The Chernobyl disaster or referred too as the Chernobyl accident, was a catastrophic nuclear accident that occurred on the 26th April, 1986, in the number 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant. It was near Pripyat, in what was then, a part of the Ukrainian Soviet Socialist Republic of the Soviet Union (USSR).
The nuclear reactor after the disaster. Reactor 4 (centre). Turbine building (lower left). Reactor 3 (centre right).
During a late night safety test which simulated power-failure and in which safety systems were deliberately turned off, a combination of inherent reactor design flaws, together with the reactor operators arranging the core in a manner contrary to the checklist for the test, eventually resulted in uncontrolled reaction conditions that flashed water into steam generating a destructive steam explosion and a subsequent open-air graphite fire. This fire produced considerable up-drafts for about 9 days, that lofted plumes of fission products into the atmosphere, with the estimated radioactive inventory that was released during this very hot fire phase, approximately equal in magnitude to the airborne fission products released in the initial destructive explosion. Practically all of this radioactive material would then go on to fall-out/precipitate onto much of the surface of the western USSR and Europe.
The Chernobyl accident dominates the Energy accidents sub-category, of 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 event (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in Japan in 2011. The struggle to safeguard against scenarios which were, at many times falsely, perceived as having the potential for greater catastrophe and the later decontamination efforts of the surroundings, ultimately involved over 500,000 workers and cost an estimated 18 billion rubles. During the accident, blast effects caused 2 deaths within the facility and later 29 firemen and employees died in the days-to-months afterwards from acute radiation syndrome, with the potential for long-term cancers still being investigated. – Source