On April 25, 1986, the worst nuclear accident in history occurred in present day Ukraine as a reactor at a nuclear power plant exploded and burned. The disaster took place near the city of Chernobyl in the former USSR, which invested heavily in nuclear power after World War II.
The accident started during a safety test on an RBMK-type nuclear reactor. The test was a simulation of an electrical power outage to help create a safety procedure for maintaining reactor cooling water circulation until the back-up electrical generators could provide power. Three tests were conducted since 1982, but they had failed to provide a solution. On this fourth attempt, an unexpected 10-hour delay resulted in, unprepared operating shift of workers were on duty. During the planned decrease of reactor power in preparation for the electrical test, the power unexpectedly dropped to a near-zero level. The operators were able to only partially restore the specified test power, which put the reactor in an unstable condition. This risk was not made evident in the operating instructions, so the operators proceeded with the electrical test. Upon test completion, the operators triggered a reactor shutdown, but a combination of unstable conditions and reactor design flaws caused an uncontrolled nuclear chain reaction.
A large amount of energy was suddenly released, and two explosions ruptured the reactor core and destroyed the reactor building. One was a highly destructive steam explosion from the vaporizing super-heated cooling water and the other explosion could have been another steam explosion or a small nuclear 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, especially Belarus, 16 km away, where around 70% landed, 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 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 when a further 68,000 people were evacuated from the wider area.
To reduce the spread of radioactive contamination from the wreckage and protect it from weathering, the protective Chernobyl Nuclear Power Plant sarcophagus was built by December 1986. It provided radiological protection for the crews of the undamaged reactors at the site, which continued operating.
The reactor explosion killed two of the reactor operating staff. A massive emergency operation to put out the fire, stabilize the reactor, and clean up the ejected nuclear core began. In the disaster and immediate response, 134 station staff and firemen were hospitalized with acute radiation syndrome due to absorbing high doses of ionizing radiation. Of these 134 people, 28 died in the days to months afterward and approximately 14 suspected radiation-induced cancer deaths followed within the next 10 years. More than 15 childhood thyroid cancer deaths were documented. Significant clean-up operations were taken in the exclusion zone to deal with local fallout, and the exclusion zone was made permanent. The World Health Organisation estimates 4000 people will eventually die as a result of the accident, from cancers and radiation poisoning.
It’s estimated that the amount of radioactive material was 400 times more than the atom bomb dropped on Hiroshima. Scientists estimate the zone around the former power plant will not be habitable for up to 20,000 years.
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