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.

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