Radionuclides leaking from a damaged canister spread into the clay surrounding the canister and then migrate through various pathways into the mobile water in the rock fractures. To quantify uncertainties regarding the release to the far field, a sensitivity analysis is performed for variations in the parameters governing the release. Variations in the size of the damage in the canister, uranium solubility, sorption capacity of the clay, and hydraulic properties of the fractured rock nearest to the canister have to be accurately determined in order to minimize uncertainties in the release.