Atomic Energy Society of Japan

The buffer material (compacted bentonite), one of the engineered barrier elements in the geological disposal of radioactive waste, develops swelling stress due to groundwater penetration from the surrounding rock mass. Mechanical and thermodynamic data on Ca-montmorillonite have been scarce in previous studies. In this study, thermodynamic data of Ca-montmorillonite were obtained by measuring relative humidity (RH) and temperature. Water activity and relative partial molar Gibbs free energy were determined from the experimental results. The swelling stress of bentonite was calculated using the thermodynamic model was approximately 200 MPa at a montmorillonite partial density of 2.0 Mg/m³ and approximately 10 MPa at a montmorillonite partial density of 1.5 Mg/m³.