Naruhiko Mukai

RESIDUAL STRESS IMPROVEMENT IN METAL SURFACE BY UNDERWATER LASER IRRADIATION

Abstract Laser shock processing of water-immersed material was developed for improving the residual surface stress of metal components. The process changes the stress field from tensile to compressive by means of impulsive pressure of laser-induced plasma generated through the ablative interaction of the intense laser pulse with the material. The plasma, generated by the irradiation of second harmonic of a Q-switched Nd:YAG laser (SH-YAG, λ = 532 nm) on an SUS304 test piece, was directly observed by imaging the plasma radiation with a gated image intensifier and a charged coupled device (CCD) camera. Comparing the observed image to the plasma expansion velocity calculated with an analytical model, we deduced that about 20% of the plasma internal energy would represent the thermal energy. The calculation of the plasma pressure with this result showed that it exceeded 2 GPa in water and the yield stress of SUS304, when a typical laser pulse of the SH-YAG impinged on a water-immersed SUS304 test piece. A residual compressive stress exceeding 200 MPa was built over 200 μm in depth, by scanning the SH-YAG focused on a spot of 0.75 mm diameter with a power density of 60 TW/m2 and a pulse duration of 5 ns.

Process and application of shock compression by nanosecond pulses of frequency-doubled Nd:YAG laser

The authors have developed a new process of laser-induced shock compression to introduce a residual compressive stress on material surface, which is effective for prevention of stress corrosion cracking (SCC) and enhancement of fatigue strength of metal materials. The process developed is unique and beneficial. It requires no pre-conditioning for the surface, whereas the conventional process requires that the so-called sacrificial layer is made to protect the surface from damage. The new process can be freely applied to water- immersed components, since it uses water-penetrable green light of a frequency-doubled Nd:YAG laser. The process developed has the potential to open up new high-power laser applications in manufacturing and maintenance technologies. The laser-induced shock compression process (LSP) can be used to improve a residual stress field from tensile to compressive. In order to understand the physics and optimize the process, the propagation of a shock wave generated by the impulse of laser irradiation and the dynamic response of the material were analyzed by time-dependent elasto-plastic calculations with a finite element program using laser-induced plasma pressure as an external load. The analysis shows that a permanent strain and a residual compressive stress remain after the passage of the shock wave with amplitude exceeding the yield strength of the material. A practical system materializing the LSP was designed, manufactured, and tested to confirm the applicability to core components of light water reactors (LWRs). The system accesses the target component and remotely irradiates laser pulses to the heat affected zone (HAZ) along weld lines. Various functional tests were conducted using a full-scale mockup facility, in which remote maintenance work in a reactor vessel could be simulated. The results showed that the system remotely accessed the target weld lines and successfully introduced a residual compressive stress. After sufficient training for operational personnel, the system was applied to the core shroud of an existing nuclear power plant.

Laser Peening without Coating to Mitigate Stress Corrosion Cracking and Fatigue Failure of Welded Components

The authors have applied laser peening without coating (LPwC) to metallic materials. Compressive residual stress nearly equal to the yield strength of the materials was imparted on the surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC had a significant effect to prevent the SCC initiation of sensitized materials of SUS304, Alloy 600 and the weld metal, Alloy 182. Push-pull type fatigue testing demonstrated that LPwC drastically enhanced the fatigue strength of fillet-welded rib-plates of SM490A.

Development and Application of Laser Peening System for PWR Power Plants

Laser peening is a process to improve residual stress from tensile to compressive in surface layer of materials by irradiating high-power laser pulses on the material in water. Toshiba has developed a laser peening system composed of Q-switched Nd:YAG laser oscillators, laser delivery equipment and underwater remote handling equipment. We have applied the system for Japanese operating BWR power plants as a preventive maintenance measure for stress corrosion cracking (SCC) on reactor internals like core shrouds or control rod drive (CRD) penetrations since 1999. As for PWRs, alloy 600 or 182 can be susceptible to primary water stress corrosion cracking (PWSCC), and some cracks or leakages caused by the PWSCC have been discovered on penetrations of reactor vessel heads (RVHs), reactor bottom-mounted instrumentation (BMI) nozzles, and others. Taking measures to meet the unconformity of the RVH penetrations, RVHs themselves have been replaced in many PWRs. On the other hand, it’s too time-consuming and expensive to replace BMI nozzles, therefore, any other convenient and less expensive measures are required instead of the replacement. In Toshiba, we carried out various tests for laser-peened nickel base alloys and confirmed the effectiveness of laser peening as a preventive maintenance measure for PWSCC. We have developed a laser peening system for PWRs as well after the one for BWRs, and applied it for BMI nozzles, core deluge line nozzles and primary water inlet nozzles of Ikata Unit 1 and 2 of Shikoku Electric Power Company since 2004, which are Japanese operating PWR power plants. In this system, laser oscillators and control devices were packed into two containers placed on the operating floor inside the reactor containment vessel. Laser pulses were delivered through twin optical fibers and irradiated on two portions in parallel to reduce operation time. For BMI nozzles, we developed a tiny irradiation head for small tubes and we peened the inner surface around J-groove welds after laser ultrasonic testing (LUT) as the remote inspection, and we peened the outer surface and the weld for Ikata Unit 2 supplementary. For core deluge line nozzles and primary water inlet nozzles, we peened the inner surface of the dissimilar metal welding, which is of nickel base alloy, joining a safe end and a low alloy metal nozzle. In this paper, the development and the actual application of the laser peening system for PWR power plants will be described.Copyright

Remote detector of alpha-ray using ultraviolet ray emitted by nitrogen in air

Alpha Camera was developed that can identify 1.5-kBq alpha emitter in 30-sec from 1-m away. The alpha camera equips mirror optics so that the alpha-induced ultraviolet lights are transferred while gamma-ray backgrounds are removed.

Development and applications of laser peening without coating as a surface enhancement technology

Laser peening without coating (LPwC) is an innovative surface enhancement technology to mitigate fatigue and stress corrosion of metallic materials by imparting a compressive residual stress. Toshiba has established a process without coating, whereas the coating is inevitably required in conventional process of laser peening to protect the surface from melting. Since the energy of laser pulses in LPwC is significantly small compared to that in the conventional process, a commercially available Nd:YAG laser can be used, and moreover, an optical fiber can be utilized to deliver the laser pulses. Compressive residual stress nearly equal to the yield strength of the materials was introduced on the surface after LPwC. The depth of the compressive residual stress reaches 1 mm or more from the surface. High-cycle fatigue tests proved that LPwC significantly prolonged the fatigue lives despite the increase in surface roughness due to ablative interaction of laser pulses with material surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC completely prevents SCC of sensitized austenitic stainless steels, nickel-base alloys and their weld metals. LPwC has been used since 1999 to prevent SCC of core shrouds or nozzle welds of ten nuclear power reactors of both boiling water reactor (BWR) and pressurized water reactor (PWR) types, already covering nearly one fifth of the existing nuclear power plants (NPPs) in Japan.

Laser-Based Maintenance and Repair Technologies for Reactor Components

Stress corrosion cracking (SCC) is the major factor to reduce the reliability of aged reactor components. Toshiba has developed various laser-based maintenance and repair technologies and applied them to existing nuclear power plants. Laser-based technology is considered to be the best tool for remote processing in nuclear power plants, and particularly so for the maintenance and repair of reactor core components. Accessibility could be drastically improved by a simple handling system owing to the absence of reactive force against laser irradiation and the flexible optical fiber. For the preventive maintenance, laser peening (LP) technology was developed and applied to reactor components in operating BWR plants. LP is a novel process to improve residual stress from tensile to compressive on material surface layer by irradiating focused high-power laser pulses in water. We have developed a fiber-delivered LP system as a preventive maintenance measure against SCC. Laser ultrasonic testing (LUT) has a great potential to be applied to the remote inspection of reactor components. Laser-induced surface acoustic wave (SAW) inspection system was developed using a compact probe with a multi-mode optical fiber and an interferometer. The developed system successfully detected a micro slit of 0.5mm depth on weld metal and heat-affected zone (HAZ). An artificial SCC was also detected by the system. We are developing a new LP system combined with LUT to treat the inner surface of bottom-mounted instruments (BMI) of PWR plants. Underwater laser seal welding (LSW) technology was also developed to apply surface crack. LSW is expected to isolate the crack tip from corrosive water environment and to stop the propagation of the crack. Rapid heating and cooling of the process minimize the heat effect, which extends the applicability to neutron-irradiated material. This paper describes recent advances in the development and application of such laser-based technologies.© 2004 ASME

Development of Remotely Operated Inspection Devices for BWR Internals

Two ultrasonic testing (UT) devices to inspect the internals of nuclear reactor have been developed. The one is a jet pump UT device to inspect the inner weld line of the jet pump, and the other is a shroud UT device to inspect the outer weld line of the shroud. The jet pump UT device is mainly composed of an inspection probe scanner and a wheeled platform with a telescopic guide. Since the inspection probe scanner has been designed slim enough to pass through the narrow opening of the jet pump nozzle, it can be remotely positioned inside the jet pump transported by the wheeled platform. The shroud UT device is mainly composed of a flat-type remotely operated vehicle (ROV) and a positioning mast of ROV. The ROV is installed remotely on the outer surface of the shroud using the positioning mast. And the ROV has been designed thin enough to pass through the narrow gap between the jet pump and the shroud, so that it can move horizontally on the surface of the shroud with automatic cable feeding. Consequently, the proposed remote and automatic inspection devices can perform the inspection work in short time without using fuel handling machine (FHM). Therefore, the inspections can be performed simultaneously with the refueling work, which contributes to the shortening of regular inspection periods of nuclear power plants.Copyright

Development of Multifunction Laser Welding Head as Maintenance Technologies Against Stress Corrosion Cracking for Nuclear Power Reactors

Multifunction laser welding head has been developed. The head is able to perform not only underwater laser welding as repair, but also laser peening as preventive maintenance and laser ultrasonic testing as inspection. By using the effect of color aberration with optics, laser beam was focused to the ideal spot size on each process. Underwater laser welding was carried out onto EDM slit with the developed head and sealing ability with deposited weld metal was confirmed. As preventive maintenance, laser peening was also performed on the material surface with the developed head, and stress improvement ability was confirmed. For inspection with the developed head, a new method of visualizing weld defects in water by laser-ultrasonics has developed. Furthermore, developing synthetic aperture focus technique for visualized inspection surfaces 2-dementionally, the inspection result like penetrant testing despite underwater environment was achieved. Therefore, practicality of the developed head on each process was confirmed.Copyright

Laser Peening Systems for Preventive Maintenance Against Stress Corrosion Cracking in Nuclear Power Reactors

Laser peening introduces compressive residual stress on metal surface by irradiating laser pulses underwater without any surface preparations. The process utilizes the impulsive effect of high-pressure plasma generated through ablative interaction of each laser pulse with material. Laser peening systems, which deliver laser pulses with mirrors or through an optical fiber, were developed and have been applied to preventive maintenance against stress corrosion cracking (SCC) in nuclear power reactors since 1999. Each system was composed of laser oscillators, a beam delivery system, a laser irradiation head, remote handling equipment and a monitor/control system. Beam delivery with mirrors was accomplished through alignment/tracking functions with sufficient accuracy. Reliable fiber-delivery was attained by the development of a novel input coupling optics and an irradiation head with auto-focusing. At present, we are developing a newer concept and the prototype system has been just completed, which is extremely small, reliable and easy-handled.Copyright