In-situ radionuclides diffusion experiment in a thermal gradient in the sandy facies of Opalinus Clay

Argillaceous rocks, such as Opalinus Clay, are considered potential hosts for high-level radioactive waste repositories due to their low permeability and ability to retard radionuclide migration. In these formations, diffusion is the primary transport mechanism for radionuclides. Previous laboratory studies have indicated that effective diffusion coefficients for non- or weakly-sorbing radionuclides increase exponentially with temperature between 0 and 70°C, suggesting that higher temperatures could enhance diffusion rates. However, the impact of temperature on radionuclide diffusion under in-situ conditions remains underexplored.

To address this gap, the DR-C experiment was initiated in 2019 at the Mont Terri Underground Research Laboratory (URL) in Switzerland. This in-situ study aims to investigate radionuclide diffusion in Opalinus Clay under a controlled thermal gradient. The experimental setup includes two 5-meter-long boreholes: one equipped with a heating module maintaining an 80°C temperature at the clay interface, and a control borehole at ambient temperature. A cocktail of radioactive tracers, including HTO, ¹²⁹I^-, ²²Na⁺, ¹³⁷Cs⁺, ⁶⁰Co²⁺, and ¹³³Ba²⁺, will be injected to monitor diffusion behavior. The injection is scheduled to commence at the beginning of 2025 and will run for one year. Upon completion, overcoring and subsequent chemical analyses will determine diffusion profiles, enhancing understanding of temperature effects on radionuclide migration in clay-rich host rocks.

This research is crucial for assessing the long-term safety of geological disposal facilities for radioactive waste, particularly concerning "worst-case scenarios" involving potential canister failure during the thermal phase. Insights gained from the DR-C experiment will inform safety assessments and contribute to public confidence in geological disposal solutions.