Solution to technical issue for practical use of concrete cask in Japan.

Hitachi Zosen provides design and fabrication of transportable storage metal cask, and is developing concrete cask system for future demand in Japanese market. During the long-term operation of austenitic stainless steel canister, the possibility of chloride induced SCC (Stress Corrosion Cracking) shall be considered. In order to introduce the concrete cask system to Japanese market, the important technical issue is to prevent the occurrence of SCC and to secure containment function of transportable storage canister made of austenitic stainless steel during long storage period.One of the measures to prevent the SCC is to convert surface tensile stress to compressive one. It is confirmed that both zirconia peening and ball burnishing are effective methods to prevent SCC. Although those methods can be expected to prevent SCC for long storage period, it is not established how to apply to the surface of canister. Furthermore, a method to ensure that SCC does not occur is not identified clearly during the storage period.To solve above issues, Hitachi Zosen proposes as follows:For the prevention of SCC occurrence, residual stress improvement (tensile to compressive) would be applied during shop fabrication and after lid welding of the canister during operating floor.In consideration of additional safety, it is proposed that helium leak detection is carried out by measuring surface temperature during the storage.Also it is necessary for us to investigate an appropriate manufacturing sequence to reduce residual tensile stress due to welding.