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Now if the coolant suddenly stops you have a large mass of radioactive fuel that continues to produce heat. If allowed to heat uncontrollably the fuel will melt eventually, usually preceded or followed by various components of the reactor core. Since the key to controlling a fission reactor is to moderate the chain reaction, melting fuel also becomes capable of causing a nuclear explosion, that while not near as bad as a nuke, can easily destroy the containment vessel and spread tons of radioactive material all over the place.