The Chernobyl disaster was a 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. It is one of only two nuclear energy disasters rated at seven—the maximum severity—on the International Nuclear Event Scale; the other being the 2011 Fukushima Daiichi nuclear disaster in Japan.
The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. The test was a simulation of an electrical power outage to aid the development of a safety procedure for maintaining cooling water circulation until the back-up generators could provide power. This operating gap was about one minute and had been identified as a potential safety problem which could cause the nuclear reactor core to overheat.
Three such tests had been conducted since 1982, but had failed to provide a solution. On this fourth attempt the test was delayed by ten hours, so the operating shift that had been prepared was not present. The test supervisor then failed to follow the procedure, creating unstable operating conditions which, combined with inherent RBMK reactor design flaws and the intentional disabling of several nuclear reactor safety systems, resulted in an uncontrolled nuclear chain reaction.
A large amount of energy was suddenly released, vapourising superheated cooling water and rupturing the reactor core in a highly destructive steam explosion. This was immediately followed by an open-air reactor core fire which released considerable airborne radioactive contamination for about nine days which precipitated onto parts of the USSR and western Europe, before being finally contained on 4 May 1986.
The fire gradually released about the same amount of contamination as the initial explosion. As a result of rising ambient radiation levels off-site, a 10-kilometre (6.2 mi) radius exclusion zone was created thirty-six hours after the accident. About 49,000 people were rapidly evacuated from the area, primarily from Pripyat. The exclusion zone was increased to 30 kilometres (19 mi) radius shortly after when a further 68,000 persons were evacuated from a wider area.
To reduce the spread of radioactive contamination from the wreckage and to protect the site from further weathering, the remains of reactor No. 4 required an enclosure, which was rapidly built and finished by December 1986. The Chernobyl Nuclear Power Plant sarcophagus would also provide radiological protection for the crews of the undamaged reactors at the site, with No. 3 continuing to produce electricity until 2000. 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 enclosure which allows the removal of both the sarcophagus and the reactor debris, while containing the radioactive contamination.
During the accident, steam-blast effects caused two deaths within the facility. In the ensuing site crisis, 134 firemen and site workers were hospitalized with acute radiation syndrome due to excessive doses of ionizing radiation, of whom 28 died within months and approximately 14 suffered radiation-induced cancer deaths within the next 10 years. Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011.
In common with attempts to estimate low level radon and air pollution exposure situations, determining the total number of exposure related deaths is subject to considerable uncertainty. Predictions with the greatest confidence values of the eventual total death toll in the decades ahead from Chernobyl releases vary, from 4,000 fatalities when solely assessing the three most contaminated former Soviet states, to about 9,000 to 16,000 fatalities when assessing the total continent of Europe. However, due to the often long Incubation periods for radiation exposure to induce cancer, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has, at multiple times, reviewed all the published research on the incident and found that at present, less than 100 documented deaths are likely to be attributable to increased exposure to radiation.
The Chernobyl disaster is considered the worst nuclear power plant accident in history, both in terms of cost and casualties. The struggle to safeguard against hazards immediately after the accident, together with later decontamination efforts of the surroundings, ultimately involved over 500,000 liquidators and cost an estimated 18 billion Soviet rubles—roughly US$68 billion in 2019, adjusted for inflation. The accident prompted safety upgrades on all remaining Soviet-designed RBMK reactors, of which 10 continue to be operational as of 2019.