Solute Transport in Bentonite Buffer Under Thermal Hydraulic Mechanical Coupling

Abstract This paper investigates solute transport processes in the bentonite buffer of a Deep Geological Repository (DGR) for used nuclear fuel, using the Canadian concept as an example. The design relies on a multi-barrier system in which the used fuel container (UFC) provides containment and the surrounding bentonite buffer protects the UFC while retarding radionuclide migration following a potential breach. Solute transport in the buffer is strongly influenced by coupled thermal-hydraulic-mechanical (THM) processes. A coupled THM-transport model was developed to evaluate (1) reactive transport of bisulfide $$\left({\text{H}\text{S}}^{-}\right)$$ HS - from the host rock to the UFC and associated UFC corrosion; and (2) radionuclide migration following a hypothetical UFC breach. The model builds on previous verification and validation and is further assessed against laboratory-scale bisulfide transport experiments and a large-scale thermal diffusion experiment at the Tournemire Underground Research Laboratory (France). The results show that temperature-dependent diffusion is the dominant transport mechanism, significantly enhancing solute migration, whereas hydraulic desaturation has limited influence and the Soret effect is negligible at repository scale. Under the reference conditions, adsorption and $$\text{F}\text{e}\text{S}$$ FeS precipitation delay detectable HS − flux to the UFC until approximately 5 × 10 5 years and limit the predicted corrosion depth to ~ 0.05 mm after one million years. In the breach scenario, the buffer effectively delays and attenuates moderately/strongly sorbing radionuclides (e.g., 135 Cs, 237 Np). The effectiveness in retarding radionuclides' migration is most sensitive to bentonite diffusivity and sorption capacity.