The Mihama Nuclear Power Plant is operated by The Kansai Electric Power Company, Inc. and is in the town of Mihama, Fukui Prefecture, about 320 km west of Tokyo. It is on a site that is 520,000 m2 of which 60% is green space.

Reactors on Mihama NPP

Unit Reactor Type Average Electric Output Electric Power Rating Commission date
Mihama - 1 PWR 320 MW 340 MW 28.11.1970
Mihama - 2 PWR 470 MW 500 MW 25.07.1972
Mihama - 3 PWR 780 MW 826 MW 01.12.1976


The Mihama Nuclear Power Plant has been notable beyond most nuclear plants due to the severity of accidents that have happened there, the 2004 steam explosion in particular.

September 2, 1991

Unit 2 steam generator had one tube fully break. This triggered a SCRAM with full activation of the Emergency Core Cooling System. Eventually, a small amount of radiation was released to the outside.

May 17, 2003

Unit 2 steam generators had two holes open simultaneously. There was no radioactive release to the environment.

August 9, 2004

On 9 August 2004, an accident occurred in a building housing turbines for the Mihama 3 reactor. Hot water and steam leaking from a broken pipe killed four workers and resulted in seven others being injured. The accident had been called Japan's worst nuclear power accident before the crisis at Fukushima I Nuclear Power Plant.

The Mihama 3 is an 826 megawatts electric, 3-loop Westinghouse type pressurized-water reactor (PWR) which has been in service since 1976. The pipe rupture occurred in a 55.9 centimeter (cm) (22 inch) outside diameter pipe in the ‘A’ loop condensate system between the fourth feedwater heater and the deaerator, downstream of an orifice for measuring single-phase water flow. At the time of the secondary piping rupture, 105 workers were preparing for periodic inspections to commence.

A review of plant parameters did not uncover any precursor indicators before the accident nor were there any special operations that could have caused the pipe rupture. An investigation concluded that water quality had been maintained since the commissioning of the plant.

Japan's previous most deadly accident at a nuclear facility took place at a uranium processing plant in Tokaimura, north of Tokyo, on September 30, 1999, when an uncontrolled nuclear chain reaction was triggered after three poorly trained workers mixed Uranium nuclear fuel in a bucket. The resulting release of radiation killed two workers, and exposed one other worker to radiation above legal limits.

The Mihama NPP started back up in January 2007 after making various changes and obtaining permission from Fukui Prefecture and industry regulators.


Another fire occurred in 2006, two workers sustained injuries. There were no fatalities and no release of radioactivity detected, though the ash involved in the fire included some low level radioactive waste.

Mihama Nuclear Power Plant
Country Japan
Locale Mihama, Fukui Prefecture
Status Operational
Construction began February 1, 1967
Commission date November 28, 1970
Operator(s) The Kansai Electric Power Company, Inc.

Reactor information
Reactors operational 1 x 340 MW
1 x 500 MW
1 x 826 MW