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Weld repair technique of helium‐containing reactor structural materials

Structures in the inside of reactors of commercial nuclear power plants are comprised of austenitic stainless steels and nickel base alloys which have excellent corrosion resistance in high temperature water. These materials accumulate nuclear transformation helium within themselves when they are irradiated with neutrons near the reactor core. In 1984, when the Savannah River reactor in the United States was welded during repair, cracking occurred at the toe of the weld of the stainless steel tank. The results of the investigation made it clear that the weld cracking was caused by the nuclear transformation helium in the material. Because 20 years have now passed since the power plants built in the 1970s started operating, a lot of projects have been carried out in recent years both in Japan and abroad to develop techniques for preventive maintenance and repair so that the long-term safe operation of these plants can be secured. What has been the subject of the attention in these studies is the weldability of the materials used for the structures in the reactors which contain helium. Therefore, this paper will outline the studies made on the weldability of stainless steels with helium as well as introduce future direction of work.