The Chernobyl disaster was a catastrophic nuclear accident that occurred on 26 April 1986 at the No. 4 nuclear reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR.
The accident occurred during a safety test on a common Soviet reactor type - the RBMK nuclear power reactor. The test, a simulation of an electrical power outage, was to aid the development of a safety procedure for keeping reactor cooling water circulating until the emergency generators could provide power. This gap was about one minute and had been identified as a potential safety problem which could cause reactor core overheating. Three such tests had been conducted since 1982 but had failed to provide a solution. On the fourth occasion, which became catastrophic, the test had been delayed by ten hours and the reactor operating shift that had specifically prepared for the test procedure was replaced by the next shift. The test supervisor then failed to follow the test procedure, creating unstable operating conditions which, combined with inherent RBMK reactor design flaws and the intentional disabling of several emergency safety systems, resulted in an uncontrolled nuclear chain reaction.[2] A huge amount of energy was suddenly released, which vapourised superheated cooling water, rupturing the reactor pressure vessel in a highly destructive steam explosion, which was instantly followed by an open-air reactor core fire.
This fire produced considerable updrafts for about nine days[5] before being finally contained on 4 May 1986. The lofted plumes of fission products released into the atmosphere by the fire precipitated onto parts of the USSR and western Europe. 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.
The total number of casualties remains disputed. Estimates of reduced life expectancy as a result of radiation released are highly uncertain and vary from 4,000 people in a United Nations study up to 200,000 reported by a Greenpeace study.[8] 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 ionizing radiation. Over the coming days and weeks, 134 servicemen were hospitalized with acute radiation syndrome (ARS), of whom 28 firemen and employees died within months.[9] Additionally, approximately 14 radiation-induced cancer deaths among this group of 134 hospitalized survivors were to follow within the next 10 years.[10] Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011.[11][12] Additional time and research is required 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.[13]
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 major accident", the maximum classification on the International Nuclear Event Scale; the other was the 2011 Fukushima disaster in Japan.[14] The struggle to safeguard against scenarios that were perceived as having the potential for greater catastrophe,[15] together with later decontamination efforts of the surroundings, ultimately involved over 500,000 liquidators and cost an estimated 18 billion rubles (roughly $30 billion USD in 1986, or $68 billion USD in 2019 adjusted for inflation).[16]
The remains of the No. 4 reactor building were soon enclosed in the sarcophagus, a large shelter designed to reduce the spread of radioactive contamination from the wreckage and to protect the site from further weathering. This was rapidly built and was finished by December 1986, when the reactor was entering the cold shutdown phase. The shelter also provided radiological protection for the crews of the other undamaged reactors at the power station, with No. 3 continuing to produce electricity until 2000.[17][18] Due to the continued deterioration of the sarcophagus both it and the No. 4 reactor were further enclosed in 2017 by the Chernobyl New Safe Confinement, a larger, state of the art enclosure, designed and built by an international team. This structure has the ability to facilitate the removal of both the sarcophagus and the reactor debris, while containing the radioactive contamination.
The accident prompted safety upgrades on all remaining Soviet-designed RBMK reactors, of which 10 continue to power electric grids as of 2019.
