Performance of the bentonite barrier at temperatures beyond 100 °C: A critical review

Many current concepts for the engineered barriers of high-level waste repositories include a bentonite buffer or backfill which will be exposed to significantly raised temperatures for a period of decades to centuries. To assess its stability, available experimental and natural analogue data on thermally-exposed bentonite is reviewed. The two main alteration processes identified are cementation by precipitation of SiO2 and transformation of smectite to non-expandable illite layers. These may lead to reduced swelling and loss of plasticity as well as increased hydraulic conductivity and diffusivity.Relevant data for repository conditions from both laboratory and field studies is scarce, but nevertheless yields a fairly consistent picture. Thus, no significant changes of hydraulic and mechanical properties have been reported for bentonite materials exposed to temperatures of at least 120 °C under wet conditions. The data suggest significant cementation and perhaps also illitisation effects occur at 150 °C and beyond. Interestingly, natural analogue bentonite samples that showed substantial cementation and illitisation effects still displayed rather favourable hydraulic properties. Under dry conditions, bentonite is stable to higher temperatures – maybe as high as 350 °C. In order to verify the findings from this work and to get more reliable information at temperatures beyond 130 °C, a series of long-term experiments examining hydraulic, mechanical and mineralogical changes under realistic conditions would be useful.