On March 11, 2011 a tsunami was triggered by a powerful earthquake off the northeast coast of the Japanese island of Honshu. The huge wave crashed into the coastal Fukushima Number 1 nuclear power plant in Okuma, Fukushima Prefecture and caused a major nuclear disaster.
The Daiichi plant contains six GE boiling water reactors operated by the Tokyo Electric Power Company. The plant was started operation 1971 and ultimately generated almost five gigawatts of electrical power making it one of the fifteen biggest commercial nuclear power plants in the world.
On March 11, 2011 reactors 5 and 6 were in what is called cold shutdown where the pressure of the coolant is at regular sea level atmospheric pressure and the temperature is under two hundred degrees Fahrenheit. Reactor 4 had been defueled (had its fuel removed.)
Following the earthquake, reactors 1, 2 and 3 automatically shut down and emergency diesel generators switched on to provide electricity for reactors controls and coolant circulation pumps. The tsunami caused by the earthquake hit the site and flooded the rooms containing the emergency generators. It also severed the connection of the plant to the electrical power grid. Access to the site was inhibited by the flooding. Newer generators had been built above the flood zone but their connection to the reactors was not protected and was flooded, preventing their use. Attempts to bring in mobile generators failed because they could not be connected to the site power grid.