Reactive transport modeling for uranium migration under environmental stress near high-level radioactive waste repository.

This study investigates the impact of decay heat and seawater intrusion on uranium migration through the bentonite-granite system in a high-level radioactive waste repository. A coupled COMSOL-IPhreeqc model (COM-PHREE) was developed and validated using three benchmark cases. The validated COM-PHREE was applied to three scenarios: 1) a standard scenario without major environmental changes, 2) a scenario with decay heat, and 3) a scenario with decay heat and seawater intrusion. A thorough analysis of environmental evolution including aqueous speciation, adsorption, and mineral reactions, was conducted to assess its impact on uranium migration. Under the standard scenario, low solubility and high adsorption affinity of uranium caused relatively low uranium concentration (<1.10 × 10 mol/L). In the second scenario, decay heat induced complex environmental evolutions such as uranium speciation changes, enhancing uranium solubility and concentration in bentonite (<9.00 × 10 mol/L). For the third scenario, seawater intrusion considerably altered the bentonite system, particularly by changing redox potential. However, due to the buffering effect of pyrite, the bentonite near the canister remained in reducing condition, effectively limiting the increase of uranium concentration (<1.00 × 10 mol/L). These findings enhance the understanding of uranium migration process in bentonite and demonstrate its effectiveness as a barrier under various conditions.