Cold Spray Ni Coatings with Hybrid Peening Treatments for Addressing Corrosion in Stainless-Steel Used Fuel Storage Containers

Abstract

With nuclear power generation projected to rise in the coming decades, increasing volumes of spent nuclear fuel (SNF) waste will be generated. Long-term safe storage of SNF is of paramount importance to regulators and to the public. Currently, after a period of residence in water pools, SNF is stored in Dry Cask Storage Systems (DCSS) which are typically located in the vicinity of nuclear power plants. The long-term plan, after interim storage in DCSS, is to permanently dispose SNF by burying it deep underground in geological repository sites. However, in the United States (and many nuclear power nations), such permanent geological repository sites have not been fully identified. Thus, SNF will have to be stored in interim DCSS for multiple decades. The DCSS essentially consists of an inner austenitic stainless-steel canister surrounded by concrete overpack with a gap between the two to circulate ambient air for cooling the canister. A confluence of Cl-ions in the environment, tensile stresses, and sensitized microstructure at the welds in the stainless-steel canister could potentially render them susceptible to chloride-induced stress corrosion cracking (CISCC) in the long-term. While no instances of CISCC have been reported in operational casks, the extended reliance on DCSS for SNF storage warrants the development of technologies for mitigation and repair of corrosion and CISCC in the DCSS stainless-steel canisters. Several technologies are being investigated to address this issue, including friction stir welding, peening, and cold spray deposition. This research is focused on corrosion of cold sprayed nickel coatings (CS-Ni) and hybrid surface treatments involving peening treatments of the CS-Ni coatings. It is noted here that CS- Ni is being considered as one of the leading candidate technologies for the DCSS application by national programs. The substrate for these surface treatments was 304H stainless-steel. ‘H’ refers to high carbon (0.04% or higher) while the ‘L’ grade used for canisters contains < 0.04%C. ‘H’ grade is more prone to the detrimental effects of sensitization and therefore was selected to amplify the effects of sensitization. Most of this research was performed on sensitized 304H stainless steel substrates. Commercially pure gas-atomized Ni powder was used, and cold spray coating deposition was performed using 100% N2 or 95% He propellant gases. Peening of the coatings was performed using two approaches, cold spray peening (CSP) and ultrasonic nanocrystal surface modification (UNSM). CSP, a process developed at UW-Madison, involves performing cold spray at powder particle velocities lower than the critical particle velocity such that the impacting particles create the necessary plastic deformation of the surface particles but rebound off rather than bonding to the surface. Particle impact modeling based on ANSYS was used to predict the particle velocity for CSP. UNSM involves tapping the surface at high frequencies with a hard tungsten-carbide pin to impart high strain rate deformation in the near surface regions of the material. For the UNSM study, load and vertical amplitude of pin were investigated as variables. Separately, in collaboration with the Electric Power Research Institute (EPRI), UNSM was also investigated for repair of prototypical CISCC created in 304H stainless steel.