Experience of Storage by Dry Storage Cask in Fukushima Daiichi Nuclear Power Plant

Spent fuel storage using Dry Storage Cask (DSC) in Japan has been experienced approximately for 20 years since 1995. In addition to the advantage of easy maintenance, storage area flexibility and convenience of transportation, the safety of DSC is increasingly focused in particular after the Great East Japan Earthquake, because DSC is cooled by natural air circulation without utilities such as electric power during storage.Kobe Steel, Ltd. manufactured 20 DSCs which have forged shell for Tokyo Electric Power Company and 9 of them have been operated since 1995 at Fukushima Daiichi Nuclear Power Station. In 2000 after 5 years’ storage and 2005 after 10 years’ storage, the primary and secondary lids were opened for one of these DSCs as a representative, and the soundness of stored fuels and the securement of containment function were confirmed.At the Great East Japan Earthquake in March 11th, 2011, these 9 DSCs were being operated at the cask storage facility in the power station. A large amount of seawater, sand, rubble, etc. were flowed due to tsunami (seismic sea wave) in the cask storage facility, then the DSCs were completely submerged in the seawater temporarily. However, it was confirmed by the detailed investigations performed later that there were no problems with the containment function, the keeping sub-criticality function, the heat removal function, the radiation shielding function and the soundness of stored fuels for all 9 DSCs affected by the tsunami. These DSCs now continue to be operated with loaded spent fuels at the new cask temporary storage facility after cleaning of sealing surface for secondary lid and replacement of metal gasket. Furthermore, the other 11 DSCs in which spent fuels from the common pool were loaded are operated at the cask temporary storage facility.This paper introduces the outline of manufacturing process for the DSC and the experience of storage in Fukushima Daiichi Nuclear Power Station.