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The Fukushima Daiichi nuclear disaster was a 2011 nuclear accident at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima Prefecture, Japan. The event was primarily caused by the 2011 Tōhoku earthquake and tsunami.
It was the most severe nuclear accident since the Chernobyl disaster in 1986. It was classified as Level 7 on the International Nuclear Event Scale (INES), after initially being classified as Level 5, joining Chernobyl as the only other accident to receive such classification. While the 1957 explosion at the Mayak facility was the second worst by radioactivity released, the INES ranks incidents by impact on population, so Chernobyl (335,000 people evacuated) and Fukushima (154,000 evacuated) rank higher than the 10,000 evacuated from the classified restricted Mayak site in rural southern Urals.
The accident was triggered by the Tōhoku earthquake and tsunami on Friday, 11 March 2011. On detecting the earthquake, the active reactors automatically shut down their normal power-generating fission reactions. Because of these shutdowns and other electrical grid supply problems, the reactors' electricity supply failed, and their emergency diesel generators automatically started. Critically, these were required to provide electrical power to the pumps that circulated coolant through the reactors' cores. This continued circulation was vital to remove residual decay heat, which continues to be produced after fission has ceased. However, the earthquake had also generated a tsunami 14 metres (46 ft) high that arrived shortly afterwards and swept over the plant's seawall and then flooded the lower parts of reactors 1–4. This flooding caused the failure of the emergency generators and loss of power to the circulating pumps. The resultant loss of reactor core cooling led to three nuclear meltdowns, three hydrogen explosions, and the release of radioactive contamination in Units 1, 2 and 3 between 12 and 15 March. The spent fuel pool of previously shut down Reactor 4 increased in temperature on 15 March due to decay heat from newly added spent fuel rods, but did not boil down sufficiently to expose the fuel.In the days after the accident, radiation released to the atmosphere forced the government to declare an ever-larger evacuation zone around the plant, culminating in an evacuation zone with a 20 km radius. All told, some 154,000 residents evacuated from the communities surrounding the plant due to the rising off-site levels of ambient ionizing radiation caused by airborne radioactive contamination from the damaged reactors.Large amounts of water contaminated with radioactive isotopes were released into the Pacific Ocean during and after the disaster. Michio Aoyama, a professor of radioisotope geoscience at the Institute of Environmental Radioactivity, has estimated that 18,000 terabecquerel (TBq) of radioactive caesium-137 were released into the Pacific during the accident, and in 2013, 30 gigabecquerel (GBq) of caesium 137 were still flowing into the ocean every day. The plant's operator has since built new walls along the coast and has created a 1.5 km long "ice wall" of frozen earth to stop the flow of contaminated water.While there has been ongoing controversy over the health effects of the disaster, a 2014 report by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and World Health Organization projected no increase in miscarriages, stillbirths or physical and mental disorders in babies born after the accident. Evacuation and sheltering to protect the public significantly reduced potential radiation exposures by a factor of 10, according to UNSCEAR. An ongoing intensive cleanup program to both decontaminate affected areas and decommission the plant will take 30 to 40 years from the disaster, plant management estimated.On 5 July 2012, the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) found that the causes of the accident had been foreseeable, and that the plant operator, Tokyo Electric Power Company (TEPCO), had failed to meet basic safety requirements such as risk assessment, preparing for containing collateral damage, and developing evacuation plans. At a meeting in Vienna three months after the disaster, the International Atomic Energy Agency faulted lax oversight by the Ministry of Economy, Trade and Industry, saying the ministry faced an inherent conflict of interest as the government agency in charge of both regulating and promoting the nuclear power industry. On 12 October 2012, TEPCO admitted for the first time that it had failed to take necessary measures for fear of inviting lawsuits or protests against its nuclear plants.

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