Neutron residual stress mapping of repaired spent nuclear fuel welded stainless-steel canisters

Stress corrosion cracking (SCC) caused by atmospheric deposition and deliquescence of aggressive compounds such as chloride-containing salts or oxidized sulfur species may pose a potential issue to the extended service life of spent nuclear fuel (SNF) storage canisters. When a chemically susceptible material is exposed to a hostile environment (e.g., coastal salt air) for a sufficient length of time, SCC may occur at locations on the canister which have undergone high tensile residual stress. These locations are often found in weld zones and the heat-affected zones (HAZ) of welded joints. Ideally, these cracks could be repaired by traditional fusion welding techniques, but the highly localized heat input of the repair welding may introduce additional high tensile residual stress and could increase the risk for future SCCs. This paper presents the results of neutron residual stress mapping of as-welded and repaired stainless steel specimens. The as-welded specimens were provided by Sandia National Laboratories from a canister mock-up manufactured using procedures similar to those used for actual canister production. ASME-qualified gas-tungsten arc welding (GTAW) was used to repair the specimens, and post-repair residual stresses were measured using neutron diffraction at the Oak Ridge National Laboratory (ORNL) High Flux Isotope Reactor (HFIR). As-welded and repaired specimen residual stress distributions are evaluated and compared. Significant residual stress profiles were identified from the received as-welded longitudinal weld sample. The generated residual stress profile from neutron diffraction is similar to that of the Sandia residual stress data obtained from deep-hole drilling. The GTAW-repaired specimen showed a notable redistribution in residual stresses and even introduced compressive stresses when compared with the as-welded condition. It appears that groove excavation and the well-controlled GTAW heat input resulted in a stress relief/annealing effect on the metal. The results of this comparison are expected to be used to guide improvements in future repair welding techniques.