As the existing light water reactor (LWR) fleet ages, the weldability of structural materials used to construct the reactor pressure vessels (RPVs) and reactor internals is diminished. The decrease in the weldability in austenitic and ferritic materials is attributed to the formation of helium in the material microstructure. Helium (He) generation occurs during the service life of irradiated reactor internals from neutron transmutation reactions of boron and nickel in these materials. Welding on irradiated materials, if performed without appropriate consideration of fluence exposure and helium generation, can result in a heat affected zone cracking phenomenon termed helium induced cracking (HeIC). The heat input associated with welding is a major factor affecting the coalescence of the generated helium along grain boundaries. As the material cools, the tensile stresses generated from welding can cause cracking to occur along grain boundaries weakened by helium bubble coalescence. In some cases, the preferred or only method of repair or replacement of a reactor internal component is welding. For components located in regions of low thermal fluence, the welding process implementation may be relatively straightforward and only heat input control may be required. However, in high thermal fluence regions, weld repair of irradiated reactor internal components is complicated by the presence of high concentrations of helium and significant care must be taken in welding process selection and heat input control. This paper highlights envisioned applications for weld repair on irradiated reactor internals. It also summarizes recently completed guidance published by the EPRI Materials Reliability Program (MRP) and EPRI BWR Vessel and Internals Project (BWRVIP) which provides an improved basis for plants to assess the weldability of components at various locations within the reactor.

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