The Great East Japan Earthquake* with magnitude 9.0 at 2.46 pm on Friday 11 March did considerable damage in the region, and the large tsunami it created caused very much more. The earthquake was centred 130 km offshore the city of Sendai in Miyagi prefecture on the eastern cost of Honshu Island (the main part of Japan), and was a rare and complex double quake giving a severe duration of about 3 minutes. Japan moved a few metres east and the local coastline subsided half a metre. The tsunami inundated about 560 sq km and resulted in a human death toll of over 20,000.
Eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the quake hit. The operating units which shut down were Tepco's Fukushima Daiichi 1, 2, 3, Fukushima Daini 1, 2, 3, 4, Tohoku's Onagawa 1, 2, 3, and Japco's Tokai, total 9377 MWe net. Fukushima Daiichi units 4-6 were not operating at the time, but were affected, total 2587 MWe net (units 4-6). Onagawa 1 briefly suffered a fire in the turbine building, but the main problem initially centred on Fukushima Daiichi units 1-3. Unit 4 became a problem on day five.
The reactors proved robust seismically, but vulnerable to the tsunami. Power, from grid or backup generators, was available to run the Residual Heat Removal (RHR) system cooling pumps at eight of the eleven units, and despite some problems they achieved 'cold shutdown' within about four days. The other three, at Fukushima Daiichi, lost power at 3.42 pm, almost an hour after the quake, when almost the entire site lost the ability to maintain proper reactor cooling and water circulation functions due to being flooded by the 15-metre tsunami. This disabled 12 of 13 back-up generators on site, located in the basements of the turbine buildings, and also the heat exchangers for dumping reactor heat to the sea. Electrical switchgear was also disabled.
Thereafter, many weeks of focused work centred on restoring heat removal from the reactors and coping with overheated spent fuel ponds. This was undertaken by hundreds of Tepco employees as well as some contractors, supported by firefighting and military personnel. Some of the Tepco staff had lost homes, and even families, in the tsunami, and were initially living in temporary accommodation under great difficulties and privation, with some personal risk. Media coverage of the Fukushima drama often ignored the context of the enormous natural disaster which greatly affected how it played out. A hardened emergency response centre on site proved very helpful in grappling with the situation.
Three Tepco employees at the Daiichi plant were killed directly by the earthquake and tsunami.
Among hundreds of aftershocks, an earthquake with magnitude 7.1, closer to Fukushima than the 11 March one, was experienced on 7 April, but without further damage to the plant. The epicenter was 120 km from Fukushima but only 20 km from Onagawa, where power supply was affected. On 11 April a magnitude 7.1 earthquake and on 12 April a magnitude 6.3 earthquake, both with epicenter at Fukushima-Hamadori, caused no further problems.
Resource:Fukushima accident
Additional:Timeline for the Fukushima Daiichi nuclear power plant accident
Picture:Fukushima Daiichi Nuclear Plant Hi-Res Photos
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