scram

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

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). In modern commercial reactor operations, however, it is often referred to as a reactor trip^[1].

Mechanisms of nuclear SCRAM

In modern pressurized water reactors (PWR), the control rods are lifted by electric motors against both their own weight and a powerful spring. Any cutting of the electric current releases the rods. 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.

In boiling water reactors (BWR) 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, again within four seconds. A typical large BWR will have 185 of these control rods.

Some 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.)

On a SCRAM for a reactor that held a constant power for a long period of time (greater than 100 hrs), 93% of the heat generation from the reaction is almost immediately stopped. The remaining 7% represents "decay heat", heat produced from the decay of fission products. For a reactor that has not had a constant power history, the exact percentage will be determined by the concentrations and half-lives of the individual fission products in the core at the time of the SCRAM. This decay heat requires cooling be continued. If the primary cooling path should fail, several Emergency Core Cooling Systems (ECCS) exist to provide cooling.

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.

Etymology of the term SCRAM

Many sources state that the term is actually an acronym, most commonly expanded to Super-Critical Reactor Axe Manila^{[dubious – discuss]}, referring in that case to a person who would use an axe to cut a manila 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.^[2] The workers at CP-1 labeled an emergency shutdown button “SCRAM,” since they would immediately be scramming (running) 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 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, recalled^[3] 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.

References

See also

• Nuclear power
• Nuclear power plant
• Nuclear safety in the U.S.

External links

• Nuclear Reactor Technology — PC-based power reactor simulator.
• NRC Glossary: Scram
• The Jargon File: Scram Switch

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