Disposal canister for spent nuclear fuel design report

The report provides a summary of the design of the canister for final disposal of Finnish spent nuclear fuel. The canister structure consists of a cylindrical massive nodular graphite cast iron insert covered by a 50 mm thick copper overlay. The canister has three versions, one for each reactor type in Finland. The fuel is sealed into the canisters in whole fuel assemblies including the possible flow channel outside the bundle. The capacity of the canister is 12 assemblies of BWR or VVER 440 fuel and 4 assemblies of EPR fuel. The canister shall be tight with a high probability for at least 100 000 years. The good and long lasting tightness requires: good initial tightness that is achieved by high quality requirements and extensive quality control; good corrosion resistance, which is obtained by the overpack of oxygen-free copper; and mechanical strength of the canister, that is ensured by analyses; the following loads are considered: hydrostatic ground water pressure, even and uneven swelling pressure of bentonite, thermal effects, and elevated hydrostatic pressure during glaciation. The allowed stresses and strains are set in such a way that reasonable engineering safety factors are obtained in all assessed design base loading cases. The canister shall limit the radiation dose rate outside the canister to minimise the radiolysis of the water in the vicinity of the canister. Moreover, the canister insert shall keep the fuel assemblies in a sub-critical configuration even if the void in the canister is filled with water due to postulated leakage. The design basis of the canister is set, the performed analyses are summarised and the results are assessed and discussed in the report.