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. 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 symptoms, of which 28 firemen and employees died in the days-to-months afterward from the effects of acute radiation syndrome (ARS). 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.





The Chernobyl accident is considered the most disastrous nuclear power plant accident in history, both in terms of cost and casualties. It is one of only two nuclear energy accidents classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in Japan in 2011. The struggle to safeguard against scenarios which were perceived as having the potential for greater catastrophe, together with later decontamination efforts of the surroundings, ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.

The remains of the No.4 reactor building were enclosed in a large cover which was named the "Object Shelter", often known as the sarcophagus. The purpose of the structure was to reduce the spread of the remaining radioactive dust and debris from the wreckage and the protection of the wreckage from further weathering. The sarcophagus was finished in December 1986 at a time when what was left of the reactor was entering the cold shut-down phase. The enclosure was not intended as a radiation shield, but was built quickly as occupational safety for the crews of the other undamaged reactors at the power station, with No.3 continuing to produce electricity up into 2000.





The disaster began during a systems test on 26 April 1986 at reactor 4 of the Chernobyl plant near Pripyat and in proximity to the administrative border with Belarus and the Dnieper River. There was a sudden and unexpected power surge. When operators attempted an emergency shutdown, a much larger spike in power output occurred. This second spike led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. For the next week, the resulting fire sent long plumes of highly radioactive fallout into the atmosphere over an extensive geographical area, including Pripyat. The plumes drifted over large parts of the western Soviet Union and Europe. According to official post-Soviet data, about 60% of the fallout landed in Belarus.





Thirty-six hours after the accident, Soviet officials enacted a 10-kilometre exclusion zone, which resulted in the rapid evacuation of 49,000 people primarily from Pripyat, the nearest large population centre. Although not communicated at the time, an immediate evacuation of the town following the accident was not advisable as the road leading out of the town had heavy nuclear fallout hotspots deposited on it. Initially, the town itself was comparatively safe due to the favourable wind direction. Until the winds began to change direction, shelter in place was considered the best safety measure for the town.





As plumes and subsequent fallout continued to be generated, the evacuation zone was increased from 10 to 30 km about one week after the accident. A further 68,000 persons were evacuated, including from the town of Chernobyl itself. The surveying and detection of isolated fallout hotspots outside this zone over the following year eventually resulted in 135,000 long-term evacuees in total agreeing to be moved. The near tripling in the total number of permanently resettled persons between 1986 and 2000 from the most severely contaminated areas to approximately 350,000 is regarded as largely political in nature, with the majority of the rest evacuated in an effort to redeem loss in trust in the government, which was most common around 1990. Many thousands of these evacuees would have been "better off staying home." Risk analysis in 2007, supported by DNA biomarkers, has determined that the "people still living unofficially in the abandoned lands around Chernobyl" have a lower risk of dying as a result of the elevated doses of radiation in the rural areas than "if they were exposed to the air pollution health risk in a large city such as nearby Kiev."



