The Chernobyl disaster was a catastrophic nuclear accident. It occurred on 25–26 April 1986 in the No. 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near the now-abandoned town of Pripyat, in northern Ukrainian Soviet Socialist Republic, Soviet Union, approximately 104 km (65 mi) north of Kiev. The event 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 the steam generating a destructive steam explosion and a subsequent open-air graphite fire. This fire produced considerable updrafts for 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 equalled in magnitude the airborne fission products released in the initial destructive explosion. This radioactive material precipitated onto parts of the western USSR and Europe. The remains of the No. 4 reactor building were enclosed in a large cover which was named the “Object Shelter,” often known as the sarcophagus. The purpose of the structure was to reduce the spread of the remaining radioactive dust and debris from the wreckage 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 shut-down 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. Credit: Wikipedia.
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