Saturated hydraulic conductivity of bentonite-sand barrier material for nuclear waste repository: effects of physical, mechanical thermal and chemical factors

Deep geological repositories (DGRs) are considered the most promising technology for the long-term management of nuclear wastes. One of the major functions of the buffer or barrier material used in DGRs for nuclear waste is to prevent the release of high-level radioactive chemicals into the environment in the event of failure. To accomplish this, the buffer is typically designed to have very low hydraulic conductivity. The effects of mix-composition, swelling condition, temperature and groundwater chemistry on the hydraulic conductivity of compacted bentonite-sand barrier material were investigated in this study. Permeability tests were carried out using flexible wall permeameter after flooding the compacted samples in simulated groundwater. The obtained results showed that the saline groundwater prevailing in the Guelph region of Canada as well as high temperature have a negative impact on the swelling potentials of the buffer material, consequently increasing the hydraulic conductivity. The saline water reduces the thickness of the diffuse double-layer which is accompanied by a weak repulsion between the clay minerals. Similarly, elevated temperature decreases the basal spacing between the bentonite clay minerals which also results in less swelling. The combination of high temperature and salt concentration have an increased negative influence on the hydraulic conductivity. Furthermore, it was observed that restricting the swelling of the bentonite-sand mixture produces a well-packed material with a very low hydraulic conductivity compared to the free swelling. High confining pressure generates higher swelling stress from the hydration of clay minerals, which expand to fill the voids between the sand particles. It was also observed that increasing the percentage of bentonite and the initial dry density of the mix reduced the hydraulic conductivity by eliminating interparticle voids in the material. The research findings presented in this manuscript provide valuable information that will contribute to gain a deeper insight into the impact of field conditions (temperature, groundwater chemistry, confinement) and material characteristics (composition, dry density) on the permeability of bentonite-sand barrier, which is essential for a safe long-term management of radioactive wastes.