Onagawa Nuclear Power Plant

The Onagawa Nuclear Power Plant (女川原子力発電所, Onagawa ( pronunciation) genshiryoku hatsudensho?, Onagawa NPP) is a nuclear power plant located on a 1,730,000 m2 (432 acres) site in Onagawa in the Oshika District and Ishinomaki city, Miyagi Prefecture, Japan. It is managed by the Tohoku Electric Power Company. It was the most quickly constructed nuclear power plant in the world.

The Onagawa-3 unit was used as a prototype for the Higashidori Nuclear Power Plant.

The plant conforms fully to ISO 14001, a set of international environmental management standards. The plant's waste heat water leaves 7 degrees Celsius higher than it came in and is released 10 meters under the surface of the water, in order to reduce adverse effects on the environment All the reactors were constructed by Toshiba.

Reactors on Site
Unit 1

Shut down manually on 25 February 2005 because it was determined that the reactor containment leaked small amounts of nitrogen. The unit was restarted once Nuclear and Industrial Safety Agency was satisfied that the countermeasures taken by the plant operator to prevent a reoccurrence were adequate.

Unit 2
  • May 2006 it was confirmed that a pipe was leaking due to debris damage.
  • June 7, 2006 Difficulties with pressure control prompted further inspections.
  • July 7, 2006 METI and the Nuclear and Industrial Safety Agency determined that the plant's performance was not satisfactory.
Unit 3
  • July 7, 2006 Due to pipe integrity concerns the reactor was shut down.
  • November 25, 2006 Following repairs the reactor was restarted.
  • March 11, 2011 2011 Tōhoku earthquake damaged the turbines after a fire broke out and was shut down.

Small fire in the administrative offices. Did not affect functioning of the plant.


The Onagawa Nuclear Power Plant was affected by the 2005 Miyagi earthquake and recorded vibrations above what the plant was designed for. Analysis after the event, however, found no damage to the reactor systems. Some people reported seeing smoke come from the plant after the earthquake and reported it, thinking that it indicated an accident, but the smoke was actually produced by the backup diesel generators.


On April 8, 2011, a leak of radioactive water spilled from pools holding spent nuclear fuel rods following the 2011 Tōhoku earthquake , the nuclear safety agency said, was reported by Kyodo News

A fire from the turbine section of the plant following the 2011 Tōhoku earthquake was reported by Kyodo News.

On March 13, 2011, levels of radiation on site reached 21μSv/hour, a level at which Tohoku Electric Power Company were mandated to declare state of emergency, and they did so at 12:50, declaring the lowest-level such state. Within 10 minutes the level had dropped to 10μSv/hour. The Japanese authorities assume the temporarily heightened values were due to radiation from the Fukushima I nuclear accidents and not from the plant itself. On March 13 20:45 UTC, the IAEA announced that radiation levels at the Onagawa plant had returned to normal background levels.

The three units remain in cold shutdown since the earthquake of 11 March. Two hundred people who lost their homes to the tsunami took refuge in the plant. The April 7th aftershock damaged 2 of the 3 external power lines to the plant but cooling was maintained through the third line.