Thermally induced modifications of clays. Final Activity Report for TIMODAZ WP 3.2

High-level waste and spent fuel emit significant amounts of heat when disposed of in a repository. The heat will modify water-rock interactions in an attempt to reach a new equilibrium. In addition, the heat effects might be coupled with other geochemical perturbations, such as alkaline plume, and alter near-field chemistry on the long-term. All these processes might affect host rock mineralogy and its favourable properties. The aim of this report is to identify and assess the importance of various thermo-chemical processes on the mineral stability of BoomClay and OpalinusClay. A special focus is given on the analysis of safety-relevant clay properties, e.g. swelling, cation-exchange capacity, surface area and hydraulic conductivity. The experiments simulating various pore water chemistries interacting with Boom Clay and Opalinus clay were studied as batch experiments at 90°C. The results indicate very limited reactivity of both clays with no major impact on their favourable properties. Low reactivity of clays is attributed to the buffering role of carbonates on one hand, and organic matter effectively protecting the clay mineral surfaces on the other hand. However, considering the real time frames of the heat phase, the results generated during one-year experiment should be regarded as indicative.