Long-term performance of contacting concrete and smectite clay in deep disposal of highly radioactive waste

Disposal of highly radioactive waste (HLW) can be environmentally acceptable if radionuclides are kept isolated from the groundwater, which has inspired planners of repositories to work out multibarrier concepts that postulate defined functions of the host rock and engineering barriers. Assessment of the role of the host rock involves groundwater flow modelling and rock mechanical analysis, which are both highly speculative and ignore future changes in rock structure, stress conditions, and groundwater flow. This makes use of engineered barriers necessary and possible technical solutions for isolating containers with HLW by clay and concrete seals have been described in the literature. The present paper examines the performance and interaction of such seals in very deep boreholes with respect to chemical reactions and their impact on the sealing functions.