Model calculations of the migration of radionuclides from a repository for a spent nuclear fuel

Release and migration calculations for all important radionuclides which potentially could escape from a repository for spent nuclear fuel have been performed. The calculations have been done for a repository in fissured crystalline rock with models which account for the leaching of the nuclides from the fuel due to dissolution in the moving water and the further transport along the fissures of the rock. Radionuclide sorption retarding the nuclides is accounted for by the concept of matrix diffusion and sorption on the microfissure surfaces. Dispersion and channeling are modeled as Fickian dispersion. Chain decay is accounted for. Calculations have been performed for migration distances of 5, 50 and 100 m. Sorption data and solubility data for oxidizing as well as reducing conditions have been used. Transport by sorption on colloidal matter in the water is also treated. In the central case the leaching starts after 100.000 years due to the long life time of the canister, but calculations have also been made for a case of some defective canisters where the leaching starts after 100 years. The results are presented as release rates to the biosphere at the various times. (author)