REFRESH — Putting the Axe to the ‘Scram’ Myth

Tom Wellock
NRC Historian

refresh leafThe NRC glossary defines a “scram” as “the sudden shutting down of a nuclear reactor usually by rapid insertion of control rods.” But where did the word come from?

One deeply engrained legend about the origin of the word dates to the first sustained chain reaction on December 2, 1942, at the Chicago Pile (CP-1), the first atomic reactor developed for the Manhattan Project. According to the legend, Enrico Fermi created the acronym, Safety Control Rod Axe Man, for Norman Hilberry. It was Hilberry’s assignment that day to kill a possible runaway reaction by using an axe to cut a rope to allow the backup safety control rod to drop into the pile.

The axe-man story now has a life of its own. A search on Google for “scram” and “axe” yields 124,000 hits. Even the NRC’s glossary attributed scram’s etymology to the axe man story. Oak Ridge National Laboratories reported a fanciful variation of this story where Fermi, presumably unimpressed with the physical prowess of his fellow physicists, recruited a lumberjack from the Pacific Northwest to do the job. That version has now spread on the internet, and the acronym itself has mutated into Super-Critical Reactor Axe Man and Start Cutting Right Away, Man.

Hilberry, as it turns out, only learned the story second-hand years later, which lends some doubt to the axe-man version. Other members of the CP-1 team recalled a different origin for the term. Leona Marshall Libby, the only female physicist present that day, wrote in her memoir that it was Volney “Bill” Wilson who called the safety rods “scram rods.” She didn’t explain why he used the term, but her credit to Wilson was supported by others involved in CP-1, including Warren Nyer.

MB900371234I contacted Nyer recently, and he was eager to tell the “scram” story, one that squares well with Wilson’s reported version of events. Nyer’s job that day was to be Hilberry’s backup. If all safety systems failed, he and the other members of the “suicide squad” were to dump a liquid cadmium solution on CP-1 to poison the reaction. The axe-man story is, he recalls, “a bunch of baloney.”

But he did offer another explanation for the word. His recollection was that Wilson was assembling an electrical panel that included a big red button. According to Nyer, someone asked Wilson the reason for the red knob. Wilson replied you’d push it if there was a problem. “Well, then what do you do?” he was asked. Wilson reportedly replied “You scram … out of here.” The word appears to have stuck.

Further indication that “scram” was associated with Wilson’s shutdown circuitry and not Hilberry’s axe wielding comes from Enrico Fermi. The AEC declassified information on the Chicago Pile in late 1950, and Fermi authored a 1951 technical report on the reactor detailing the circuitry of the “SCRAM line” designed by Wilson’s team.  (See ).

And so it seems likely that scram switches all over the world owe their names not to the nuclear industry’s later penchant for acronyms, but to the slang of 20th century America.

REFRESH is an occasional series where we revisit some of our previous posts. This post is slightly revised from the original, one of the blog’s most viewed posts, which first ran in May 2011.

Author: Moderator

Public Affairs Officer for the U.S. Nuclear Regulatory Commission

12 thoughts on “REFRESH — Putting the Axe to the ‘Scram’ Myth”

  1. In lectures at Purdue University in the early 1980s, Professor Sesonske often referred to the “Start Cutting Right Away, Man” origin of SCRAM. As with many of his lectures then, it was hard to tell when Dr. Sesonske was serious.

    Besides, everyone knows that cutting the rope holding out the control rods would be a design change to a reactor control system, and the required DCP could never be completed in time to shut down a reactor.

  2. Thanks for Dr. Wigner’s account. While it mostly accords with Norman Hilberry’s story, their memories are in conflict with at least four other CP-1 veterans. Hilberry’s account of the SCRAM story was laid out in a letter he wrote in 1981 to Professor Raymond Murray, a professor of nuclear engineering at North Carolina State University. Murry used the letter as the basis for a 1988 article in “Nuclear News” magazine about scram’s origin. Hilberry told Murray that the account was not his own. “I did not get the SCRAM story until many years after the fact. Then one day one of my fellows who had been on [CP-1 physicist Walter] Zinn’s construction crew called me Mr. Scram. I asked him, ‘How come?’ And then the story.” That Hilberry, the lone axe man that day, was unaware of the meaning of SCRAM and only learned of the story after the passage of many years leaves some doubt to its accuracy.

    Hilberry himself deferred to Leona Marshal Libby’s account of that day. Though he had not read her 1979 book, “The Uranium People, “ he told Murray it “should be the most authentic recounting on record” because she had been part of “the Fermi ‘thinking family’ and [was] in some ways more intimately involved with the Fermi thought processes during those days than were any others . . . .”

    What did Libby say about SCRAM? On page 122, she described the first ever SCRAM of a chain reaction in which she attributes the the word to Volney Wilson whose team had designed the rod control circuitry:

    “Fermi waited—and waited—to test Volney Wilson’s control circuits. Suddenly bells rang, startling everybody; the ion chamber flux had passed the level preset for safety. Wilson reached over and tore the connecting wires out of the wall to stop the bells.

    Electric motors drove in the safety rods. . . . Volney Wilson called these ‘scram’ rods. He said that the pile had ‘scrammed,’ the rods had scrammed into the pile. This word has entered the vocabulary of power-plant operation, and also of nuclear submarines, so that any shutdown of the reactors is called a scram.”

    Libby did not explain why Wilson used that term, but Wilson did. Raymond Murray’s 1988 article in “Nuclear News” did not accord with Warren Nyer’s memory of events. He contacted Wilson and another member of Wilson’s team, Hugh Barton. Nyer sent their response to “Nuclear News:”

    “The word arose in a discussion Dr. Wilson, who was head of the instrumentation and controls group, was having with several members of his group. The group had decided to have a big button to push to drive in both the control rods and the safety rod. What to label it? ‘What do we do after we punch the button?,’ someone asked. ‘Scram out of here!,’ Wilson said. Bill Overbeck, another member of that group said, ‘OK I’ll label it SCRAM.’ ”

    “I was part of a different group,” Nyer wrote, “but that accords with my memory.”

    In the Fermi technical report I mentioned in the blog, “SCRAM” only appears in Appendix II and its accompanying circuit diagrams. Appendix II was written not by Fermi but by Wilson and his team, including Barton.

    Tom Wellock

  3. I heard the exact same story years ago 1978 from a Mr. Herb Diegel who had worked as a medic on the USS Enterprise and was sent into Hiroshima as a medic following Japans surrender. He later became a Health Physics technician at Los Alamos following the war. The only other thing I remember about it is the signal. When asked what the signal was, Fermi said when I drop. Other than that the story was as pretty much the same.
    He was the one that also said Health Physics came to be the name because of national security. They did not want the term radiation associated with any job, so the term health physics was adopted in place of radiation protection.

  4. I grew up in Oak Ridge, Tennessee, graduating from high school there in 1971. Given the town’s association with the Manhattan Project and my own personal interest in history, I had an ongoing desire to learn as much as I could about the town and its historic purpose.

    At some time during my high school years, I had the chance to be in the same place as Dr. Eugene Wigner, although I don’t remember the purpose of the specific gathering. I seem to remember that he was in Oak Ridge for an extended period of time, which is consistent with my learning later that he remained a consultant to the national laboratory long after WWII. I spoke to Dr. Wigner for several minutes asking him about early Oak Ridge during the Manhattan Project. During a conversation about the Chicago graphite pile being removed to Oak Ridge, I remember him asking me if I knew what SCRAM stood for, and I responded that I did not. He then shared the following…

    At some moment before commencing the CP-1 experiment, Enrico Fermi made the comment (may be only to Wigner or a few in addition to Wigner) that those assigned with the wielding the axes (notice that that he indicates more than one) stood a reasonable chance of dying as a result of the experiment. Fermi noted that if axe-men cut the rope too soon, they would have to personally answer to him in severe terms, and if they cut the rope too late, they might die from an uncontrolled chain reaction from the pile. There was some conversation that Fermi and only Fermi would signal if an when the ax-men should cut the rope. From that conversation the phrase, “Start Cutting Rope Axe Men,” was used and the SCRAM acronym was born. I note that the the various wordings of the acronym the moderator cites above are different from the one that Wigner shared with me. I also note that the narrative above makes no mention of Fermi’s ax-men comment prior to the CP-1 experiment.

    The SCRAM story told to me by Wigner is something that can never be proven, but I find it very hard to believe that I was the only person over the course of Wigner’s life to hear the story from him. I do not find Fermi’s notation in a 1951 technical report at all compelling in disproving the SCRAM acronym account that Wigner shared with me. By 1951, SCRAM had become the common shorthand description for the emergency shutdown of a nuclear reactor, no matter what that method of shutdown might be.

    I have not read Wigner’s book, and have no explanation as to why the story he shared with me does not appear there (assuming it doesn’t). I am compelled to believe Wigner’s story because he was a nobel prize winner, a confidant of Fermi, present at the CP-1 experiment and had no reason to create a story that was not factual. For all of the “evidence” presented in the moderator’s narrative above, I do not think we should dismiss the possibility that a private conversation between Wigner, Fermi and may be one or two others subsequently led to the clever SCRAM acronym. The acronym would be a natural outcome reflecting Fermi’s concern that the axe-men not screw up the CP-1 experiment, and his insistence that no one should do anything unless he issued the command, “Start cutting rope axe men!” Once the acronym was in use among a broader group of people as shorthand for the emergency shutdown of a chain reaction, the more obvious, correct and coincidental meaning of the word — “get the hell out” — would certainly come into play. As time has passed, it is not unusual that many different wordings would be assigned to the acronym. As for my Wigner story, we should never forget one of the fundamental axioms of history — there is an element of truth to any story that persists through time.

  5. It DID mean “run like hell!”; there’s no other way to take “scram out of here”.

  6. Way I heard it, SCRAM was what those present should do, after trying their best to stop the reaction.

    As it beat it, or in far more modern 60’s terminology SPLIT…

  7. Another myth regarding nuclear power is that when Lewis Strauss predicted electricity would become “too cheap to meter” he was referring to nuclear fission. In fact, his autobiography (“No Sacrifice Too Great”) says he was actually referring to hydrogen fusion research, which was a then secret offshoot of cold war thermonuclear weapons projects. According to the DOE authorized history of the US fusion program by Joan Bromberg, Strauss and the AEC commissioners were so excited about the prospects of cheap fusion energy that Strauss even proposed dangling a million dollar prize to the Project Sherwood group that harnessed fusion power first.

  8. Please note the last sentence, which states the post was originally published May 2011.


  9. Not being one to just accept what I read on the internet as fact and the fact I have heard the axe story from people all my career. So I am not so quick to dismiss it as some people might be, up until today it was FACT, not it is unclear.
    First you recently contacted Mr. Nyer, and the date on your article is 2/18/16. Warren Nyer passed away recently at 94 years of age. Nobody ever challenged the origin of the term SCRAM until now. At the time Mr. Nyer was an undergraduate physics student (probably a bright one) to get selected for this project. Not to be disrespectful of Mr. Nyer but he may not have been in on every detail of the project.

  10. Thanks for the research on this one! The mother of an acquaintance of mine was one of the “adding machine girls” on the Manhattan Project and worked at the University of Chicago campus during the time that CP-1 first achieved criticality. She has Alzheimer’s now, which sometimes results in her mind replaying (and her repeating) conversations and memories she had / has regarding her work on the Project. My acquaintance has been trying to coax information out of his mother regarding the “scram” story for quite some time. I’ll be greatly curious to see whether this account brings out any memories from her.

  11. Lol, I would have thought it meant “run like hell!” The stories and narrations of our existence … fun. Thanks NRC.

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