Masataka Tamura

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

Development of Underwater Laser Cladding and Underwater Laser Seal Welding Techniques for Reactor Components (II)

Stress corrosion cracking (SCC) is one of the major reasons to reduce the reliability of aged reactor components. Toshiba has been developing underwater laser welding onto surface of the aged components as maintenance and repair techniques. Because most of the reactor internal components to apply this underwater laser welding technique have 3-dimensional shape, effect of welding positions and welded shapes are examined and presented in this report.Copyright

Temper-Bead Weld by Underwater Laser Beam Welding

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.© 2009 ASME

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

Plasma behaviors in laser cutting

Behaviors of the plasma, the emission intensity, distribution, source and species were evaluated using a high-speed video camera, a color CCD camera, and a spectrum multi-channel analyzer. It was found that a strong blue radiation from iron atoms of workpiece, nitrogen molecules or ions of a gas jet, when the cut surface quality was not acceptable. As cutting speed increased, plasma formation region where a laser beam was focused was more extended. It was found that flow speeds of molten material at bottom in which the plasma was induced became slower than those of top and middle points. Also, scattering angles of molten material droplets at a bottom exit of a kerf became larger and, at the same time, reattachment of molten material at the bottom was observed. Moreover, it was found that additional branches of the flows of molten material with smaller and darker droplets were formed intermittently, and that parts of the branches of the flowing droplets were being attached as dross to the bottom edges. From these results, it was confirmed that the plasma formation (region) and sizes and angles of striations (cut surface quality) were strongly correlated. Possibilities of the in-process evaluation of the cut surface quality by monitoring the plasma sizes and/or brightness in laser beam cutting were discussed.

A Basic Study on High-Temperature Protective Coating Design of Single Crystal Ni Based Super Alloy.

A computer-aided interactive system for coating desing has been developed, which enables to analyze coveniently the reaction diffusion of bonded materials. The object of this study is the overlay coatings of MCrAlY alloy sprayed by a low-pressure-plasma spray (LPPS) process for protection against high-temperature corrosion and oxidation in the field of gas turbine components. However, the reaction diffusion behavior at the interface between the MCrAlY coating and the substrate, which has an important effect on coating degradation, has not been fully clarified. Four kinds of low-pressure-plasma sprayed MCrAlY alloys, namely CoCrAlY, NiCrAlY, CoNiCrAlY and NiCoCrAlY, and single-crystal CMSX-2 were selected for the experiments.The experimental results showed that the reaction diffusion layers consisted of aluminum compound layer and low aluminum layer. In case of the NiCoCrAlY and CoNiCrAlY coatings, the aluminum compound layer could not be observed clearly. It was also indicated that each diffusion thickness could be expressed by the parabolic time dependence. The order of reaction diffusion rate was NiCrAlY>CoCrAlY>CoNiCrAlY>NiCoCrAlY. It was also clarified by the simulation analysis that the diffusion distance during heating process can not be ignored in comparison with the total diffusion distance, and the estimation of long time diffusion was made.

Development of Coating System for Gas Turbine. III. Reaction Diffusion Analysis for Interface between Ni Based Super Alloy and MCrAlY Coating.

A computer-aided interactive system for coating design has been developed, which enables to analyze conveniently the reaction diffusion of bonded materials. The object of this study is the overlay coatings of MCrAlY alloy sprayed by a low-pressure-plasma spray (LPPS) process for protection against high-temperature corrosion and oxidation in the field of gas turbine components. However, the reaction diffusion behavior at the interface between the MCrAlY coating and the substrate, which has an important effect on coating degradation, has not always been clarified. Three kinds of substrate, such as equiaxis IN738LC, directional solidified CM247LC and single-crystal CMSX-2, and four kinds of low-pressure-plasma sprayed MCrAlY coating were selected for the experiments.The experimental results showed that the reaction diffusion layers consisted of aluminum compound layer and aluminum decrease layer, basically. However, the aluminum decrease layer could not be observed in the cases of CoNiCrAlY and NiCoCrAlY coatings. It was also indicated that each diffusion thickness changed in parabolic time dependence. And the order of reaction diffusion rate was NiCrAlY>CoCrAlY>CoNiCrAlY>NiCoCrAlY independent of the kind of substrate. It was also clear that the estimation of long time diffusion behavior by simulation analysis was possible in comparison with the experiments.

Study of Gas Reaction by Electron Beam Irradiation.

In a batch system with an electron beam irradiation, experiments have been carried out for accelerating chemical gas reactions. Carbon dioxide (CO2) is a low free energy compound, and either a high energy input of electron beams or a use of other high free energy reactants is needed for radiation chemical reactions. In this paper, the effects of content of added gas elements and an exposure dose of the electron beam irradiation on the CO2 gas reactions were investigated by monitoring the variations of gas temperature and gas pressure. As a result, there was a tendency that the CO2 gas reactions with an added acetylene gas (C2H2) increased with increasing the content of C2H2, the exposure does of electron beams and the gas temperature. Also, in this experiment, it was confirmed that carbon monoxide, methanol and some solid polymer could be produced under the condition of room temperature and atomospheric pressure.