The Effect of Helium on Welding Irradiated Materials

Energetic neutron irradiation and associated transmutation reactions are known to cause significant damage to materials. The resultant deterioration in the mechanical properties and corrosion resistance plays a decisive role in the life expectancy of nuclear reactor components. With the first generation of nuclear power plants reaching their life expectancy, the I repair and replacement of degraded reactor components will be required. Hence, joining using conventional welding techniques will be needed. However, during welding of irradiated materials, a major difficulty must be overcome. Helium generated in the material from transmutation reactions is almost insoluble in a solid matrix. Under the high temperatures and high thermal stresses generated during welding, helium bubbles will grow and coalesce along the grain boundaries. This will cause intergranular cracking and fracture of the components during welding repair. This article reviews the investigations that have occurred involving welding postirradiated materials over the last 25 years. The effect of helium on the weldability and postweld properties of the irradiated structural materials, such as stainless steels, is summarized. Theories that have been developed to describe the helium-induced cracking are discussed along with possible techniques to suppress cracking and improve the weldability of irradiated materials.