially robotic.

In addition to low permeability, another important characteristic of bentonite is its high self-healing ability. As important ingredients for evaluating self-healing capacity, swelling pressures (p_s) during saturation, including pressures at peak (p_peak), valley (p_valley), re-peak (p_re-peak) and equilibrium (p_eq) points in swelling curve had been studied by experiments. Among those swelling pressures, most scholars examined p_eq instead of p_peak, p_valley, and p_re-peak. However, experimental conditions strongly affect p_peak, p_valley and p_re-peak. Meanwhile, findings from the study of p_peak, p_valley, and p_re-peak can elucidate the swelling mechanisms during saturation. Devoting greater attention and making greater effort to assess p_peak, p_valley, and p_re-peak are necessary.

In this study, swelling pressure tests were conducted at 25, 50, and 80 °C to elucidate temperature effect on swelling pressures at peak, valley, re-peak and equilibrium points (p_peak, p_valley, p_re-peak andp_eq) in swelling pressure evolution curve. A pellet bentonite, Kunigel-GX, was used to investigate p_peak, p_valley, p_re-peak, and p_eq at different temperatures. Experimentally obtained results were the followings: 1) p_peak, p_valley, p_re-peak, and p_eq of Kunigel-GX decreased as temperature rises and increased as dry density increases; 2) Kunigel-GX was found to have larger values of (p_eq-p_valley) at higher dry density and lower temperature conditions.