Activity release calculation from the near-field of a repository for spent fuel

It is essential for the safety assessment of a repository to analyze the release and transport of radionuclides from the near-field to the geosphere and subsequently to the surface environment. A new model has been developed in COMSOL Multiphysics based on input data from the safety case for the disposal of spent nuclear fuel at Olkiluoto. Complex processes, for example, nuclide decay, material corrosion, diffusion, sorption, and advection, are coupled in COMSOL Multiphysics. Under reasonable assumptions, the release rates of C-14, Cl-36, I-129, Cs-135, and Ni-59 are compared with results from simulations by GoldSim. Higher release rates are observed for these five isotopes. This discrepancy may arise from two factors. On the one hand, an excessively coarse grid in GoldSim enhances nuclide transport properties, leading to a greater accumulation of nuclides in the buffer in areas below the failed canister. On the other hand, important information is omitted in the 2D model. This suggests that the release rates calculated by GoldSim may require reconsideration. Although higher release rates were confirmed in this 3D model, the release rates of Tc-99, Np-237, Pu-239, and Pu-242 remain at low levels after 10 6 years (the timeframe for safety assessment in Finland and Sweden). Further development is possible in COMSOL Multiphysics, such as the replacement of spent fuel with vitrified glass and the implementation of a chemical reaction module. With this new model, greater confidence is achieved in the calculation of radionuclide release and transport. This is crucial for the safety assessment of repositories.