Flaw Stability Analysis of Surface Cracks in DOE Standard Canisters Under Operation Loads and Welding Residual Stresses

Abstract Many commercial spent nuclear fuel (SNF) storage canisters are made of stainless steel and located in coastal regions for long-term storage. These canisters are fabricated by welding and contain large welding residual stresses (WRS). As a result, the canisters may be susceptible to chloride induced stress corrosion cracking (CISCC). The flaw stability analysis of these canisters was performed (i.e., PVP2015-45935 and PVP2016-63887) for through-wall flaws and for surface flaws, respectively, based on the fracture mechanics method codified by API 579-1/ASME FFS-1-2007 Edition was adopted. This paper extends the codified fracture mechanics method to perform the flaw stability analysis for DOE standard SNF canisters with mechanical surface flaws, where the normal and accident operation loading conditions combined with WRS are considered. It is noted that the newest version of API 579-1/ASME FFS-1, 2021 Edition and the old API 579-1/ASME FFS-1, 2007 Edition recommend completely different through thickness WRS distributions for both axial and hoop flaws existing in a longitudinal or a circumferential weld in a cylindrical vessel. Accordingly, the newly recommended WRS distributions are adopted in this work. In addition, an equivalent residual stress distribution is proposed with WRS reduction with distance from the weld centerline to reduce the over-conservatism embedded in the previous work. Moreover, two sets of new closed-form solutions of the stress intensity factor K are developed in terms of the tabular data of the K factor provided in API 579-1/ASME FFS-1, 2021 Edition and used to determine more accurate flaw sizes at flaw instability. With these three updates, the flaw instability is performed for the outside surface flaws in the DOE (US Department of Energy) standard canister (DOESC) and the flaw stability assessment results are obtained for the DOESC.