SCR4M

Scram is usually cited as being an acronym for safety control rod axe man, however the term is probably a backronym. The actual axe man at the first chain-reaction was Norman Hilberry. In a letter to Dr. Raymond Murray (January 21, 1981), Hilberry wrote:

When I showed up on the balcony on that December 2, 1942 afternoon, I was ushered to the balcony rail, handed a well sharpened fireman’s ax and told that was it, “if the safety rods fail to operate, cut that manila rope.” The safety rods, needless to say, worked, the rope was not cut… I don’t believe I have ever felt quite as foolish as I did then. …I did not get the SCRAM [Safety Control Rod Axe Man] story until many years after the fact. Then one day one of my fellows who had been on Zinn’s construction crew called me Mr. Scram. I asked him, “How come?” And then the story.

The US Nuclear Regulatory Commission verifies that this etymology of SCRAM is the correct one in their glossary, stating:

“Also known as a “reactor trip,” “scram” is actually an acronym for “safety control rod axe man,” the worker assigned to insert the emergency rod on the first reactor (the Chicago Pile) in the United States.”

SCRAM – Safety Control Rod Alarm Mechanism. Alerternative Acronym. Primary Source from a undisclosed source.

Articles from Oak Ridge National Laboratory (ORNL) indicate that the term stands for “safety cut rope axe man”, referring in that case to the early neutronic safety mechanism of using a person equipped with an axe to cut the rope suspending the control rods over the Chicago Pile nuclear reactor, at which point the rods would fall by gravity into the reactor core, shutting the reactor down. Specifically, Wallace Koehler, a technician working for the Manhattan Project at Chicago Pile 1, under Stagg Field at the University of Chicago, and later a research physicist at ORNL, reportedly said that Fermi coined the term as this acronym. Although Koehler did not serve as a rope-cutting control rod axe-man, he was responsible for dumping a bucket of aqueous cadmium solution into the reactor if reactor period entered into the sub-optimal range.

Leona Marshall Libby, who was present that day at the Chicago Pile, recalled[6] that the term was coined by Volney Wilson:

The safety rods were coated with cadmium foil, and this metal absorbed so many neutrons that the chain reaction was stopped. Volney Wilson called these “scram” rods. He said that the pile had “scrammed,” the rods had “scrammed” into the pile.

Others[who?] have different theories as to the origins of “SCRAM”. An alternative derivation is that it stood for Simulated Chicago Reactor Axe Man, while, allegedly, U.S. Navy reactor operator circles have defined SCRAM as “super critical reactivity abatement mechanism”, while further sources suggest that SCRAM refers to “Start Cutting Right Away Man”. One common myth is that “SCRAM” refers to operators either running (“scramming”) from the premises (or to their emergency stations) in the event that the “SCRAM” switch is actuated. This explanation is unlikely, since – especially in modern reactors – operators do neither. Instead they remain in the control room in the event the reactor is scrammed, or a more serious event happens, as persons are needed to monitor and control the reactor at all times.

SCRAM Defined Wikipedia