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A rapid shut-down of a nuclear reactor, usually by the quick inserting of emergency rods. The rods could be inserted manually by the operator or automatically. The scram is used to quickly terminate the fission reaction and shut down the reactor in the event of an incident that could compromise nuclear safety.

The etymology of the term “scram” is unclear. It is probably a slang term for “to leave quickly and urgently”, but the widespread interpretation is that it was an abbreviation for Safe Control Rod Axe Man, for the axe-man who, in the first nuclear reactor built by Enrico Fermi, was supposed to cut, in an emergency, the rope on which the control rod hung above the reactor.