On March 11, 2011 a major earthquake created a tsunami measuring over 50 feet that hit the coast of Japan most severely in Fukushima Daiichi. Upon impact the tsunami disabled the power supply and cooling systems of three nuclear reactors, causing all three cores to melt within three days and high radioactive releases. The most imparative ongoing struggle regarding the incident was to prevent the release of radioactive materials from contaminated water that leaked from the three reactors. Over 100,000 people had to be evacuated from their homes, though there have been no reported deaths or cases of radiation sickness. According to an expert report commisioned by the World Health Organization the nuclear disaster has increased the risk of cancer in the radiation affected areas.
The earthquake that caused the disaster was measured as a 9.0 on a 10 point scale. The resulting tsunami caused even more damage than the original quake, which happened about 100 miles off the coast of the city of Sendai. Furthermore, the quake was a actually a rare and complex occurance known as a double quake giving a severe duration of about 3 minutes. Japan moved a few meters East.
The Daiishi reactors proved structurally stable after the quake but were flooded when the resulting tsunami hit the coast. Generators were used to power the Residual Heat Removal (RHR) system, which serve to cool the cores. In some reactors the generators on site were flooded as well. Without heat removal by circulation through an exhaust system, pressure gathered from steam in the cores. Water was used to try and cool the plant along with the Emergency Core Cooling System. However, these systems failed and the fuel in the core rose to 2800 degrees Celcius or about 5100 degrees Fahrenheit, which caused the core to melt. Also, as pressure rose there was an attempt to vent the gas, but without power the gases backflowed to the service floor at the top of the reactor. Here noble gas, aerosols and hydrogen mixed with air and ignited causing a hydrogen explosion on the service floor of the Unit 1 reactor. Eventually the melted core of Unit 1 eroded through 65 cm of the drywell concrete below the reactor, which is a total of about 260 cm thick. In the concrete the intensity of the heat was reduced and the mass solidified.
References:
http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Fukushima-Accident-2011/
http://www.abc.net.au/news/2013-03-07/water-from-the-tsunami-that-hit-the-fukushima/4558338
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