Laser based maintenance technology for PWR power plants

摘要

Stress corrosion cracking (SCC) is the majorfactor to reduce the reliability of aged reactorcomponents. Toshiba has developed variouslaser-based maintenance technologies and alreadyapplied them to several existing nuclear powerplants. Recently, we have developed themaintenance system for the inner surface ofbottom-mounted instruments (BMI) of PWR plants.This system performs nondestructive testing (NDT)and preventive maintenance against SCC by usingYAG lasers .Laser ultrasonic testing (LUT) has a greatpotential to be applied to the remote inspection ofreactor components. Laser-induced surface acousticwave (SAW) inspection system was developed byusing a compact probe with a multi-mode opticalfiber and an interferometer. This system is used forboth detection and depth measurement ofsurface-breaking cracks. It is confirmed throughlaboratory studies that the developed systemsuccessfully detected and sized micro slits of around1.0mm depth on weld metal and heat-affected zone(HAZ). SCCs produced by chemical method werealso tested by the system.For the preventive maintenance treatment,laser-peening (LP) technology was developed andalready applied to several reactor components inoperating BWR plants. LP is a novel process toimprove residual stress from tensile to compressiveon material surface layer by irradiating focused high-power laser pulses in water. We havedeveloped a fiber-delivered LP (FLP) system as apreventive maintenance against SCC. For checkingthe effect of FLP, we carried out FLP experimentson the inner surface of a small tube-shaped Alloy600 by using this system. After FLP, residual stresswas measured by X-ray method for radial and axialdirections on the inner surface of the tube, andeffectiveness of stress improvement was proved.Based on these experiences, LUT and FLP wereapplied to Ikata unit-1 of Shikoku Electric PowerCompany Inc. and successfully treated the innersurface of BMIs.