Fukushima Dai-ichi Nuclear Power Station Unit 1, 2 and 3, which is in rated power operation, have automatically shut down around 14:46 (JST) in March 11, 2011 because of the Tohoku regional Pacific Ocean Offshore Earthquake.
After that, the power station was attacked by tsunami.
Electricity rooms and seawater pump rooms were flooded, and the cooling function became only the coolant injection of the turbine driven pump. The turbine driven pump operation increased the pressure of the suppression chamber, and it became impossible to continue the operation.
It means loss of all cooling function, and the declaration of a state of emergency was done.
The release valve was opened several times and injections of the sea water into the reactor vessel were carried out with fire extinguishing pumps. The gas in the containment vessel was also vented.
Then, the radioactive dose rate at site boundary had exceeded the limit value of 0.5 mSv/h.
This value is thought smaller than the value resulting in the release of a few per cent of core inventory of a power reactor from the fuel assemblies.
But the radioactive dose rate at the suppression chamber was high. And it is thought that hydrogen gas was generated enough to damage the reactor building.
The behavior of the pressure of the reactor vessel and the containment vessel, and the behavior of the water level of the reactor vessel were complicated. Some measurements were not possible because of failures of measurement equipment. As a result, the detailed estimate can not be done.
But these values are thought the values resulting in the release of more than a few per cent of core inventory of a power reactor from the fuel assemblies.
The inhabitants of the range of 20 km took refuge. At least, one worker received a dose (about 106 mSv) in excess of the annual limit (100 mSv). But we can not rate on impact on people and the environment because the correspondence work still continues.
Event date: Fri, 11-03-2011