Kansai Electric Power stopped the reactor at Mihama Power Station Unit 3 on the evening of September 17 after an employee doing equipment checks tied to a power increase found water dripping from insulation near a valve fitting at three drops a second. The unit was in its 29th periodic inspection, and the drip was found at around 15:45 on insulation around the air vent valve on the outlet piping of the 6A high-pressure feedwater heater's normal water-level control valve.
The utility said the leak did not immediately threaten safe operation. The problem was that the affected section of piping could not be isolated from the rest of the system while running. To be certain about future operation, Kansai Electric decided at 17:45 the same day to stop the reactor and inspect the area, and began reducing output at 20:30.
Kansai Electric said the event caused no radiological impact on the environment, an assessment the company itself made in the disclosure rather than one confirmed by an independent inspector. The company has not identified what caused the drip, and the filing describes only the discovery and its response, not a root cause.
What happens next is inspection of the leaking section, which Kansai Electric said it will carry out before deciding on further steps. The company gave no restart date. It also said it will promptly disclose any material developments, including earnings impact, once they become known, leaving open how the extended outage affects the unit's contribution to results for the year.
Mihama Unit 3 was raising power as part of its return from the 29th periodic inspection when the leak was found, and Kansai Electric's decision to stop the reactor is what triggered the load reduction that began at 20:30. The company's contact for the disclosure is Mitsuhiro Iwata, head of its corporate planning office.