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 Soviet Socialist Republic. It is considered the worst nuclear disaster in history and 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 that 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 10 hours, so the operating shift that had been prepared was not present. The test supervisor then failed to follow procedure, creating unstable operating conditions that, 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 that released considerable airborne radioactive contamination for about nine days that 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 36 hours after the accident. About 49,000 people were evacuated from the area, primarily from Pripyat. The exclusion zone was later increased to 30 kilometres (19 mi) radius when a further 68,000 people were evacuated from the 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 a protective enclosure. The Chernobyl Nuclear Power Plant sarcophagus was built and finished by December 1986, and 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 that allows the removal of both the sarcophagus and the reactor debris, while containing the radioactive hazard. Nuclear clean-up is scheduled for completion in 2065.The reactor explosion killed two of the reactor operating staff. In the emergency response that followed, 134 firemen and station staff were hospitalized with acute radiation syndrome due to absorbing high doses of ionizing radiation. Of these 134 men, 28 died in the days to months afterward and approximately 14 suspected radiation-induced cancer deaths followed within the next 10 years. Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011. 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, fewer than 100 documented deaths are likely to be attributable to increased exposure to radiation.Determining the total eventual number of exposure related deaths is based on the linear no-threshold model, a contested statistical model, which has also been used in estimates of low level radon and air pollution exposure. Model 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.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 more than 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.
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