Development of Ultrasonic Shot Peening Technique for Reactor Components to Improve Structural Integrity Against Stress Corrosion Cracking

The ultrasonic shot peening (USP) technique has been developed for boiling water reactor (BWR) components as a countermeasure against stress corrosion cracking. The effects on residual stress of USP for type 316L stainless steel and alloy 600 were evaluated. Compressive residual stress layer of 0.5 mm from the surface were formed on the specimens after USP using stainless steel ball with a diameter of 3 mm. Cross-sectional hardness measurement revealed that the increase of hardness due to USP is not significant compared with shot peening (SP). The FEM calculation showed the plastic strain induced by the impacts of 3 φ shot with 5 m/s is lower than those of 0.6 φ shot with 50 m/s. It suggests USP process suppresses the degree of work hardening in comparison with SP process. Dissimilar weld joint specimens which simulate the material and dimension of the shroud weld line H7 were examined to confirm the applicability of USP. The experimental result reveals that USP technique is applicable to reactor internal components as stress modification process.