The impact of redox changes on uranium migration using a field tracer test and reactive transport modeling

Safety assessment of a deep geological repository is essential to ensure the long-term isolation of spent nuclear fuel and protection of the environment. Uranium migration in groundwater is a key factor in evaluating the safety of the spent nuclear fuel disposal, and uranium mobility is significantly influenced by redox conditions. However, quantifying the effects of redox changes at the field-scale remains challenging. This study investigates uranium migration under changing redox conditions using a tracer test and reactive transport modeling. The study site consists of uranium-bearing black slate and has weakly oxidizing groundwater conditions (1.62 mg/L DO). Strongly oxidizing conditions were induced by injecting DO-saturated groundwater (10.16 mg/L DO) during the tracer test. Uranium migration during the tracer test was quantified using a reactive transport model (RTM) that couples a discrete fracture-matrix (DFM) model implemented in COMSOL Multiphysics with the geochemical code PhreeqcRM. The tracer test showed that uranium mobility increased under shifting redox conditions, with the maximum uranium concentration rising by 85.2% and the cumulative mass increasing by 66.0%. The RTM results indicate that desorption primarily contributes to the enhancement of uranium mobility, and that the oxidative dissolution rate of UO₂ varies by up to six-fold depending on redox conditions. These findings highlight that redox changes can increase uranium mobility and the need to consider redox evolutions in the safety assessment of spent nuclear fuel disposal.