Nuclear or radiation accidents are occasions on which irradiation or radioactive contamination occurs in accidental circumstances; and cases of irradiation or contamination which are likely to lead to overexposure are usually considered to be accidental. 'Overexposure' is to be understood as meaning any radioactive irradiation or contamination such that the maximum permissible levels laid down in the relevant regulations are likely to be exceeded. The consequences of such events depend on the circumstances and extent of the irradiation or contamination.
Nuclear accidents can involve X-ray equipment, radioactive isotopes, particle accelerators, nuclear fuel production plants, nuclear power plants and atomic and hydrogen bombs. Some accidents threaten public health, contaminate crops and animals and could destroy the country side for hundreds of square kilometres. There have been more accidents involving nuclear weapons or reactors than the authorities have officially admitted.
It is extremely difficult to predict exactly what kinds of accidents will occur and thus build in safeguards. All major nuclear accidents in the USA have involved events completely unforeseen in any detailed government study of reactor safety. Furthermore, operator error is likely to play a major role in future accidents.
In addition to the accidents at Chernobyl, Three Mile Island and a number of other nuclear facilities, radiation accidents are increasingly common from non-nuclear facilities. Radium needles are leaking in a hospital in Khartoum. In Brazil, an unwanted radiocaesium source was stolen, the thieves cut it open, there was contamination, several people died. There has been a similar incident in Mexico. There are unwanted teletherapy sources in Jordan and Zaire, old radium needles in Afghanistan and Bolivia, gauging sources abandoned at a mine in Zambia, a neutron source in broken-down equipment in Uganda.
The 1986 Chernobyl explosion played a critical role in opening up admissions of previous disasters in the former Soviet Union: the secret dumping of millions of tonnes of highly radioactive waste in the River Techa since the early 1950's by a plutonium plant -- 2.75 million curies of waste, equivalent to half the fallout from the bomb that fell on Hiroshima, some tens of kilometres upstream of drinking and bathing water supplies of unknowing villagers; a series of accidents at a plutonium-producing plant near the southern Urals city of Chelyabinsk between 1948 and 1967; a waste storage tank which exploded at the Mayak plutonium plant in 1957, releasing 20 million curies of radiation; a decade later a drought dried up nearby Lake Karachai which had been used as a reservoir for 120 million curies of waste products. Winds scattered radioactive dust over a wide area.
Nuclear accidents, like the Challenger disaster, have repeatedly demonstrated that fools of sufficient magnitude can be found to overcome any foolproof system.
The specific accident which occurred at Chernobyl (a water-cooled, graphite moderated RBMK reactor) could not -- as a matter of physics -- occur in any Western European rector, and the operating cultures in western European countries would make it practically impossible for any accident to occur which had catastrophic consequences outside the plant.