<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophic nuclear accident . It occurred on 26 April 1986 in the No. 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near Pripyat, a town in northern Ukrainian Soviet Socialist Republic which was part of the Soviet Union (USSR). According to Nuclear Technology magazine, there is a scientific hypothesis that the failure was inititiated by a true nuclear blast, which emitted a blue flash prior to the steam blast . </P> <P> The event occurred during a late - night safety test which simulated a station blackout power - failure and in which safety systems were deliberately 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 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 . 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 . </P>

When did the nuclear accident happen in chernobyl