The ultimate nuclear reactor accidents would result from an uncontrollable chain reaction, although the possibility of such a reaction arising is virtually nil, due to control and protection systems. Ultimate nuclear reactor nightmare is a meltdown, that occurs when overheated reactors boil away the massive water reservoirs, and the molten core burns its way deep into the earth's crust with repercussions on a scale never experienced before. However, localized brief periods of criticality may arise which in themselves can cause serious threats to health.
The following nuclear reactor accidents have occurred: 1957, Windscale, UK; 1965, Idaho Falls ID, USA; 1966, Detroit MI, USA; 1969, Saint-Laurent, France; 1975, Brown's Ferry AL, USA; 1979, Three Mile Island PA, USA; 1982, Kozluduj, Bulgaria; 1983, Constituyentes, Argentine; 1984; Greifswald, Germany; 1986, Gore, OK, USA; 1986, Chernobyl, Russia, and Fukushima, Japan in 2011. According to the International Atomic Energy Agency (IAEA) more than 250 nuclear reactor accidents have been kept secret. The costs of such accidents are very high - the Three Mile Island accident was estimated to cost $500 million to clean-up, plus the costs to those inhabitants who lived nearby and left the area either permanently or for a short time, which amounted to $19 million. Nuclear explosion at Chernobyl caused evacuation of 135,000 people, death of 31 workers and immediate fall-out and the poisoning of agricultural land in the Soviet Union and Europe from relatively long-lived radioactive isotopes. The number of fatal cancers that might develop worldwide as a result of the accident could range from zero to 17,400, among local population between 5,000 and 25,000.
Nuclear reactor accidents may cause severe diseases or death, as well as profound psychological stress.