Geoenvironmental Application of Bentonites in Underground Disposal of Nuclear Waste: Characterization and Laboratory Tests

In the context of underground storage of nuclear waste, this paper presents the characterization and laboratory tests on compacted MX80 bentonite. Various laboratory testing techniques were used for establishing the water retention behavior of the bentonite. Compacted bentonite specimens were hydrated with fluids of low and high salt concentrations both at ambient and an elevated temperature of 70°C. The swelling behavior of the bentonite was also studied by stepwise wetting compacted specimens at various suctions under isochoric condition. The influence of thermal and thermohydraulic gradients on the mechanical behavior of the bentonite was studied in a thermohydraulic column cell. The investigation showed that the development of swelling pressure in compacted bentonites upon a decrease in the applied suction is accompanied by a decrease in the magnitude suction stress, which gets manifested in the development of the interparticle repulsive pressure and a decrease in the effective stress. The influence of an elevated temperature and an increase in concentration of salt in the hydrating fluid was found to reduce the swelling pressure of the bentonite. Both thermal and thermohydraulic gradients were found to be manifested on the development of axial pressures. Applications of thermal and thermohydraulic gradients were found to cause redistributions of water content, dry density, degree of saturation, and suction within compacted bentonites.