A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor operations, this type of shutdown is often referred to as a "scram" at boiling water reactors and a "reactor trip" at pressurized water reactors. In many cases, a scram is part of the routine shutdown procedure which serves to test the emergency shutdown system.
Etymology
There is no definitive origin for the term. United States Nuclear Regulatory Commission historian Tom Wellock notes that scram is English-language slang for leaving quickly and urgently (as in scrambling to get away), and he cites this as the original and most likely accurate basis for the use of scram in the technical context. Scram is sometimes cited as being an acronym for safety control rod axe man or safety cut rope axe man. This was, according to urban legend, coined by Enrico Fermi when he oversaw the construction of the world's first nuclear reactor. 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. The axe man at the first chain reaction was Norman Hilberry. In a letter to 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 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... 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. Leona Marshall Libby, who was present that day at the Chicago Pile, recalled that the term was coined by Volney Wilson who led the team that designed the control rod circuitry:
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. Other witnesses that day agreed with Libby's crediting "scram" to Wilson. Wellock wrote that Warren Nyer, a student who worked on assembling the pile, also attributed the word to Wilson: "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," Nyer wrote. "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.'" The earliest references to "scram" among the Chicago Pile team were also associated with Wilson's shutdown circuitry and not Hilberry. In a 1952 U.S. Atomic Energy Commission (AEC) report by Fermi, the AEC declassified information on the Chicago Pile. The report includes a section written by Wilson's team shortly after the Chicago Pile achieved a self-sustaining chain reaction on December 2, 1942. It includes a wiring schematic of the rod control circuitry with a clearly labeled "SCRAM" line (see image on the right and pages 37 and 48). The Russian name, AZ-5 (АЗ-5, in Cyrillic), is an abbreviation for аварийная защита 5-й категории (avariynaya zashchita 5-y kategorii), which translates to "emergency protection of the 5th category" in English.
Mechanisms
In any reactor, a scram is achieved by inserting large amounts of negative reactivity mass into the midst of the fissile material, to immediately terminate the fission reaction. In light-water reactors, this is achieved by inserting neutron-absorbing control rods into the core, although the mechanism by which rods are inserted depends on the type of reactor. In pressurized water reactors the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. A scram is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, rapidly halting the nuclear reaction by absorbing liberated neutrons. Another design uses electromagnets to hold the rods suspended, with any cut to the electric current resulting in an immediate and automatic control rod insertion. In some boiling water reactor types, the control rods are inserted up from underneath the reactor vessel. In this case a hydraulic control unit with a pressurized storage tank provides the force to rapidly insert the control rods upon any interruption of the electric current. In both the PWR and the BWR there are secondary systems (and often even tertiary systems) that will insert control rods in the event that primary rapid insertion does not promptly and fully actuate.
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