The Chernobyl disaster, also referred to as the Chernobyl accident or simply Chernobyl, 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 in the city of Pripyat, in what was then known as the Ukrainian Soviet Socialist Republic of the Soviet Union (USSR). During an impatient 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", that "burned" for 9 days releasing plumes of radioactive isotopes into the atmosphere, spreading over much of the western USSR and Europe. The Chernobyl disaster was the worst nuclear power plant accident in history in terms of cost and casualties. It is one of only two 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 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 itself 2 deaths occurred and 31 people died in the following months from acute radiation syndrome, with long-term effects such as 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 the year 2000. The accident motivated safety upgrades on all remaining Soviet designed reactors in the RBMK(Chernobyl No.4) family, of which eleven continue to power electric grids as of 2013.

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