Deployment of Laser Peening to Prevent CISCC of Nuclear Fuel Dry Storage Canisters

By generating very deep levels of compressive stress, laser peening can eliminate stress corrosion cracking of materials of use in the nuclear industry; tested materials include Alloys N06600 (Inconel 600), N06690 (Inconel 690), N06022 (Alloy C22) and stainless steels K44220 (300M), S30400 (304) and S31600 (316L). In this work we take a further step in direct application to the welds of multi-purpose canisters (MPCs) employed for dry storage of spent fuel at nuclear plants. This storage is a temporary approach awaiting interim or permanent storage.. For storage in coastal, lakeside and even humid environments, air-entrained chlorides can make the canister welds susceptible to pitting and chloride induced stress corrosion cracking (CISCC). Using ASTM G36 (2013) accelerated corrosive testing we evaluated CISCC lifetimes of welded 316L stainless steel canister plates configured to MPC geometry showing in excess of 19 times lifetime increase of laser peened panel sections vs. those left as-welded. Specifically cracks never developed within and were arrested when propagating into the laser peened area. We measured residual stress in test plates and related it to calculations of stress intensity and depth expected in the full canister geometry. We discuss the relevance of stress depth to pitting depth and crack growth rates. Our two-dimensional stress mapping shows that high energy laser peening provides deep (> 5 mm) plastic compression in the canister material and geometry. In parallel, as we gained customer and NRC approvals, we developed and deployed the hardware and control technology that enabled a transportable system to peen canisters welds during the fabrication process. The canisters are now loaded or in process of being loaded at the San Onofre Nuclear Power Plant.