As a countermeasure against high residual stress, we have developed some residual stress improvement methods, as 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 being able to approach from outer surface only. 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 BMI because BMI inner surface is very narrow space (inner diameter; approximately 10–15mm) and BMI J-weld is 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 mitigation. And the compressive stress induced by WJP is 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 the compressive can prevent the occurrence of PWSCC.

No abstract

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.

PWSCC incidents of Alloy 600 in vapor phase environment of pressurizer have been confirmed at several PWR plants. Vapor phase of pressurizer is filled with vapor from primary water, and the inner surface is covered with liquid film. Chemistry of the liquid film may be different from primary water, and this may cause different PWSCC susceptibility. Therefore the chemistry of liquid film of vapor phase has been investigated using simulated mock-up tests, and PWSCC susceptibility of 152 weld metal and TT600 (SG tube) has been investigated under the chemistry of the liquid film of vapor phase and primary water. According to the result of the chemistry investigation tests using mock-up of pressurizer, the liquid film environment was evaluated as follows: DH2 concentration: 300cc/kg·H2O, B:150ppm, Li<0.1ppb, pH320°C:5.6 under the primary water chemistry condition is DH2 concentration:30cc/kg·H2O, B:1950ppm, Li:3.7ppm, pH340°C:6.9. DH2 concentration of the liquid film is ten times higher and pH is lower than that of primary water. PWSCC susceptibility tests have been performed under the environment of the liquid film and primary water. No PWSCC crack propagation of 152 weld metal is confirmed in vapor phase environment. Crack growth rate of TT600 in vapor phase environment of pressurizer is not particularly high compared with that in primary water environment. It is confirmed that Alloy 690 (152 weld metal) has no PWSCC susceptibility under vapor phase environment of pressurizer. The difference of PWSCC susceptibility for Alloy 600 between vapor phase of pressurizer and primary water environment is not significant.

It has long been known that the most effective in a countermeasure for stress corrosion cracking in pipe and nozzle welds is by reducing the residual stress in the portion of the weld exposed to the corrosive environment. An irradiated laser stress improvement process (L-SIP) was introduced as a method to improve residual stress inside steel pipes and nozzles. L-SIP has been applied to the pressurizer nozzles in actual plant, Tsuruga unit 2 Japan, for the first time in the world. The nozzles to which this process was applied are the surge nozzle (September 2007), safety nozzles, relief nozzle and spray line nozzle (April 2010). L-SIP can be applied without inner surface cooling because the high power laser beam can generate the sufficient temperature difference without such cooling. Where necessary to achieve optimum temperarure difference, water cooling may also be applied at the inner surface. At Tsuruga unit 2, L-SIP was successfully applied to the spray line nozzle in air-cooling mode, and the surge nozzles, 3 safety nozzles and relief nozzle in water-cooling mode.