The accident occurred during a late-night safety test which simulated a station blackout power-failure, in the course of which safety systems were intentionally turned off. A combination of inherent reactor design flaws and the reactor operators arranging the core in a manner contrary to the checklist for the test, eventually resulted in uncontrolled reaction conditions. Water flashed into steam generating a destructive steam explosion and a subsequent open-air graphitefire.[note 1] This fire produced considerable updraftsfor about nine days. These lofted plumes of fission products into the atmosphere. The estimated radioactive inventory that was released during this very hot fire phase approximately equaled in magnitude the airborne fission products released in the initial destructive explosion. This radioactive material precipitated onto parts of the western USSR and other European countries.
During the accident, steam-blast effects caused two deaths within the facility: one immediately after the explosion, and the other compounded by a lethal dose of radiation. Over the coming days and weeks, 134 servicemen were hospitalized with acute radiation syndrome (ARS), of which 28 firemen and employees died in the days-to-months afterward. Additionally, approximately fourteen radiation induced cancer deaths among this group of 134 hospitalized survivors were to follow within the next ten years (1996). Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011. It will take further time and investigation to definitively determine the elevated relative risk of cancer among the surviving employees, those that were initially hospitalized with ARS, and the population at large.
The Chernobyl accident is considered the 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 disaster in Japan in 2011. The struggle to safeguard against scenarios that were perceived as having the potential for greater catastrophe, together with later decontaminationefforts of the surroundings, ultimately involved over 500,000 workers (called liquidators) and cost an estimated 18 billion rubles.
The remains of the No. 4 reactor building were enclosed in a large cover which was named the “Object Shelter”, often known as “sarcophagus“. The purpose of the structure was to reduce the spread of the remaining radioactive dust and debris from the wreckage, thus limiting radioactive contamination, and the protection of the wreckage from further weathering. The sarcophagus was finished in December 1986 at a time when what was left of the reactor was entering the cold shutdown phase. The enclosure was not intended as a radiation shield, but 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 up into 2000.
The accident motivated safety upgrades on all remaining Soviet-designed RBMK reactors, the same type as Chernobyl No. 4, of which ten continue to power electric grids as of 2019.