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The was a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011.〔(【引用サイトリンク】url=http://spectrum.ieee.org/tech-talk/energy/nuclear/explainer-what-went-wrong-in-japans-nuclear-reactors )〕 It is the largest nuclear disaster since the Chernobyl disaster of 1986.〔("Analysis: A month on, Japan nuclear crisis still scarring" ) ''International Business Times'' (Australia). 9 April 2011, retrieved 12 April 2011; excerpt, According to James Acton, Associate of the Nuclear Policy Program at the Carnegie Endowment for International Peace, "Fukushima is not the worst nuclear accident ever but it is the most complicated and the most dramatic...This was a crisis that played out in real time on TV. Chernobyl did not."〕 ==Overview== The plant comprises six separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. Immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to control electronics and coolant systems. However the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through a Generation II nuclear reactor for several days in order to keep it from melting down after being shut down. As the pumps stopped, the reactors overheated due to the normal high radioactive decay heat produced in the first few days after nuclear reactor shutdown (smaller amounts of this heat normally continue to be released for years, but are not enough to cause fuel melting). At this point, only prompt flooding of the reactors with seawater could have cooled the reactors quickly enough to prevent meltdown. Salt water flooding was delayed because it would ruin the costly reactors permanently. Flooding with seawater was finally commenced only after the government ordered that seawater be used, and at this point it was too late to prevent meltdown.〔F. Tanabe, Journal of Nuclear Science and Technology, 2011, volume 48, issue 8, pages 1135 to 1139〕 As the water boiled away in the reactors and the water levels in the fuel rod pools dropped, the reactor fuel rods began to overheat severely, and to melt down. In the hours and days that followed, Reactors 1, 2 and 3 experienced full meltdown. In the intense heat and pressure of the melting reactors, a reaction between the nuclear fuel metal cladding and the remaining water surrounding them produced explosive hydrogen gas. As workers struggled to cool and shut down the reactors, several hydrogen-air chemical explosions occurred.〔〔(Hydrogen explosions Fukushima nuclear plant: what happened? )〕 Concerns about the repeated small explosions, the atmospheric venting of radioactive gasses, and the possibility of larger explosions led to a -radius evacuation around the plant. During the early days of the accident workers were temporarily evacuated at various times for radiation safety reasons. At the same time, sea water that had been exposed to the melting rods was returned to the sea heated and radioactive in large volumes for several months until recirculating units could be put in place to repeatedly cool and re-use a limited quantity of water for cooling. The earthquake damage and flooding in the wake of the tsunami hindered external assistance. Electrical power was slowly restored for some of the reactors, allowing for automated cooling.〔(Stricken reactors may get power Sunday ), ''The Wall Street Journal'', 19 March 2011〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Accident rating of the Fukushima Daiichi nuclear disaster」の詳細全文を読む スポンサード リンク
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