Electrochemical migration-induced mineral reactions at the bentonite-concrete interface in multi-barrier systems

Abstract

This study investigates the effect of electrochemical migration on the geochemical stability of multi-barrier nuclear waste repositories. We used a multicomponent reactive transport modeling approach to conduct two simulations: one considering only Fickian diffusion (FD) and another incorporating both Fickian diffusion and electrochemical migration using the Nernst-Planck equation (NP). The model domain consisted of bentonite, concrete, and granite barriers. The NP simulation results revealed Ca²⁺ fluxes moving against their concentration gradients across the bentonite-concrete interface (BCI), driven by Coulombic interactions. This uphill diffusion of Ca²⁺ is coupled with the downhill diffusion of SO₄²⁻, resulting in the concurrent migration of both ions from bentonite to concrete. This process triggers slow gypsum dissolution and precipitation near the BCI, even though local equilibrium with respect to gypsum was reached. These unexpected mineral reactions across the BCI persist even after gypsum is completely depleted in the bentonite near the interfaces, as long as the pore connection near the BCI is maintained. Our findings highlight the significance of electrochemical effects for the long-term stability of multi-barrier systems in nuclear waste repositories.