Scram 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. It could also stand for "Safety Control Rods Activation Mechanism" or "Control Rods Actuator Mechanism". Both of these are probably backronyms from the original, non-technical usage. In any reactor, a SCRAM is achieved by inserting large amounts of negative reactivity mass
Enrico Fermi December. The shape of the pile was intended to be roughly spherical, but as work proceeded Fermi calculated that criticality could be achieved without finishing the entire pile as planned. This experiment was a landmark in the quest for energy, and it was typical of Fermi's approach. Every step was carefully planned, every calculation meticulously done. When the first self-sustained nuclear chain reaction was achieved, Compton made a coded phone call to James B. Conant, the chairman of the National Defense Research Committee. To continue the research where it would not pose a public health hazard, the reactor was disassembled