Hydro-mechanical, geochemical and mineralogical characteristics of the bentonite buffer in a heater experiment: The HE-B project at the Mont Terri Rock Laboratory

A small scale heater experiment was realized at the underground laboratory in Mont Terri, Switzerland. The effects of heat-producing waste in a repository for high-level waste and spent fuel were simulated by a heater element of 10 cm diameter, held at a constant surface temperature of 100 °C. The heater element was embedded in an engineered barrier of compacted bentonite blocks that were water saturated for 35 months before the 18 months heating phase was started. The host rock is a highly consolidated stiff Jurassic clay stone (Opalinus Clay). Extensive on-site and laboratory tests were performed to determine the hydro-mechanical, geochemical and mineralogical characteristics of the bentonite buffer to identify variations in the material properties in response to heating. Other studies deal with the host rock material.The results indicate that the bentonite shows only very weak modifications during the heater experiment. Whereas mineralogical alterations (e.g. transformations and formation of new phases) could not be measured, very weak changes in layer charge density and interlayer chemical composition of the smectite were found. Changes in the porosity and surface area of the bentonite may be explained as result of cementing or aggregation processes. These processes affect some THM characteristics like porosity, water uptake and thermal properties.