The Study on the Applicability of Laser Surface Modification Technology to Irradiated Stainless Steel

Abstract

Recently, occurrence of stress corrosion cracking has been reported at core shrouds in Boiling Water Reactor (BWR) nuclear power plants. Yttrium aluminum garnet (YAG) laser surface modification technologies (i.e. Laser Surface Melting Technology (LSM), Laser Cladding Technology (LC)) have been developed as promising preventive maintenance technologies to stress corrosion cracking (SCC) of austenitic stainless steel structures and components. On the other hand, it has been also well-known that the helium transmuted from nickel and boron is accumulated to neutron irradiated stainless steel, and that helium related cracks may occur at weld heat affected zone which were attributed to nucleation along grain boundaries, coalescence and growth of helium bubbles due to thermal cycle and thermal stress during welding. Then, the laser surface modification technologies to the irradiated stainless steels was developed and the applicability of these technologies was evaluated based on the results of various tests (e.g. dye-penetrant test, micro structure observation and bending test) to the laser surface modified Type 304 and Type 316L specimens containing up to about 10 appm helium. The laser surface modification applicability diagram was developed as a function of weld heat input and helium concentration, which was supported by numerical simulation on helium bubble formation and growth during welding for irradiated stainless steels.