Spent nuclear fuel (SNF) in the United States is stored onsite at the commercial reactors where it is generated. The dry storage systems consist of welded 304/316 stainless steel canisters enclosed in passively-ventilated concrete overpacks. The ambient cooling air comes from the surrounding environment and contains typical atmospheric aerosols, including chloride-rich salts (e.g. Cl-
), which accumulate on the storage canister surface over time. When the canister surface cools sufficiently, surface salts can absorb water from the air and form corrosive surface electrolyte droplets. Over time, these droplets may interact with weld regions that have susceptible microstructures and high through-thickness tensile stresses to promote stress corrosion cracking (SCC). A study is underway to understand atmospheric corrosion mechanisms and morphology that leads to SCC initiation on metals currently used for SNF dry storage canisters and on other corrosion-resistant alloys (CRA) that could be used for future storage. Austenitic SS304H (both annealed and sensitized), PH13-8 Mo SS, and Ni-based Alloy 600 corrosion test coupons have been coated with finely disseminated synthetic sea salt deposits using an ink-jet printer, simulating deposition of naturally occurring sea-salt aerosols, and exposed to expected atmospheric conditions with the aim of studying the resulting corrosion morphology, surface electrolyte chemistry, and SCC crack initiation behavior. Corrosion morphology (pit size distribution, pit number density, and shape) will be characterized as a function of salt surface load density, relative humidity, temperature, and exposure time using optical profilometry, FIB-SEM, microXCT, and robomet (a robotic cross-sectional metallographic examination technique). The surface electrolyte and corrosion products will be characterized by SEM-EDS, TOF-SIMS, and microRaman spectroscopy. Identical salt loading and atmospheric conditions will be applied to dog-bone style tensile samples used for SCC studies where intermittent high R ripple-style fatigue loading will be applied to mark the progression of fracture after crack initiation has occurred. These marks on the fracture surface will be used to identify the critical crack initiating features. Ultimately, the results of this study will be used for modeling initiatives aimed at designing new CRA alloys for next generation SNF tanks and to aid in lifetime assessments for current SNF temporary storage canisters.
This work is supported at The Ohio State University by the Center for Performance and Design of Nuclear Waste Forms and Containers (WastePD), and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. The work conducted under WastePD is all stress corrosion cracking analysis and corrosion morphology investigations utilizing optical profilometry. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.