Scram

A scram or SCRAM is an emergency shutdown of a nuclear reactor, though the term has been extended to cover shutdowns of other complex operations, such as server farms. In commercial reactor operations, this type of shutdown is often referred to as a "SCRAM" at boiling water reactors (BWR), and as a "reactor trip" at pressurized water reactors (PWR). In many cases, a SCRAM is part of the routine shutdown procedure as well.

The term is usually cited as being an acronym for safety control rod axe man, which was supposedly coined by Enrico Fermi when the world's first nuclear reactor was built under the spectator seating at the University of Chicago's Stagg Field, but NRC Historian Tom Wellock calls the axe-man story "a bunch of baloney". It could also stand for "Safety Control Rods Activation Mechanism" or "Control Rods Actuator Mechanism", both of which are probably backronyms. "Scram" is American English slang for leaving quickly and urgently, which has been cited as the original basis for the use of scram in the technical context.

In any reactor, a SCRAM is achieved by inserting large amounts of negative reactivity mass(es) into the midst of the fissile material.

In light water reactors, this is achieved by inserting neutron-absorbing control rods into the core, although the mechanism by which rods are inserted depends on the type of reactor. In PWRs, the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. Any cessation of the electric current releases the rods. Another design uses electromagnets to hold the rods suspended, with any cut to the electric current resulting in an immediate and automatic control rod insertion. A SCRAM is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, rapidly halting the nuclear reaction by absorbing liberated neutrons.

...
Wikipedia