Solute transport processes in the bentonite buffer of a deep geological repository for used nuclear fuel with thermal-hydraulic-mechanical coupling

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, where the used fuel container (UFC) ensures containment, and the surrounding bentonite buffer seals gaps between the container and host rock. The buffer serves two key functions: (i) protecting the UFC against processes that may compromise containment, and (ii) retarding radionuclide migration in the event of UFC breach. Solute transport in the buffer is strongly influenced by coupled thermal-hydraulic-mechanical (THM) processes. A fully coupled THM-transport model was developed to evaluate two scenarios: (1) reactive transport of bisulfide \(\:\left({HS}^{-}\right)\) from the host rock to the UFC and its implications for long-term corrosion; and (2) radionuclide migration toward the host rock following a hypothetical breach. The model builds on previous verification and validation and is further validated here against laboratory-scale bisulfide transport experiments and a large-scale thermal diffusion experiment at the Tournemire Underground Research Laboratory (France). Model–data agreement demonstrates that adsorption and \(\:FeS\) precipitation significantly retard bisulfide migration, delaying detectable flux to the UFC by ~ 5×10⁵ years and limiting copper corrosion to ~ 0.05 mm over 1 Myr. Temperature-dependent diffusion is the dominant transport mechanism, while the Soret effect is negligible at repository scale. In the breach scenario, the buffer effectively delays and attenuates moderately/strongly sorbing radionuclides (e.g., ¹³⁵ Cs, ²³⁷ Np). Overall performance is most sensitive to bentonite diffusivity and sorption capacity, highlighting key parameters for design and safety assessment.