As a countermeasure against high residual stress, we have developed some residual stress improvement methods; Water Jet Peening (WJP) [1] [2] for components installed in water, Shot Peening by Ultrasonic-wave vibration (USP) for components installed in air, and outer surface irradiated Laser Stress Improvement Process (L-SIP)[3] for components that can only be accessed from the outer surface.

WJP is applied to Reactor Vessel (RV) outlet/inlet nozzle safe-end joints (Alloy600 weld metal), RV Bottom Mounted Instrument (BMI) inner surface and J-weld. Especially, it is difficult to apply the technology to BMI because BMI inner surface is a very narrow space (inner diameter; approximately 10–15mm) and BMI J-weld configuration is a complicated 3-dimensional form.

On the occasion of actual application, we carry out the verification tests and check that a stress improvement was effective as one of PWSCC mitigations. And the compressive stress induced by the WJP was verified to continue to exist under actual plant operation conditions.

Thus, in addition to replacing the material with Alloy 690, converting the residual stress to compressive one can prevent the occurrence of PWSCC.

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