Search

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 © 2017 by ASME

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.

Decommissioning of aged nuclear reactors is planned, and cutting technologies for thick structure are necessary to reduce storage space of radioactive wastes. Though thermal cutting technology is suitable for cutting thick materials, radioactive fume is one of the problems due to increase the environmental dose. A water jet-guided laser cutting technology is one of the solutions for cutting irradiated materials, because radioactive fume is confined in the water and doze level won’t be increased. However, this technology was developed for precision machining like dicing and slotting of silicon wafers, cutting thick materials by using this process is very diffcult. In this study, cutting technology for thick material with a water jet-guided laser was discussed. Phenomenon during cutting thick stainless steel was observed by using high speed camera and optimum conditions for both water jet and laser cutting were derived. Finally, 50mm thick stainless steel plate was successfully cut by using this technology.

Bonding strength of coating to substrate in low pressure plasma spraying was investigated and the effect of reverse transferred arc treating before spraying was studied. The results are summarized as follows, (1) For the bonding strength test in low pressure plasma spraying, instead of the conventional adhesive, vacuum brazing process using Ag-Cu-In-Ti active filler metal at 1023 K was proposed and the propriety has been confirmed.(2) By the above test method, it was proved that the bonding strength of low pressure plasma sprayed coating is over 100 MPa. However, the bonding strength of coating pretreated by blasting before spraying fell down and fluctuated when the average roughness, Ra of the substrate surface was less 2μm.(3) Reverse transferred arc treating after blasting has enhanced the bonding strength of low pressure plasma sprayed coating, especially when the roughness, Ra of substrate before spraying is less than 2 μm. It is considered that the projections formed on the substrate surface by reverse transferred arc treating were buried into coating and performed the pile effect.

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 © 2011 by ASME

In repair welding for nuclear reactor vessel, low alloy steels are affected by heat input during welding process. The conventional repair welding for wall steel constructions requires post weld heat treatment (PWHT) to achieve the desired microstructure properties. However, post weld heat treatment is very difficult for some structures in operating plants. In such case, temper-bead welding technique is available as a repair welding method. Temper-bead welding employs a multi-pass deposition of welding metal. Each layer of beads provides heat for thermal treatment of the previous weld bead or layer, which lowers hardness of the heat affected zone (HAZ) and improves mechanical properties like the toughness. Toshiba has developed underwater laser cladding and laser seal welding techniques for reactor components repair welding. In this report, some experimental results of laser based underwater temper-bead welding are presented.

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 © 2006 by ASME

In nuclear power plants, irradiated materials like Control Rod (CR) should be stored underwater after service. Due to reducing the storage space, underwater cutting technology is expected. In this study, we developed underwater cutting technology of thick stainless steel with YAG laser in order to cut used CR. Preliminary tests were performed with flat plate test-pieces to optimize the cutting conditions. Due to creating a local dry area between nozzle and test-piece, high-pressure air was blown from the nozzle. Underwater laser cutting was carried out by laser irradiation power of 4 kW, changing the parameters of cutting speed, distance between the nozzle and test-piece, and thickness of the test-piece. We also investigated the wastes like dross and aerosols by laser cutting. Amount of dross was approximately 0.1 kg/m after cutting a 14 mm thick stainless steel plate, which is estimated to be less than other cutting method. Based on these results, we developed underwater cutting system of CR test-piece with YAG laser as a mock-up test. In the cutting torch, there was tracking system was introduced to keep the distance between the nozzle and the test-piece constant, and cutting monitor was also set-in to detect whether the test-piece was successfully cut or not. We have already tried to cut the CR test-piece with this facility and successfully cut in half.