Koji Okimura

10-kW-class YAG laser application for heavy components

The authors have put the YAG laser of the kW class to practical use for repair welding of nuclear power plant steam generator heat exchanger tubes, all-position welding of pipings, etc. This paper describes following developed methods and systems of high power YAG laser processing. First, we apply the 6 kW to 10 kW YAG lasers for welding and cutting in heavy components. The beam guide systems we have used are optical fibers which core diameter is 0.6 mm to 0.8 mm and its length is 200 m as standard one. Using these system, we can get the 1 pass penetration of 15 mm to 20 mm and multi pass welding for more thick plates. Cutting of 100 mm thickness plate data also described for dismantling of nuclear power plants. In these systems we carried out the in-process monitoring by using CCD camera image processing and monitoring fiber which placed coaxial to the YAG optical lens system. In- process monitoring by the monitoring fiber, we measured the light intensity from welding area. Further, we have developed new hybrid welding with the TIG electrode at the center of lens for high power. The hybrid welding with TIG-YAG system aims lightening of welding groove allowances and welding of high quality. Through these techniques we have applied 7 kW class YAG laser for welding in the components of nuclear power plants.

PWSCC of Nickel Base Alloys in Vapor Phase Environment of Pressurizer

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·H2 O, B:150ppm, Li<0.1ppb, pH320°C :5.6 under the primary water chemistry condition is DH2 concentration:30cc/kg·H2 O, 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.Copyright

Reliability of Water Jet Peening as Residual Stress Improvement Method for Alloy 600 PWSCC Mitigation

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.Copyright

Development of Outer Surface Irradiated Laser Stress Improvement Process (L-SIP)

Improvement of residual stress is effective in a countermeasure to deal with the stress corrosion cracks in pipe welds. A irradiated laser stress improvement process (L-SIP) will be introduced as a method to improve residual stress inside steel pipes. This work method is to improve inner surface residual stress from tensile stress to compressive stress by irradiating laser beam around the welds of steel pipe and utilizing the temperature differences between inner and outer surface.Copyright

Advancement of Lining Inspection Technology Inside Seawater Piping

Seawater piping employed for cooling of emergency diesel generator and various components at Nuclear power plant, are internally lined for protection against seawater corrosion. (Fig. 1)However, the lining materials such as rubber or polyethylene tend to incur peel-off or crack due to aged deterioration occurred by the operation, resulting in corrosion of the piping. (Fig. 2)Internal piping integrity of seawater piping is usually performed by periodic visual inspection. But, for the 4B to 20B pipes it has been a challenge to detect the initial degradation because the inspector cannot get into the pipes.Pinhole detection technology that enables detection of microscopic damages and cracks is available to use as means to detect its initial state of degradation, which seems effective as preventive maintenance of the lining.Such being the case, we are developing Seawater Piping Inside Inspection Equipment applicable to the 4B to 20B sizes of pipes by evolving the conventional pinhole detection technology.© 2014 ASME

Reliability of Water Jet Peening as Residual Stress Mitigation

As a countermeasure against high residual stress, some residual stress improvement methods have been developed; 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, the verification tests have been carried out and checked 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.Copyright

Reliability of Water Jet Peening as Residual Stress Improvement

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.Copyright

Reliability of Water Jet Peening for Alloy 600 PWSCC Mitigation

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.Copyright

Application of L-SIP to Pressurizer Nozzles

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.Copyright

Application of Outer Surface Irradiated Laser Stress Improvement Process (L-SIP) to Pressurizer as Residual Stress Improvement Method for Alloy 600 PWSCC Mitigation

Improvement of residual stress is effective in a countermeasure to deal with the stress corrosion cracks in pipe welds. A irradiated laser stress improvement process (L-SIP) will be introduced as a method to improve residual stress inside steel pipes. This work method is to improve inner surface residual stress from tensile stress to compressive stress by irradiating laser beam around the welds of steel pipe and utilizing the temperature differences between inner and outer surface. Recently this method is applied to PWR pressurizer surge nozzle on TRUGA unit 2.Copyright