In nuclear physics, what term means the rapid, emergency shutdown of a nuclear reactor?

A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor operations, this type of shutdown is often referred to as a "SCRAM" at boiling water reactors (BWR), a "reactor trip" at pressurized water reactors (PWR) and an Emergency Poison Injection System (EPIS) at a Canada Deuterium Uranium (CANDU) reactor. In many cases, a SCRAM is part of the routine shutdown procedure, which serves to test the emergency shutdown system.

The etymology of the term is a matter of debate. United States Nuclear Regulatory Commission historian Tom Wellock notes that 'scram' is English-language slang for leaving quickly and urgently, and cites this as the original and mostly likely accurate basis for the use of 'scram' in the technical context. A persistent alternative explanation posits that 'scram' is 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.