Throwback Thursday: The legend of SCRAM

Readers of the Nuclear News Daily newsletter would be correct in guessing that today’s jaunt back in time is inspired by yesterday’s Trivia Tuesday question: “What does the term SCRAM refer to in the context of December 2, 1942?” It’s not the first time we at ANS have pondered the lore surrounding that word. In the August 1988 issue of Nuclear News, North Carolina State University professor emeritus Raymond Murray wondered the same thing.

While updated research into the meaning of the term SCRAM in relation to nuclear power, including comments from NRC historian Thomas Wellock, have debunked the myths discussed in the story below around the origin of the word, though members were trying to sort through the facts over thirty years ago.

The etymology of “scram”

by Raymond L. Murray

The word “scram” is commonly used in place of “trip” in reactor operation; in fact, Webster's Ninth New Collegiate Dictionary defines scram as “a rapid emergency shutdown of a nuclear reactor.” No origin of the word is provided.

One might conjecture that it was an adaptation of the conventional usage of the slang word that means “to go away at once,” as if operators suddenly left the scene of a reactor accident. But the dictionary also notes that “scram” is an abbreviation for “scramble,” one definition of which is “a rapid emergency takeoff of fighter interceptor planes.” This analogy is too farfetched, and thus there must be another explanation.

Textbooks on nuclear engineering, reactor physics, and reactor safety vary in use of the word. Some avoid it, perhaps because it is undignified; others use it without explanation; and none of the books consulted gives its origin. For example, Glasstone¹ uses the term freely in connection with safety rod action, but in the later edition, Glasstone and Sesonske,² apparently a little embarrassed, say “the term ‘scram,’ which dates from the early days of reactor development, is now being replaced by ‘trip.’” The word appears prominently in the title of recent proceedings,³ however. Okrent⁴ defines it as “a fast insertion of all safety rods,” and gives a detailed account of the controversy about ATWS (anticipated transient without scram). At a 1983 conference⁵ on transients, Hendrie provided his opinions on ATWS. Harper⁶ has an excellent account of the first reactor startup under the stands of Stagg Field at the University of Chicago in December 1942, but he merely notes that “scram” was emergency shutdown by the safety rods. Leona Marshall Libby,⁷ in describing the startup of the first reactor, says, “. . . safety rods were used to scram the pile.”

The best source of information on the derivation of "scram" is Norman Hilberry,⁸ now deceased. He was a member of Enrico Fermi’s team on the first reactor, later was director of Argonne National Laboratory, and on retirement became professor of nuclear engineering at the University of Arizona. In a January 1981 letter, he explains that on the original pile, there was a shim rod going through the middle of the pile and two safety rods, plus carboys of cadmium solution to be dumped on the pile if a blue glow appeared. A third safety rod was attached to a rope over a pulley. In the design sketches of the system, there was the legend SCRAM, standing for “Safety Control Rod Ax Man.” Hilberry had the assignment of standing with the ax, ready to cut the rope if necessary. He learned much later that his colleagues had called him “Mr. Scram.” No photographs of the startup appear to be available, but from accounts by participants, one can draw the accompanying cartoon.

Hilberry in his letter mused about the situation: “I don't believe I have ever felt quite as foolish as I did then. Clearly if there were any real reason for my standing there with that ax, we should not be doing what we were doing-at least not where we were doing it. The fact that any misadventure there would almost cer­tainly result in tipping off the Germans as to what we were doing and how far we had gotten was at least as worrisome as was any possible damage to our neigh­bor's health. Concerned as we all were with the absolute need for secrecy, none of us would have been there had we had any doubt as to the outcome.

“By that afternoon, there had been some forty exponential experiments with successively larger geometries done. The nuclear physics was well established, the microprocesses well known . . . . The possibility of a power failure, of course, did exist but as I stood there it sure seemed remote. The argument then as now was if the scandalously remote should occur and you had not taken every conceivable safety precaution, whether reasonably justifiable or not, you would certainly be adjudged to have been remiss. The philosophical roots of some of our problems extend way back to our very early days.”

Finally, he says, “ . . . the real safety measure that day was that the night before Fermi had computed what the activity readings should be for every position of the shim rod that he called for so that if any conceivable deviation from the accepted theoretical behavior should occur it would become obvious well before crit­icality would be reached.”

The story has two lessons for us. The first is that there is an ancient precedent for redundancy in safety equipment that can respond to apparently inconceivable events. The second is that we cannot expect our operators and supervisors to have the stature of Fermi, but we can expect them to be extremely knowledgeable of the fundamentals of the behavior of the reactor system.

References

1. Samuel Glasstone, Principles of Nuclear Reactor Engineering, D. Van Nostrand Co., Inc., Princeton, N.J. (1955).
2. Samuel Glasstone and Alexander Sesonske, Nuclear Reactor Engineering, Van Nostrand Reinhold Co., New York (1981).
3. Reducing the Frequency of Nuclear Reactor Scrams, proceedings of a conference held April 14–16, 1986, in Tokyo, Japan. Published by the American Nuclear Society, La Grange Park, Ill. (1988).
4. David Okrent, Nuclear Reactor Safety: On the History of the Regulatory Process, The University of Wisconsin Press, Madison (1981).
5. Pamela L. Lassahn, Debu Majumdar, and George F. Brockett, Eds., Anticipated and Abnormal Plant Transients in Light Water Reactors, proceedings of an American Nuclear Society Topical Meeting held September 26–29, 1983, in Jackson Hole, Wyo., Plenum Press, New York (1984).
6. W. R. Harper, Basic Principles of Fission Reactors, Interscience Publishers, Inc., New York (1961).
7. Leona Marshall Libby, The Uranium People, Crane, Russak & Co., Charles Scribner's Sons, New York (1979).
8. Norman Hilberry, personal letter to Raymond L. Murray (January 21, 1981).