The DR-C Mont Terri Project: Inferring Effects of a Thermal Gradient on the Diffusion of Radionuclides in Opalinus Clay

In the context of the Mont Terri Project, the DR-C experiment investigates the impact of a thermal gradient on the diffusion of radionuclides in the sandy facies of Opalinus Clay. While current safety assessments for high-level radioactive waste disposal often assume containment throughout the thermal phase, this experiment addresses a more conservative scenario: radionuclide release during the thermal phase due to premature canister failure.The experiment compares two in situ setups: one reference borehole at ambient temperature and one heated borehole maintained at 80°C. Both are equipped with tracer injection systems and monitoring tools. A suite of tracers, including HTO, ¹²⁹I⁻, ⁶⁰Co²⁺, ¹³⁷Cs⁺, ²²Na⁺, and ¹³³Ba²⁺, will be used to study diffusion under these contrasting thermal conditions. Observation boreholes monitor temperature, strain, and porewater pressure to capture thermo-hydro-mechanical effects.Heating is expected to stabilize by Q3 2025, with tracer injection planned thereafter. The diffusion phase will last a minimum of two years, followed by over-coring and analysis. This experiment aims to generate critical in situ data to improve understanding of temperature effects on radionuclide migration, supporting more robust safety cases and addressing concerns linked to retrievability and public acceptance.