Scram

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This article is about nuclear reactors. For other meanings, see Scram (disambiguation).

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A scram or SCRAM is an emergency shutdown of a nuclear reactor—though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads (see Tech Model Railroad Club).

1 Mechanisms of nuclear SCRAM
2 Etymology of the term SCRAM
3 References
4 External links

[edit] Mechanisms of nuclear SCRAM

In modern nuclear power plants, the control rods are lifted by electric motors against both their own weight and a powerful spring. A SCRAM rapidly (less than four seconds, by test) releases the control rods from those motors and allows their weight and the spring to drive them into the reactor core, thus halting the nuclear reaction as rapidly as possible. A typical large boiling water reactor will have 185 of these control rods. Modern naval nuclear power reactors have, in addition to scramming, the ability to automatically run the electric motors in reverse at high speeds for a few seconds, thus driving the rods into the core a short distance while leaving them latched to their motors. This "fast insertion" partially shuts down the reactor while leaving it ready to quickly restart — a consideration much more important in a warship than in a commercial power plant. (Also see Nuclear navy.)

Liquid neutron absorbers are also used in emergency shutdown systems. During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various solutions, including sodium polyborate and gadolinium nitrate, are used. For example, Sizewell B has an Emergency Boration System (EBS), four large tanks of highly borated water, which can be run into the main Reactor Pressure Vessel by circuit pressure differences during pump-rundown.

[edit] Etymology of the term SCRAM

Much sources state that the term is actually an acronym, most commonly expanded to safety control rod axe man, referring in that case to a person who would use an axe to cut a wire or rope to drop a control rod into a reactor to shut it down. This became another meaning of the word SCRAM after people working at the first nuclear reactor pile in Chicago, Illinois, known as CP-1, incorporated it into their emergency procedures. (An alternative derivation is that it stood for Simulated Chicago Reactor Axe Man). Many attribute the usage to Enrico Fermi, who supposedly wrote the “axe man” phrase into the original reactor design. There were multiple safety systems in place at the Chicago pile, with some electrically-controlled control rods as well as vessels containing a cadmium solution available to stop any reactions if necessary. Therefore, the job of the “SCRAM” to drop another control rod by the force of gravity was most likely superfluous.

Other sources indicate that the term stands for safety cut rope axe man. (Source:NRC Glossary: Scram) The workers at CP-1 labeled an emergency shutdown button “SCRAM,” since they would immediately be scramming from the premises (or to their emergency positions) as soon the button was hit. (In modern nuclear power plants, the operators do not leave the Control Room in the event of a SCRAM or even a major accident.)

The term is probably a backronym. The actual "safety control rod 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 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 Ax 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, recalled ^[1] that the term was coined by Volney Wilson:

[T]he 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.