Assessment of repair welding technologies of irradiated materials

The damages of reactor internals of stainless steels caused by SCC and fatigue were identified in aged BWR plants. Repair-welding is one of the practical countermeasure candidates to restore the soundness of components and structures. The project of 'Assessment of Repair welding Technologies of Irradiated Materials' is being carried out to develop the technical guideline regarding the repair-welding of reactor internals. In fiscal year of 2010, weldabilities of irradiated SUS316L stainless steel by the tungsten inert gas (TIG) welding and fatigue properties of irradiated SUS316L stainless steel containing 0.2appm helium were studied. Multi-pass TIG welding (heat input 2 MJ/m) of irradiated SUS316L stainless steel containing 0.02∼0.1appm helium were carried out. Optical microscopic cross-sectional observations of the welds showed cracks and degradations of grain boundaries in heat affected zone (HAZ). The degree of cracks and degradation of grain boundaries depended on the amount of helium, many cracks were observed in the amount of 0.1appm helium, and no crack observed in the amount of 0.02appm helium. Fatigue strength of irradiated SUS316L stainless steel containing 0.24appm helium was more than equal to non-irradiated material. The trend showed a good consistency with the general empirical formula that shows the fatigue limit depends on the Vickers hardness. (author)