Masaki Yoda

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

Underwater Laser Beam Welding for Nuclear Reactors

Toshiba has developed underwater laser beam welding (ULBW) technology as a maintenance measure for stress corrosion cracking.For an application to reactor vessel nozzles of PWR, ambient temperature temper bead welding technique and actual welding tools were developed. The ambient temperature temper bead welding technique can mitigate the degradation of toughness of low alloy steel of the nozzle due to high heat input of welding. Applicability of welding tools to actual PWR plants was confirmed through full-scale mockup tests in water tank at the depth of 10m. As ULBW dose not need to seal up and drain the work area, the new system can reduce the work period to less than half of the conventional system which needs draining.For an application of core shroud support of BWR, a prototype welding tool was developed to seal cracks and its performance was confirmed through mockup tests.ULBW enables significant reductions in radiation dose associated with maintenance efforts and also reduces impact on nuclear plant outage schedules. We will utilize this cutting-edge technology at nuclear plants both in Japan and abroad.Copyright

Multipoint radiation monitor using waveguide scintillators and optical fiber

A novel multipoint radiation monitor using waveguide scintillators and optical fiber connections is described. A new design of waveguide scintillator consisting of a scintillating material and a wavelength-shifting fiber (WLSF) has been deeveloped. The WLSF is embedded in the scintillator, and each end is fitted with an optical connector for connection to a transparent optical fiber. Multiple waveguide scintillators can be connected in a series, and the ends of the transparent optical fiber loop are terminated with a pair of photodetectors. This new technique for radiation monitoring dispenses with electric apparatus and improves noise resistance. Furthermore, it offers the advantages of multipoint measurement, meaning that the radiation incident on each scintillator is monitred by two photodetectors. We have examined the light output characteristics and time resolution of a prototype arrangement of waveguide scintillators and confirmed the feasibility of multipoint measurements in a system of five waveguide scintillators connected in series using a 200 m optical fiber loop.

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

Laser Peening Qualification for the U.S. Application

Primary water stress corrosion cracking (PWSCC) is a degradation process that has plagued nickel alloy components and welds in the nuclear industry for decades. Numerous mitigation techniques have been developed over the years that help reduce the potential for cracking in nickel alloy components exposed to the primary water environment. One such method is Laser Peening (LP), which improves the stress properties and helps to reduce the potential for crack initiation. The LP process has been applied in Japan to both boiling water reactors (BWR) and pressurized water reactors (PWR) for stress corrosion mitigation. The first application of LP in the US for the nuclear industry was applied in the fall of 2016 to the bottom mounted instrumentation (BMI) nozzles of a PWR. The bottom mounted nozzles are made from Alloy 600 tubing and attached with Alloy 82/182 welds, which are known to be susceptible to PWSCC. In order to prevent crack initiation, it is important for the peening mitigation process to induce sufficient compressive stress on the surface of the susceptible materials. However, it is not practical to take stress measurements directly on the reactor components in order to verify compression. Thus, the magnitude of compression induced by the LP process was verified prior to the application at the plant using mockups of the BMI nozzles. As a part of the qualification process, test coupons were peened and stress measurements were taken using X-ray diffraction (XRD). The results of the stress measurements demonstrate that sufficient surface compression was achieved by the LP process in order to provide PWSCC mitigation. This paper presents and discusses key stress measurement results taken during the qualification process for the first application of LP at a U.S. nuclear plant. Although not directly applicable in this case, the guidance in ASME Code Case N-729 Mandatory Appendix II and MRP-335 for PWR upper head nozzles was generally followed.Copyright

Laser Peening Technology as the SCC Mitigation for Reactor Internals

Stress Corrosion Cracking (SCC) is one of the major concerns for aged nuclear reactors. SCC-susceptible materials have been employed in a wide variety of applications in the nuclear industry. Laser Peening (LP) is a method for the SCC mitigation that eliminates surface tensile stress using impulsive effect of high-pressure plasma induced by irradiation with high-power laser pulses in the water. To apply LP to nuclear reactor internals, Toshiba has developed a new process which needs no protective coating on the materials and optimizes the conditions for the laser irradiation. Its effects for stress improvement and SCC-mitigation of laser-peened materials were confirmed through SCC tests for austenitic stainless steels and nickel-based alloys. Also integrity of the laser-peened materials was confirmed through various examinations. Toshiba developed the LP system for the core shroud and the reactor bottom part of BWRs, and has been applying it to actual Japanese nuclear reactors since 1999. For PWRs, Toshiba developed the system for Bottom-Mounted Instrumentation (BMI) nozzles and other Reactor Vessel (RV) nozzles, and has been applying it to Japanese PWRs since 2004. So far Toshiba has already completed LP operations for 2 PWR plants and 8 BWR plants in Japan. In consideration of extending the LP system to BWRs and PWRs overseas, a portable LP system equipped with a small size laser oscillator has been developed. We confirm the possibility that the portable LP system makes the outage period shorter.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