What does it take to cause a historic nuclear meltdown? How about a 9.0 magnitude earthquake off the coast of eastern Japan, followed by a consequentially larger tsunami? The Fukushima Daiichi nuclear disaster of March 2011 joins the Chernobyl disaster as one of the worst nuclear meltdowns in history. The earthquake caused much damage to the plant, decommissioning 3 of 6 reactors. The reactors are designed to shut down automatically in the case of emergency at which point emergency generators would power electronics and coolant system. However, It was the 15-meter tsunami triggered by the earthquake that caused the disaster. The tsunami flooded the rooms that contained the emergency generators used for cooling the reactor systems. Upon the generator failure, pumps critical to maintaining the continuous circulation of coolant throughout the reactors to prevent meltdown lost power. As the pumps stopped, the reactors overheated as a result of to the normal high amount of radioactive decay heat. Due to this high radioactive release the accident was rated a 7 on the INES (International Nuclear and Radiological Event Scale) scale. The Chernobyl disaster is the only other nuclear disaster to receive such a rating. The World Nuclear Association reports “there have been no deaths or cases of radiation sickness from the nuclear accident, but over 100,000 people had to be evacuated from their homes to ensure this.” Efforts to clean up and repair the damage are ongoing, however it is projected to take up to 40 years and could cost up to $12 billion to close the reactors. Apart from cooling, a large part of ongoing on-site tasks are to prevent release of radioactive materials, especially in the form of contaminated water leaked from the three units.
http://www.bbc.co.uk/news/world-asia-21737910
http://rt.com/news/fukushima-nuclear-plant-disaster-013/
http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster
http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Fukushima-Accident-2011/#.UVOtmlvwJc8