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12 March 2011

topical, informative: SCRAM = Safety Control Rod Axe Man



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 commercial reactor operations, this emergency shutdown is often referred to as a "SCRAM" at boiling water reactors (BWR), and as a "reactor trip" at pressurized water reactors (PWR).[1]


In any reactor, a SCRAM is achieved by a large insertion of negative reactivity. 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 PWRs, the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. Any cutting of the electric current releases the rods. Another design uses electromagnets to hold the rods suspended, with any cut to electric current resulting in an immediate and automatic control rod insertion. A SCRAM rapidly (less than four seconds, by test on many reactors) 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 (by absorbing neutrons) as rapidly as possible. In BWRs, 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. 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.

Liquid neutron absorbers are also used in rapid shutdown systems for light water reactors. Following SCRAM, if the reactor (or section(s) thereof) are not below the shutdown margin (they are still critical), the operators can inject solutions containing neutron poisons directly into the reactor coolant. Neutron poisons are water-based solutions that contain chemicals that absorb neutrons, such as common household borax, sodium polyborate, boric acid, or gadolinium nitrate, causing a decrease in neutron multiplication, and thus shutting down the reactor without use of the control rods. In the PWR, these neutron absorbing solutions are stored in pressurized tanks (called accumulators) that are attached to the primary coolant system via valves; a varying level of neutron absorbent is kept within the primary coolant at all times, and is increased using the accumulators in the event of a failure of all of the control rods to insert, which will promptly bring the reactor below the shutdown margin. In the BWR, soluble neutron absorbers are found within the Standby Liquid Control System, which uses redundant battery-operated injection pumps, or, in the latest models, high pressure nitrogen gas to inject the neutron absorber solution into the reactor vessel against any pressure within. Because they may delay the restart of a reactor, these systems are only used to shut down the reactor if control rod insertion fails. This concern is especially significant in a BWR, where injection of liquid boron would cause precipitation of solid boron compounds on fuel cladding,[2] which would prevent the reactor from restarting until the boron deposits were removed.

Some modern naval nuclear power reactors have, in addition to scramming, the ability to automatically run the electric motors in the inward direction 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).

Reactor response

Most neutrons in a reactor are prompt neutrons; that is, neutrons produced directly by a fission reaction. On average, these neutrons live for about 13 μs, which allows the insertion of neutron absorbers to affect the reactor quickly. As a result, once the reactor has been scrammed, the reactor power will drop significantly almost instantaneously. However, a small fraction (about 0.65%) of neutrons in a typical power reactor comes from the radioactive decay of a fission product. These delayed neutrons will limit the rate at which a nuclear reactor will shut down.[3]

Decay heat

On a SCRAM for a reactor that held a constant power for a long period of time (greater than 100 hrs), about 7% of the steady-state power will initially remain after shutdown due to the decay of these 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. The power produced decay heat slowly falls with the decay of fission products.


Scram is usually cited as being an acronym for safety control rod axe man, however the term is probably a backronym. The actual 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.

The US Nuclear Regulatory Commission verifies that this etymology of SCRAM is the correct one in their glossary, stating:

    "Also known as a “reactor trip,” “scram” is actually an acronym for “safety control rod axe man,” the worker assigned to insert the emergency rod on the first reactor (the Chicago Pile) in the United States."[4]

Articles from Oak Ridge National Laboratory (ORNL) indicate that the term stands for "safety cut rope axe man", referring in that case to the early neutronic safety mechanism of using a person equipped with an axe to cut the rope suspending the control rods over the Chicago Pile nuclear reactor, at which point the rods would fall by gravity into the reactor core, shutting the reactor down. Specifically, Wallace Koehler, a technician working for the Manhattan Project at Chicago Pile 1, under Stagg Field at the University of Chicago, and later a research physicist at ORNL, reportedly said that Fermi coined the term as this acronym. Although Koehler did not serve as a rope-cutting control rod axe-man, he was responsible for dumping a bucket of aqueous cadmium solution into the reactor if reactor period entered into the sub-optimal range.[5]

Leona Marshall Libby, who was present that day at the Chicago Pile, recalled[6] 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.

Others [who?] have different theories as to the origins of "SCRAM". An alternative derivation is that it stood for Simulated Chicago Reactor Axe Man, while, allegedly, U.S. Navy reactor operator circles have defined SCRAM as "super critical reactivity abatement mechanism", while further sources suggest that SCRAM refers to "Start Cutting Right Away Man". One common myth is that "SCRAM" refers to operators either running ("scramming") from the premises (or to their emergency stations) in the event that the "SCRAM" switch is actuated. This explanation is unlikely, since – especially in modern reactors – operators do neither. Instead they remain in the control room in the event the reactor is scrammed, or a more serious event happens, as persons are needed to monitor and control the reactor at all times.


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Cladding said...

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