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In this study, Johnson-Cook fracture strain model considering the effect of stress triaxiality and strain rate is determined for austenitic stainless steel 304. Tensile test data of four different stress triaxiality and six different strain rate conditions are used to determine the parameters in the J-C fracture strain model. To see the effect of local variation of stress triaxiality and strain rate in the specimen, the J-C fracture models are determined in two different ways. The first case uses the initial stress triaxiality and nominal strain rate, and the second case uses the average value of local stress triaxiality and strain rate obtained from finite element analysis. The use of initial stress triaxiality gives conservative estimate of fracture strain at low stress triaxiality, and non-conservative estimate at high stress triaxiality. The use of nominal strain rate gives overall conservative estimate of fracture strain.

In this paper, a strain-based failure assessment is performed on a canister made of stainless steel when a spent nuclear fuel dry storage system goes through a drop accident, to investigate the effects of strain rate on strain-based failure assessment results. The KORAD-21 multi-purpose dry storage container system developed for interim storage and transportation at the Korea Radioactive Waste Agency (KORAD) is considered. A finite element (FE) analysis is performed on a 1m puncture drop of the KORAD-21 model. Based on the FE results, the canister under a 1m puncture drop is evaluated by two different criteria: (1) strain-based acceptance criteria suggested in ASME Boiler and Pressure Vessels Code Section III, Appendix FF, “Strain-based acceptance criteria for energy-limited events” and (2) the Johnson-Cook fracture strain model based on experimental data. The difference between the two criteria is that the Johnson-Cook fracture strain model expresses the true fracture strain as a function of stress triaxiality and strain rate, whereas the formula in App. FF establishes strain limit (combination of uniform strain and true fracture strain) as a function of stress triaxiality only. In this study, the safety margins of Appendix FF are analyzed by comparing the failure assessment results for canister drop simulation with those applying the Johnson-Cook fracture strain model.

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