Development of concrete cask storage technology for spent nuclear fuel

Need of spent fuel storage in Japan is estimated as 10,000 to 25,000 t by 2050 depending on reprocessing. Concrete cask storage is expected due to its economy and risk hedge for procurement. The CRIEPI executed verification tests using full-scale concrete casks. Heat removal performances in normal and accidental conditions were verified and analytical method for the normal condition was established. Shielding performance focus on radiation streaming through the air outlet was tested and confirmed to meet the design requirements. Structural integrity was verified in terms of fracture toughness of stainless steel canister for the cask of accidental drop tests. Cracking of cylindrical concrete container due to thermal stress was confirmed to maintain its integrity. Seismic tests of concrete cask without tie-down using scale and full-scale model casks were carried out to confirm that the casks do not tip-over and the spent fuel assembly keeps its integrity under severe earthquake conditions. Long-term integrity of concrete cask for 40 to 60 years is required. It was confirmed using a real concrete cask storing real spent fuel for 15 years. Stress corrosion cracking is serious issue for concrete cask storage in the salty air environment. The material factor was improved by using highly corrosion resistant stainless steel. The environmental factor was mitigated by the development of salt reduction technology. Estimate of surface salt concentration as a function of time became possible. Monitoring technology to detect accidental loss of containment of the canister by the stress corrosion cracking was developed. Spent fuel integrity during storage was evaluated in terms of hydrogen movement using spent fuel claddings stored for 20 years. The effect of hydrogen on the integrity of the cladding was found negligible. With these results, information necessary for real service of concrete cask was almost prepared. Remaining subject is to develop more economical and rational technology for stress corrosion cracking of canister. (author)