The Kansai Electric Power Company, Incorporated announced that, at approximately 3:45 p.m. on September 17, 2026, while Mihama Nuclear Power Station Unit 3 (Pressurized Water Reactor, rated electric output: 826 MW) was in Commissioning Operation, water dripping at a rate of three drops per second was observed from the thermal insulation around the air vent valve on the outlet piping of the normal level control valve for the No. 6A high-pressure feedwater heat exchanger.* To investigate the cause, the Company manually shut down the reactor at 4:03 a.m. on September 18. The investigation identified a through-wall hole in the section of piping between the outlet of the normal level control valve for the No.
6A high-pressure feedwater heat exchanger and the air vent valve. The Company determined that the likely cause was as follows: When the thermal insulation and cover were installed during work in 2010, the cover directly above the location of the through-wall hole was installed upside down. Rainwater subsequently entered through the seam in the cover, causing external corrosion of the piping to progress until the pipe wall was penetrated.
As corrective actions, the Company replaced the affected piping with stainless steel piping and revised its “Maintenance Work Guide” to clarify precautions concerning the installation orientation of the cover. With preparations for reactor startup now complete, the Company plans to start up the reactor on October 10 and resume commissioning operations on the same day. Reactor Startup and Criticality: October 10, 2026.
Commissioning Operation: October 10, 2026. Resumption to Full Operation: November 4, 2026.
Source: Market Screener