The accident occurred during a late-night safety test which simulated a station blackout power-failure, in the course of which both emergency safety and power-regulating systems were intentionally turned off. A combination of inherent reactor design flaws as well as 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. [note 1] This fire produced considerable updrafts for about nine days. The heat was so intense that it melted firefighters' boots. The fire was finally contained on 4 May 1986. The lofted plumes of fission products released into the atmosphere by the fire precipitated onto western Europe and parts of the USSR. 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.
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