Modelling of the Radionuclide Release from an Initially Defective Canister

To investigate the effect of geochemical conditions of the repository on the release rate of radionuclides from an initially defective canister, a mathematical model is proposed. The model is based on the concept that ligands diffuse through the damage into the canister and form complexes with uranium, increasing its solubility. The complexes then diffuse out to the surroundings. Accordingly, the model combines a chemical equilibrium model for the solubility of uranium under strongly oxidizing conditions, and a transport model for the release rate of uranium. The transport model takes into account the diffusion resistance in the amage as well as in the surrounding compacted bentonite clay and the water flow in the rock. Using this model, the sensitivity of the release rate of aqueous uranium species to the concentrations of various groundwater component species, to the diffusion coefficients and to other parameters is systematically investigated. The results show that, the concentrations of carbonate, phosphate, silicate and calcium species in deep groundwaters play the most important role in determining the release rate of radionuclides. The results help identify the key diffusion coefficients that are needed to obtain more reliable estimates of the release rate.