The Onagawa Nuclear Power Plant is a nuclear power plant 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 is the most modern reactor in all of Japan. It 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.

Onagawa Nuclear Power Plant Reactors

Unit 1

Since November 11, 2006 this unit has been shut down due to the result of a test.

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.

Earthquake Events

• 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. These generators were designed to start up in such an event so that off-site power can still be provided to certain reactor systems. A tank of sulfuric acid on the roof did burst releasing some sulfuric acid into the air, but it was retrieved and processed later.
• A fire at the plant from the turbine section of the plant following the 2011 Sendai earthquake was reported by Kyodo News.
• On 13 March the lowest-level state of emergency was declared regarding the Onagawa plant by TEPCO, as radioactivity readings temporarily exceeded allowed levels in the area of the plant. TEPCO stated this was due to radiation from the Fukushima I nuclear accidents and not from the Onagawa plant itself.