Core internals replacement method in Japanese PWR

A technology for replacing whole of reactor vessel internals, together upper internals and lower internals, has been developed. The new internals for the replacement is designed from aspects of adapting the latest internals design, getting negative feedback from operational experiences worldwide, preparing for applying high burn up fuels and keeping compatibility to the existing components such as a reactor vessel in case of replacing 2 loop plants in Japan. In particular, special baffle structure with the newly developed L-type baffle plate and other applicable design improvements is applied as countermeasure against IASCC of the baffle former bolts. Since it is feasible to apply design improvements for the new internals systematically and flexibly in consideration of total performance, it can be concluded that replacing whole of the reactor vessel internals is the integrated and diversified solution that can solve several concerns on the internals at the same time. The replacement works using the developed technology have been already applied and successfully completed at three PWR plants in Japan since 2004, for objectives of proactive maintenance against IASCC of the baffle former bolts and of installing additional guide tubes as a preparation for applying high burn up fuels. In the replacement works at the site, special replacement method, so-called All-in-One piece extraction method, has been applied and it has greatly contributed to achievement of short operation period of about 70 days and lower radiation exposure to workers. (authors)