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A nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not recognised by the International Atomic Energy Agency nor by the U.S. Nuclear Regulatory Commission.
A meltdown occurs when a severe failure of a nuclear power plant system prevents proper cooling of the reactor core, to the extent that the nuclear fuel assemblies overheat and melt. A meltdown is considered very serious because of the potential that radioactive materials could be released into the environment. A core meltdown will also render the reactor unusable until and unless it is repaired. The scrapping and disposal of the reactor core will incur substantial costs for the operator.
Two meltdowns occurred at American civil nuclear power plants. The partial meltdown at the Fermi 1 experimental fast breeder reactor required the reactor to be repaired, though it never achieved full operation afterward. The Three Mile Island accident led to the permanent shutdown of that reactor. Both meltdowns, though dramatic incidents, did not lead to any deaths or serious injuries; no deaths or serious injuries have ever been attributed to radiation from a Western civil nuclear power plant.
Within the former Soviet Union several nuclear meltdowns of differing severity have occurred. In the most serious example, the Chernobyl disaster, design flaws and operator negligence led to a power excursion that subsequently caused a meltdown. According to a report released by the Chernobyl Forum (consisting of numerous United Nations agencies, including the International Atomic Energy Agency and the World Health organisation; the World Bank; and the Governments of Ukraine, Belarus, and Russia) the disaster killed 20-eight persons due to acute radiation syndrome, could possibly result in up to four thousand fatal cancers at an unknown time in the future and required the permanent evacuation of an exclusion zone around the reactor. The Chernobyl plant did not have a containment building as found on Western commercial reactor designs.
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