Planned Reactor Startup and Commencement of Adjustment Operation at Mihama Nuclear Power Station Unit 3 (Water Dripping from Around the Air Vent Valve on the Outlet Piping of the 6A High-Pressure Feedwater Heater Normal Water-Level Control Valve)
The Kansai Electric Power Company,Incorporated Mihama Nuclear Power Station Unit 3
Around the air vent valve on the outlet piping of the 6A high-pressure feedwater heater's normal water-level control valve at Mihama Nuclear Power Station Unit 3
It is presumed that, when the insulation and exterior panels were installed during construction work in 2010, the exterior panel directly above the penetration was installed upside down. Rainwater subsequently entered through the joint in the exterior panel, causing external corrosion of the piping to progress until it penetrated the pipe wall.
Company response
Reactor operation at Mihama Nuclear Power Station Unit 3
Reactor startup and criticality: 10 October 2026; commencement of adjustment operation: 10 October 2026; resumption of full operation (completion of the comprehensive load performance inspection): 4 November 2026
The piping was replaced with stainless steel piping, and the precautions concerning the installation direction of exterior panels were clarified in the "Repair and Maintenance Work Guide."
The reactor is scheduled to start up on 10 October 2026, with adjustment operation scheduled to begin on the same day. Resumption of full operation (completion of the comprehensive load performance inspection) is scheduled for 4 November 2026.
| Item | Details |
|---|---|
| Event name | Planned Reactor Startup and Commencement of Adjustment Operation at Mihama Nuclear Power Station Unit 3 (Water Dripping from Around the Air Vent Valve on the Outlet Piping of the 6A High-Pressure Feedwater Heater Normal Water-Level Control Valve) |
| Date of occurrence | Around 15:45 on 17 September 2026 |
| Date recognized / date notice received | Around 15:45 on 17 September 2026 |
| Subject Entity | The Kansai Electric Power Company,Incorporated Mihama Nuclear Power Station Unit 3 |
| Target Locations / Products | Around the air vent valve on the outlet piping of the 6A high-pressure feedwater heater's normal water-level control valve at Mihama Nuclear Power Station Unit 3 |
| Cause | It is presumed that, when the insulation and exterior panels were installed during construction work in 2010, the exterior panel directly above the penetration was installed upside down. Rainwater subsequently entered through the joint in the exterior panel, causing external corrosion of the piping to progress until it penetrated the pipe wall. |
| Suspended operations | Reactor operation at Mihama Nuclear Power Station Unit 3 |
| Scheduled resumption date | Reactor startup and criticality: 10 October 2026; commencement of adjustment operation: 10 October 2026; resumption of full operation (completion of the comprehensive load performance inspection): 4 November 2026 |
| Measures to prevent recurrence | The piping was replaced with stainless steel piping, and the precautions concerning the installation direction of exterior panels were clarified in the "Repair and Maintenance Work Guide." |
| Outlook | The reactor is scheduled to start up on 10 October 2026, with adjustment operation scheduled to begin on the same day. Resumption of full operation (completion of the comprehensive load performance inspection) is scheduled for 4 November 2026. |
Items this document does not state (15)
These are items the company does not state in this document. We do not leave them blank, nor fill them in from other sources or by inference.
The reasons the company gives
The company's own explanation, not something inferred from the numbers.It is presumed that, because the exterior panel was installed upside down during construction work in 2010, rainwater entered through the joint, causing external corrosion of the piping to progress until it penetrated the pipe wall.