Kansai Electric Power has named a cause for the leak that shut Mihama Unit 3 and set a date to bring the reactor back. In a disclosure dated October 9, 2026, the utility said it plans to start the reactor and begin adjustment operation on October 10, with full operation resuming on November 4 once the comprehensive load performance inspection ends.
A plate fitted the wrong way up
The unit is a pressurized-water reactor rated at 826,000 kilowatts. During adjustment operation on September 17, 2026, at about 3:45pm, staff saw water dripping at three drops a second from insulation around an air vent valve. The valve sits on the outlet piping of the normal water level control valve for the 6A high-pressure feedwater heater. Kansai Electric stopped the reactor by hand at 4:03am on September 18 to investigate, as the company disclosed that day. Tokyo Brief's earlier article on the manual shutdown covers that stoppage.
The investigation found a through-hole in the pipe between the control valve outlet and the air vent valve. Kansai Electric estimates the cause as follows: when insulation and an outer cover plate were installed during 2010 work, the plate directly above the hole was mounted upside down. Rainwater then entered through the plate's joint, and external corrosion of the pipe progressed until it broke through. The company describes this as an estimate, not a confirmed finding.
The fix
As countermeasures, Kansai Electric replaced the affected piping with stainless steel and clarified a precaution on cover-plate orientation in its maintenance work guide.
The defect sits in the feedwater heater system, on the secondary side. The notice explains that feedwater flows through the heater's heat-transfer tubes and is warmed with exhaust steam from the high-pressure turbine. The steam condenses into drain water, which returns to the feedwater system and flows through the affected piping.
The schedule
| Milestone | Planned date |
|---|---|
| Reactor startup and criticality | October 10, 2026 |
| Adjustment operation begins | October 10, 2026 |
| Full operation resumes (comprehensive load performance inspection ends) | November 4, 2026 |
Both milestones are plans, not completed events. The October 10 startup follows what the company calls completed preparations. The November 4 date is tied to the end of the comprehensive load performance inspection.