Fukushima Dai-Ichi was the first nuclear plant to be constructed and operated entirely by Tokyo Electric Power Company (TEPCO).
Units 1, 2, 3 and 4 of the nuclear complex were damaged in a series of events after the 11 March 2011 earthquake (Tohoku-Chihou-Taiheiyou-Oki Earthquake) and tsunami that struck the nation.
The earthquake had cutoff the power supply needed to pump cooling water into the damaged reactors. A portion of the fuel rods which create heat through nuclear reaction was exposed due to the failure of the cooling system caused by the tsunami. This failure resulted in nuclear explosion in the reactors.
Efforts to cool the reactor vessels with seawater and boric acid failed. The evacuation zone around the nuclear complex was doubled from six to 12 miles and was further extended to 30km radius after the explosion of Unit 3.
The total installed capacity of the six boiling water reactor units is 4,696MW. Unit 1 has an installed capacity of 460MW, Units 2, 3, 4 and 5 each have 784MW capacity and Unit 6 is rated at 1,100MW.
“The total installed capacity of the six boiling water reactor units is 4,696MW.”
Units 1 to 5 are Mark I type while Unit 6 is a Mark II built with containment structures. All the reactors except Unit 3 continued using low enriched uranium (LEU). Unit 3 was being fed with mixed-oxide (MOX) fuel since September 2010.
Unit 1 was scheduled for closure in 2011. It was however granted an extension of further 10 years by the Japanese regulators in February 2011.
The six reactors were designed by GE. The architectural design was provided by Ebasco.GE also supplied the Units 1, 2 and 6. Units 3 and 5 were supplied by Toshiba and Unit 4 by Hitachi.The nuclear complex was constructed by Kajima.
The reactor units 1, 3 and 4 were automatically shut down following the earthquake.
The earthquake measured 8.9 on the Richter magnitude scale, which was much more than the plant’s bearing capacity. The remaining two units 5 and 6 were also shut down for regular inspection.
The emergency onsite generation had failed to provide the necessary backup power needed to support the critical instruments and control systems. Even the special cooling system known as the reactor core isolation cooling system that uses waste heat to run the critical systems could not provide the power needed to operate the control systems.
“Fukushima Dai-Ichi was the first nuclear plant to be constructed and operated by TEPCO.”
Unit 1 exploded on 12 March 2011 knocking down the external concrete building. However, the reactor and the steel containment structure remained intact. The radiation levels rose to 1015 microsievert, which is equivalent to the maximum permissible level for a year in a single day.
The explosion of Unit 1 has been classified as a ‘level 4 accident with local consequences’ on the International Nuclear and Radiological Event Scale (INES). This scale measures 0-7 (from deviation-no safety significance – major accident) and is used to communicate the safety significance of events associated with radiation sources.
Unit 3 exploded due to hydrogen ignition on 14 March 2011. Pressure in the reactor was built-up to 530 kiloPascals (kPa) even while sea water was being injected into the reactor to control the radiation.
On the same day, the fifth floor of Unit 4 building was damaged. Fire was sighted in the north-west part of the fourth floor and efforts to put it out were initiated immediately.
The reactor Core Isolation system of Unit 2 had also stopped functioning, resulting in a third explosion in the suppression chamber.