Argillaceous formations have been studied for decades as potential candidates for the deep geological disposal of high-level and/or long-lived radioactive waste (HLW) in several countries. They present intrinsic physicochemical and thermo-hydro-mechanical properties that are favourable for ensuring containment and isolation safety functions. Numerous in-situ diffusion experiments have been conducted in several Underground Research Laboratories (URL), including the Mont Terri URL within the shaly facies of the Opalinus Clay. Nevertheless, most existing in-situ experiments do not account for the influence of therally induced gradients. The influence of such thermal gradients is particularly important in the context of safety assessments for geological disposal. These assessments generally assume that radionuclides remain fully contained within high-level waste canisters throughout the entire heat-generating phase. However, the potential early failure of waste canisters during the thermal phase, and the associated premature release of radionuclides, is a scenario that must be considered, especially when waste retrievability is required. These alternative scenarios highlight the need to improve our understanding of radionuclide transport under non-isothermal conditions. Thermally induced gradients can indeed modify porewater properties and alter diffusive transport mechanisms. Available surface-laboratory results indicate an exponential increase in effective diffusion coefficients between 0 and 70 °C for certain species, suggesting that elevated temperatures may enhance the diffusion of non- or weakly sorbing radio-nuclides in argillaceous formations. Furthermore, the presence of thermal gradients may also induce thermodiffusion. This physical phenomenon, also known as the Soret effect, generates an additional diffusive flux that superimposes on the flux driven by concentration gradients. The DR-C experiment at the Mont Terri URL was initiated to address this experimental gap. It aims to investigate radionuclide diffusion and sorption in Opalinus Clay under a controlled thermal gradient and is expected to provide valuable new insights for safety assessments of geological disposal concepts.
Pochet et al. (Thu,) studied this question.