The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophic nuclear accident that occurred on 26 April 1986 in the No.4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near Pripyat, in what was then part of the Ukrainian Soviet Socialist Republic of the Soviet Union (USSR).
During a hurried late night power-failure stress test, 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 stress 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 updrafts 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 afterward from acute radiation syndrome, with the potential for long-term cancers still being investigated.
The remains of the No.4 reactor building were enclosed in a large sarcophagus (radiation shield) by December 1986, at a time when what was left of the reactor was entering the cold shut-down phase; the enclosure was built quickly as occupational safety for the crews of the other undamaged reactors at the power station, with No.3 continuing to produce electricity into 2000.
The accident motivated safety upgrades on all remaining Soviet-designed reactors in the RBMK (Chernobyl No.4) family, of which eleven continued to power electric grids as of 2013.