Stiffening of smectite buffer clay by hydrothermal effects

Smectite clay is a major engineered “buffer” barrier in repositories for high-level radioactive waste since it provides the waste canisters with a low-permeable embedment that minimizes canister stresses caused by earth shocks and tectonics by being ductile and homogeneous. The hydrothermal conditions prevailing in deep repositories cause some loss and degradation of the buffer and stiffening by precipitation of cementing matter, like quartz, cristobalite and iron compounds emanating from the clay or from the canisters. Thus, chemical reactions leading to cementation can prevent self-sealing of fissures caused by desiccation in the early stage of maturation of the buffer and increase its stiffness so that critical stress conditions may be generated in the canisters.