The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophic nuclear accident. It occurred on 25-26 April 1986 in the No. 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near the now-abandoned town of Pripyat, in northern Ukrainian Soviet Socialist Republic, Soviet Union, approximately 104 km (65 mi) north of Kiev.
The event occurred during a late-night safety test which simulated a station blackout power-failure, in the course of which safety systems were intentionally turned off. A combination of inherent reactor design flaws and the reactor operators arranging the core in a manner contrary to the checklist for the test, eventually resulted in uncontrolled reaction conditions. Water flashed into steam generating a destructive steam explosion and a subsequent open-air graphite fire.[note 1] This fire produced considerable updrafts for about nine days. These lofted plumes of fission products into the atmosphere. The estimated radioactive inventory that was released during this very hot fire phase approximately equaled in magnitude the airborne fission products released in the initial destructive explosion. This radioactive material precipitated onto parts of the western USSR and Europe.
During the accident, steam-blast effects caused two deaths within the facility; one immediately after the explosion, and the other, compounded by a lethal dose of radiation. Over the coming days and weeks, 134 servicemen were hospitalized with acute radiation sickness(ARS), of which 28 firemen and employees died in the days-to-months afterward from the radiation effects. In addition, approximately fourteen radiation induced cancer deaths among this group of 134 hospitalized survivors, were to follow within the next ten years (1996). Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011. It will take further time and investigation to definitively determine the elevated relative risk of cancer among the surviving employees, those that were initially hospitalized with ARS and the population at large.