EXPERIMENT REGARDING MONTMORILLONITE CONTENT AND STRENGTH PROPERTIES IN BENTONITE UNDER THE INFLUENCE OF HEAT

Abstract

At present, underground geological disposal at depths greater than 300 m is considered a viable disposal option for high-level radioactive waste generated from the reprocessing of spent fuel used in nuclear power plants. According to the Japan Nuclear Cycle Development Institute [1] and Agency for Natural Resources and Energy [2], in geological disposal, bentonite is employed as the primary component of buffer material to fill gaps between waste and geologic rock. However, specification details have yet to be completely determined. It is anticipated that the buffer material will be exposed to high temperature due to the heat generated by the vitrified solid. This study aims to understand the mechanical properties of bentonite buffer material by employing uniaxial compression tests with temperatures between 30°C to 90°C applied to bentonite specimens having received a temperature history treatment of 200°C applied over six months. Results show that the maximum compressive strength of the sample given the temperature history is lower than that of the sample without the temperature history. Furthermore, the montmorillonite content of the sample given a temperature history of 200°C also decreased. The reason for the decrease in strength seems to be that the compaction energy during fabrication decreased due to the decrease in the montmorillonite content. This is because the specimens were fabricated so that the dry density was constant. It seems that the maximum compressive stress decreased with increasing temperature regardless of temperature history. The reason for this seems to be microcracks, which appear to decrease rigidity and, therefore, compressive strength.