An Instrument Platform for Controlling Weld Microstructures to Minimize Stress Corrosion Cracking in Dry Storage Canister

Stress corrosion cracking (SCC) is a potential safety concern for welded stainless-steel dry storage canisters (DSC) for spent nuclear fuel (SNF). The welding procedure introduces high tensile residual stresses and changes in material properties in the heat-affected zone (HAZ) in the DSC. This might promote pitting and SCC crack initiation and growth when exposed to an aggressive chemical environment. Therefore, mitigation technologies are important for enhancing the reliability of long-term storage and maintenance of DSCs. Spectral Energies & Penn State University propose to develop a technology (processing platform and analysis software) for post-processing the welding microstructure that will have tremendous future for commercialization. The basic principle is highly localized annealing of the welded region, while leaving rest of the DSC undisturbed. The goal is to convert the non- equilibrium microstructure in the HAZ to a recrystallized one with minimum defect and residual stress. In addition, the microstructure should preferably be high angle grain boundary dominant, which further reduces risks of radiation damage. This is simply not possible with conventional annealing or conventional post- processing technologies. Conventional annealing requires very high temperatures and cannot specifically target the welded region. Post processing techniques (such as hot isostatic pressing) are also not feasible for the same reason. We propose an out of the box concept in materials processing, where tensile residual stress in distorted microstructure is eliminated by the momentum of electrons (also known as electron wind force). During Phase-I effort, we propose to explore the followings: (1) Demonstrate the proposed basic process of electron wind force by recrystallizing distorted metal specimen cut out from a welded region, (2) Map residual stress and grain boundary character of the processed specimen and compared that with the weld region to show the effectiveness of the microstructural and chemical defect elimination, (3) Parametric study of microstructural process control (residual stress as function of current density, time, temperature) to pave the foundation for a scaling up operation by projecting the instrument size, cost and operating economics, and (4) Design the prototype welding stress removal platform with robotic articulated arm and explore commercial partnership with the interested parties. We envision a robust robotic product capable of performing localized electro-thermal operation with the processing software as the ultimate outcome of this effort. This comprehensive effort will guide the development of a product that will have immediate application not only for nuclear storage canisters but for ship-building or other welding-heavy industries such as oil/gas fields.