Fukushima Dai-ni Nuclear Power Station Units 1, 2 and 4, which is in rated power operation, have automatically shut down around 14:46(JST) on March 11, 2011, because of the Tohoku regional Pacific Ocean Offshore Earthquake. Following the earthquake, the big tsunami attacked the power station and the seawater pump rooms were flooded. The licensee lost the function operability of sea water pumps for last heat removal. The turbine driven pump started operation and the exhausted steam increased the pressure of the suppression pool. The turbine driven pump became inoperable.
As a result of the efforts by the people on site, some seawater pumps were recovered and restarted. The suppression pools were cooled and the pressure was decreased. And the sea water pumps became to discharge the heat to the ultimate heat sink through the residual heat removal system.
Finally, the temperature of the reactors was decreased below 100 degree C and the reactors became cold shut down condition.