Design Features that Enhance Spent Fuel Canister Integrity Under Drop Impact

In many spent fuel dry cask storage systems a welded steel canister provides the confinement boundary that prevents the release of radionuclides, not only during normal conditions of storage, but also, in the unlikely event of a drop or tip-over accident. Transfer and storage casks that contain steel canisters are typically lifted and moved, and the canister transferred, in a vertical upright position. Any drop of the cask or canister, therefore, induces high longitudinal compressive stresses in the canister shell and, depending on the drop height, possible buckling of the shell at the base of the canister. The buckling of the canister shell, in turn, can introduce high bending strains in the circumferential weld joining the shell and base plate. (This weld and the longitudinal welds in the canister shell are 100% radiographed and hydrostatically tested.) However, the strains at this location, caused by such an accidental drop, can be significantly increased by local design features, such as welded attachments and basket supports, which may act to constrain the free buckling of the shell. This paper evaluates one such design feature, longitudinal basket supports, and shows how a simple modification to the design can reduce the maximum plastic strain at the circumferential weld by more than 20 percent and, in turn, reduce the probability of the initiation of a weld failure by more than a factor of 5.