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.

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In repair welding for nuclear reactor vessel, low alloy steels are affected by heat input during welding process. The conventional repair welding for wall steel constructions requires post weld heat treatment (PWHT) to achieve the desired microstructure properties. However, post weld heat treatment is very difficult for some structures in operating plants. In such case, temper-bead welding technique is available as a repair welding method. Temper-bead welding employs a multi-pass deposition of welding metal. Each layer of beads provides heat for thermal treatment of the previous weld bead or layer, which lowers hardness of the heat affected zone (HAZ) and improves mechanical properties like the toughness. Toshiba has developed underwater laser cladding and laser seal welding techniques for reactor components repair welding. In this report, some experimental results of laser based underwater temper-bead welding are presented.