I asked him, "How come?" And then the story. Then one day one of my fellows who had been on Zinn's construction crew called me Mr. I don't believe I have ever felt quite as foolish as I did then.I did not get the SCRAM story until many years after the fact. When I showed up on the balcony on that Decemafternoon, I was ushered to the balcony rail, handed a well sharpened fireman's axe and told, "If the safety rods fail to operate, cut that manila rope." The safety rods, needless to say, worked, the rope was not cut. In a letter to Raymond Murray (January 21, 1981), Hilberry wrote: The axe man at the first chain-reaction was Norman Hilberry. The core, which was built under the spectator seating at the University of Chicago's Stagg Field, had an actual control rod tied to a rope with a man with an axe standing next to it cutting the rope would mean the rods would fall by gravity into the reactor core, shutting the reactor down. This was supposedly coined by Enrico Fermi when he oversaw the construction of the world's first nuclear reactor. Scram is sometimes cited as being an acronym for safety control rod axe man or safety cut rope axe man. The 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 most likely accurate basis for the use of scram in the technical context. There is no definitive origin for the term. In many cases, a scram is part of the routine shutdown procedure, which serves to test the emergency shutdown system.Įtymology Norman Hilberry (left) and Leó Szilárd at Stagg Field, site of the first self-sustaining nuclear chain-reaction. 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 and at a CANDU reactor. It is also the name that is given to the manually operated kill switch that initiates the shutdown. Sometimes the switch will have a flip cover to prevent inadvertent operation.Ī scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. SCRAM button at the Experimental Breeder Reactor I in Idaho, USA. For other uses, see Scram (disambiguation).
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