Alloy 600 aging issue within a PWR plant is one of the important aging issues especially for aged plants in terms of susceptibility of PWSCC and impact on integrity of critical components. As for Reactor vessel, alloy 600 material is used for reactor vessel head (RVH) penetrations, bottom mounted instrumentation (BMI) penetrations and nozzle safe-end weld joints. RVH replacement work has been and will be conducted at many PWR plants all over the world to manage RVH penetration aging issue, and aging countermeasures for BMI penetrations and nozzle safe-end weld joints may become still more important taking into account of leakage events at South Texas Project (2003, BMI penetrations), at V. C. Summer (2000, nozzle safe-end weld joint of Reactor vessel) and at Tsuruga Unit 2 (2003, nozzle safe-end weld joint of Pressurizer safety & release nozzle). Mitsubishi Heavy Industries, Ltd. (MHI) has developed and also is developing maintenance technology such as inspection, repair, replacement and mitigation measures in order to cope with this issue effectively and economically. This paper describes MHI’s maintenance technology as aging countermeasures for Reactor Vessel’s regions using alloy 600, especially for BMI penetrations and nozzle safe-end weld joints. As for BMI penetrations, MHI has already developed water jet peening (WJP) technology as mitigation measure and applied it to several plants since 2001, also MHI has developed repair technology in case of leakage and cracking events. As for nozzle safe-end weld joints, MHI has been developing mitigation, repair and replacement technology, and has already applied the nozzle safe-end replacement technique to Pressurizer safety & release nozzle of Tsuruga unit 2 in 2003.

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