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The reactor Core Isolation system of Unit 2 had also stopped functioning, resulting in a third explosion in the suppression chamber. Fire was sighted in the north-west part of the fourth floor and efforts to put it out were initiated immediately. On the same day, the fifth floor of Unit 4 building was damaged. 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. Unit 3 exploded due to hydrogen ignition on 14 March 2011. 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. 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). The radiation levels rose to 1015 microsievert, which is equivalent to the maximum permissible level for a year in a single day. However, the reactor and the steel containment structure remained intact. Unit 1 exploded on 12 March 2011 knocking down the external concrete building. 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. The emergency onsite generation had failed to provide the necessary backup power needed to support the critical instruments and control systems. The remaining two units 5 and 6 were also shut down for regular inspection. The earthquake measured 8.9 on the Richter magnitude scale, which was much more than the plant’s bearing capacity. This brief provides some technical background on nuclear reactors and the major accident that occurred at the Fukushima Daiichi Nuclear Power Plant (NPP) on 11. The reactor units 1, 3 and 4 were automatically shut down following the earthquake. Units 3 and 5 were supplied by Toshiba and Unit 4 by Hitachi. Ebasco provided the architectural design, and Kajima constructed the nuclear complex. The Units 1-4 were disabled in April 2012, followed by the Units 5 and 6 in January 2014 in accordance with the Electric Utility Industry Law article 9. Unit 3 was being fed with mixed-oxide (MOX) fuel since September 2010. All the reactors except Unit 3 continued using low enriched uranium (LEU). Units 1 to 5 are Mark I type while Unit 6 is a Mark II built with containment structures. 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.

The evacuation zone around the nuclear complex was increased from 9km to 19km and was further extended to 30km radius after the explosion of Unit 3. This failure resulted in nuclear explosion in the reactors.Įfforts to cool the reactor vessels with seawater and boric acid failed.

A portion of the fuel rods that create heat through nuclear reaction was exposed due to the failure of the cooling system caused by the tsunami. The earthquake had cut off the power supply needed to pump cooling water into the damaged reactors.