- 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 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). In many cases, a SCRAM is part of the routine shutdown procedure as well.
The term is usually cited as being an acronym for safety control rod axe man; however, it is probably a backronym.
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 mechanism is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, in four seconds or less, thus rapidly halting the nuclear reaction by absorbing liberated neutrons. 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 (that is, 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, which would prevent the reactor from restarting until the boron deposits were removed.
In most reactor designs, the routine shutdown procedure also uses a SCRAM to insert the control rods, as it is the most reliable method of completely inserting the control rods, and prevents the possibility of accidentally withdrawing them during or after the shutdown.
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. These neutrons move at a high velocity, so they are likely to escape into the moderator before being captured. On average, it takes about 13 μs for the neutrons to be slowed by the moderator enough to facilitate a sustained reaction, 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, which are emitted at lower velocities, will limit the rate at which a nuclear reactor will shut down.
Decay heat 
For a reactor that is SCRAMed after holding a constant power level for an extended period (greater than 100 hrs), about 7% of the steady-state power will remain after initial shutdown due to fission product decay. For a reactor that has not had a constant power history, the exact percentage is determined by the concentrations and half-lives of the individual fission products in the core at the time of the SCRAM. The power produced by decay heat decreases as the fission products decay.
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, "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 gives contradictory information. Their glossary supports this etymology of SCRAM, 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.
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 Enrico 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.
[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.
See also 
- Nuclear safety systems
- Nuclear power
- Nuclear reactor
- The China Syndrome (1979 film)
- Scram (video game)
- Iodine pit
- Sergei Preminin, Russian sailor who was posthumously awarded the title of hero of the Russian Federation for performing a SCRAM on board a submarine in 1986
- "Reactor Protection & Engineered Safety Feature Systems". The Virtual Nuclear Tourist. Retrieved 2007-02-25.
- Shultis, J. Kenneth; Richard E. Faw (2002). Fundamentals of Nuclear Science and Engineering. Marcel Dekker. ISBN 0-8247-0834-2.
- Duderstadt, James J.; Louis J. Hamilton (1976). Nuclear Reactor Analysis. Wiley-Interscience. p. 245. ISBN 0-471-22363-8.
- Staff, USNRC (2010-01-14). "NRC: Glossary — Scram". Official Glossary. Beltsville, Maryland, United States: United States Nuclear Regulatory Commission, Federal Government of the United States. Retrieved 2010-01-24.
- "Reactor veteran recalls account of the birth of a key word in the nuclear vernacular". Retrieved 2009-05-05.
- The Uranium People, Crane, Rusak & Co., 1979
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