The Earthquake and its Immediate Aftermath
On the afternoon of Friday, March 11, 2011, an earthquake of Richter magnitude 9 occurred with its epicenter off the northeast coast of the Japanese island of Honshu. The Japanese infrastructure withstood the earthquake well but was devastated by the tsunami that followed, with unprecedented loss of life and property.
The Fukushima I nuclear complex is owned and operated by Tokyo Electric Power Company (TEPCO) and consists of six boiling water reactors (BWR5) of General Electric Company (GE) design. The first reactor was commissioned in 1971. At the time the event took place, units 1, 2 and 3 were operating and units 4, 5 and 6 were shut down for periodic inspection. Units 1, 2 and 3 started the process of automatic shutdown (SCRAM) when the earthquake struck.
None of the reactor containment vessels was compromised despite the earthquake's magnitude being considerably greater than the maximum design specifications for the reactors. The tsunami arrived 50 minutes later and, at a height of 13m, overwhelmed the 5.7m sea wall. The diesel generators providing emergency power were flooded and put out of action, leaving the cooling water pumps dependent upon batteries. The pumps shut down after depletion of the batteries.
This loss of cooling water led to partial meltdowns in reactors 1, 2 and 3 in the hours and days following the accident. The inability to control temperatures also caused the metallic cladding, which contains the uranium dioxide fuel, to react with the residual water and evolve hydrogen. This was released from the reactor pressure vessels and collected in the roof space above the reactors. It detonated on March 12, 14 and 15. Unit 4 also suffered explosive roof damage on March 15.
It is now thought that the hydrogen source was unit 3 rather than a source in unit 4. The situation was stabilized over several weeks using a series of ad hoc measures, including seawater flooding.
The complex of four reactors at Fukushima II is situated 7.5 kilometers from Fukushima I. It too experienced cooling water systems failures but achieved cold shutdown by March 15.