In the Chernobyl disaster, the moderator was not responsible for the primary event. Instead, a massive power excursion during a mishandled test caused the catastrophic failure of the reactor vessel and a near-total loss of coolant supply. The result was that the fuel rods rapidly melted and flowed together while in an extremely-high-power state, causing a small portion of the core to reach a state of runaway prompt criticality and leading to a massive energy release,[22] resulting in the explosion of the reactor core and the destruction of the reactor building. The massive energy release during the primary event superheated the graphite moderator, and the disruption of the reactor vessel and building allowed the superheated graphite to come into contact with atmospheric oxygen. As a result, the graphite moderator caught fire, sending a plume of highly radioactive fallout into the atmosphere and over a very widespread area.[