Chernobyl Nuclear Accident
The destroyed reactor Source: Chernobyl Forum
The Chernobyl nuclear facility is located in Ukraine about 20 km south of the border with Belarus. At the time of the accident, the plant had four working reactors (units 1, 2, 3, and 4).
The accident occurred in the very early morning of 26 April 1986 when operators ran a test on an electric control system of unit 4. The accident happened because of a combination of basic engineering deficiencies in the reactor and faulty actions of the operators. The safety systems had been switched off, and the reactor was being operated under improper, unstable conditions, a situation which allowed an uncontrollable power surge to occur. This power surge caused the nuclear fuel to overheat and led to a series of steam explosions that severely damaged the reactor building and completely destroyed the unit 4 reactor.
The explosions started numerous fires on the roofs of the reactor building and the machine hall, which were extinguished by firefighters after a few hours. Approximately 20 hours after the explosions, a large fire started as the material in the reactor set fire to combustible gases. The large fire burned during 10 days. Helicopters repeatedly dumped neutron-absorbing compounds and fire-control materials into the crater formed by the destruction of the reactor and later the reactor structure was cooled with liquid nitrogen using pipelines originating from another reactor unit.
The radioactive materials from the damaged reactor were mainly released over a 10-day period. An initial high release rate on the first day resulted from the explosions in the reactor. There followed a five-day period of declining releases associated with the hot air and fumes from the burning graphite core material. In the next few days, the release rate increased until day 10, when the releases dropped abruptly, thus ending the period of intense release. The radioactive materials released by the accident deposited with greatest density in the regions surrounding the reactor in the European part of the former Soviet Union. More...