Scram

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

SCRAM button at the Experimental Breeder Reactor I.

A scram or SCRAM is an emergency shutdown of anuclear reactor, though the term has been extended tocover shutdowns of other complex operations, such asserver farms and even large model railroads. In commer-cial reactor operations, this emergency shutdown is of-ten referred to as a “SCRAM” at boiling water reactors(BWR), and as a “reactor trip" at pressurized water reac-tors (PWR).[1] In many cases, a SCRAM is part of theroutine shutdown procedure as well.The term is usually cited as being an acronym for safetycontrol rod axe man, which was supposedly coinedby Enrico Fermi when the world’s first nuclear reactorwas built under the spectator seating at the University ofChicago’s Stagg Field, but NRC Historian Tom Wellockcalls the axe-man story “a bunch of baloney.”[2]

1 Mechanisms

In any reactor, a SCRAM is achieved by a large inser-tion of negative reactivity mass(es) into the midst of thefissile material. In light water reactors, this is achievedby inserting neutron-absorbing control rods into the core,although the mechanism by which rods are inserted de-pends on the type of reactor. In PWRs, the control rodsare held above a reactor’s core by electric motors againstboth their own weight and a powerful spring. Any cuttingof the electric current releases the rods. Another design

uses electromagnets to hold the rods suspended, with anycut to electric current resulting in an immediate and au-tomatic control rod insertion. A SCRAM mechanism isdesigned to release the control rods from those motorsand allows their weight and the spring to drive them intothe reactor core, in four seconds or less, thus rapidly halt-ing the nuclear reaction by absorbing liberated neutrons.In BWRs, the control rods are inserted up from under-neath the reactor vessel. In this case a hydraulic controlunit with a pressurized storage tank provides the force torapidly insert the control rods upon any interruption ofthe electric current, again within four seconds. A typicallarge BWR will have 185 of these control rods. In boththe PWR and the BWR there are secondary systems (andoften even tertiary systems) that will insert control rods inthe event that primary rapid insertion does not promptlyand fully actuate.

SCRAM button in the control room of the NS Savannah

Liquid neutron absorbers are also used in rapid shutdownsystems for light water reactors. Following SCRAM,if the reactor (or section(s) thereof) are not below theshutdown margin (that is, they could return to a criti-cal state due to insertion of positive reactivity from cool-ing, poison decay, or other uncontrolled conditions), theoperators can inject solutions containing neutron poi-sons directly into the reactor coolant. Neutron poisonsare water-based solutions that contain chemicals that ab-sorb neutrons, such as common household borax, sodiumpolyborate, boric acid, or gadolinium nitrate, causinga decrease in neutron multiplication, and thus shuttingdown the reactor without use of the control rods. Inthe PWR, these neutron absorbing solutions are stored inpressurized tanks (called accumulators) that are attachedto the primary coolant system via valves; a varying levelof neutron absorbent is kept within the primary coolantat all times, and is increased using the accumulators in

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2 3 ETYMOLOGY

the event of a failure of all of the control rods to insert,which will promptly bring the reactor below the shutdownmargin. In the BWR, soluble neutron absorbers are foundwithin the Standby Liquid Control System (SLCS), whichuses redundant battery-operated injection pumps, or, inthe latest models, high pressure nitrogen gas to inject theneutron absorber solution into the reactor vessel againstany pressure within. Because they may delay the restartof a reactor, these systems are only used to shut downthe reactor if control rod insertion fails. This concern isespecially significant in a BWR, where injection of liq-uid boron would cause precipitation of solid boron com-pounds on fuel cladding,[3] which would prevent the reac-tor from restarting until the boron deposits were removed.In most reactor designs, the routine shutdown procedurealso uses a SCRAM to insert the control rods, as it is themost reliable method of completely inserting the controlrods, and prevents the possibility of accidentally with-drawing them during or after the shutdown.

2 Reactor response

Most neutrons in a reactor are prompt neutrons; that is,neutrons produced directly by a fission reaction. Theseneutrons move at a high velocity, so they are likely to es-cape into the moderator before being captured. On aver-age, it takes about 13 μs for the neutrons to be slowed bythe moderator enough to facilitate a sustained reaction,which allows the insertion of neutron absorbers to affectthe reactor quickly. As a result, once the reactor has beenSCRAMed, the reactor power will drop significantly al-most instantaneously. However, a small fraction (about0.65%) of neutrons in a typical power reactor comes fromthe radioactive decay of a fission product. These delayedneutrons, which are emitted at lower velocities, will limitthe rate at which a nuclear reactor will shut down.[4]

2.1 Decay heat

Further information: Decay heat

For a reactor that is SCRAMed after holding a constantpower level for an extended period (greater than 100 hrs),about 7% of the steady-state power will remain after ini-tial shutdown due to fission product decay. For a reac-tor that has not had a constant power history, the exactpercentage is determined by the concentrations and half-lives of the individual fission products in the core at thetime of the SCRAM. The power produced by decay heatdecreases as the fission products decay.

Norman Hilberry (left) and Leó Szilárd at Stagg Field, site of thefirst self-sustaining nuclear chain-reaction.

3 Etymology

Scram is usually cited as being an acronym for safetycontrol rod axe man; however, the term is probably abackronym. The actual axe man at the first chain-reactionwas Norman Hilberry. In a letter to Dr. Raymond Mur-ray (January 21, 1981), Hilberry wrote:

When I showed up on the balcony on thatDecember 2, 1942 afternoon, I was ushered tothe balcony rail, handed a well sharpened fire-man’s axe and told, “if the safety rods fail tooperate, 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 asI did then. ...I did not get the SCRAM [SafetyControl Rod Axe Man] story until many yearsafter the fact. Then one day one of my fel-lows who had been on Zinn’s construction crewcalled me Mr. Scram. I asked him, “Howcome?" And then the story.

In aMay 17, 2011, entry on theNuclear Regulatory Com-mission’s official blog, NRC historian Tom Wellock ar-gues that this account is effectively an urban legend andarose many years after the event.[2]

Articles from Oak Ridge National Laboratory (ORNL)indicate that the term stands for “safety cut rope axeman”, referring in that case to the early neutronic safetymechanism of using a person equipped with an axe tocut the rope suspending the control rods over the Chicago

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Pile nuclear reactor, at which point the rods would fall bygravity into the reactor core, shutting the reactor down.Specifically, Wallace Koehler, a technician working forthe Manhattan Project at Chicago Pile 1, under StaggField at the University of Chicago, and later a researchphysicist at ORNL, reportedly said that Enrico Fermicoined the term as this acronym. Although Koehler didnot serve as a rope-cutting control rod axe-man, he wasresponsible for dumping a bucket of aqueous cadmiumsolution into the reactor if reactor period entered into thesub-optimal range.[5]

Leona Marshall Libby, who was present that day at theChicago Pile, recalled[6] that the term was coined byVolney Wilson:

[T]he safety rods were coated with cad-mium foil, and this metal absorbed so manyneutrons that the chain reaction was stopped.Volney Wilson called these “scram” rods. Hesaid that the pile had “scrammed,” the rods had“scrammed” into the pile.

4 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 posthu-mously awarded the title of hero of the Russian Fed-eration for performing a SCRAMon board a subma-rine in 1986

5 References[1] “Reactor Protection & Engineered Safety Feature Sys-

tems”. The Virtual Nuclear Tourist. Retrieved 2007-02-25.

[2] Wellock, Tom (May 17, 2011). "“Putting the Axe to the‘Scram’ Myth”. United States Nuclear Regulatory Com-mission. Retrieved 26 May 2015.

[3] Shultis, J. Kenneth; Richard E. Faw (2002). Fundamen-tals of Nuclear Science and Engineering. Marcel Dekker.ISBN 0-8247-0834-2.

[4] Duderstadt, James J.; Louis J. Hamilton (1976). NuclearReactor Analysis. Wiley-Interscience. p. 245. ISBN 0-471-22363-8.

[5] Blackburn, Edwin (September 2000). "“Scram!” - Reac-tor veteran recalls account of the birth of a key word in thenuclear vernacular”. ORNL Reporter (Oak Ridge NationalLaboratory) 19. Retrieved 25 October 2014.

[6] The Uranium People, Crane, Rusak & Co., 1979

6 External links• NRC Glossary: Scram

• "Scram Switch" entry in The Jargon File

4 7 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

7 Text and image sources, contributors, and licenses

7.1 Text• Scram Source: https://en.wikipedia.org/wiki/Scram?oldid=675460114 Contributors: The Epopt, The Anome, Leandrod, Tregoweth, Jpa-tokal, Julesd, Mulad, Przepla, Katana0182, Maximus Rex, Furrykef, Pstudier, Kadin2048, Auric, HaeB, DocWatson42, Ab762, Subsailor,Alistair1978, Goochelaar, Cuppysfriend, Rlaager, Kjkolb, Rajah, Nsaa, Linuxlad, Anthony Appleyard, Burzum, AzaToth, Yvh11a, Taint-edMustard, Stephan Leeds, Pauli133, DV8 2XL, Distantbody, Woohookitty, Sburke, Royan, Lairor, Srleffler, Tofergregg, Simesa, Yurik-Bot, Limulus, Shaddack, Bachrach44, Drewt, Ospalh, BOT-Superzerocool, Lanker, Clayhalliwell, Petri Krohn, AtomCrusher, SmackBot,Septegram, Patrickdepinguin, Nyckname, Bluebot, OrangeDog, Kurtm, Cybercobra, SmilingFace, Will Beback, Ajnosek, John, NJMau-thor, ILorbb, Fromeout11, DouglasCalvert, Dakart, CmdrObot, Grsing, Fumblebruschi, Nbach, UncleBubba, Adolphus79, Slo186, Mmck-night4, Guy Macon, Lumbercutter, Ingolfson, Demonkey36, Acroterion, Pencefn, RogerOAS, Fredrosse, Bdmccray, Johnfos, TXiKiBoT,Rdfox 76, BlackSlivers, Afernand74, Chem-awb, Acdoyle2000, ClueBot, Boing! said Zebedee, Alexbot, Nikwong, DumZiBoT, Internet-Meme, Scostigan, Addbot, Amirobot, AnomieBOT, Materialscientist, ArthurBot, Gerntrash, Kakabel, FrescoBot, A little insignificant,Bluefist, Bigtony1994, Orphan Wiki, Zerohourrct, ZéroBot, Dkevanko, Bomazi, ChuispastonBot, Ivolocy, ClueBot NG, Dylantv, Griffbo,Bcastell, David O. Johnson, Helpful Pixie Bot, OrbiterSpacethingy, Asauers, AllenZh, Khazar2, Joeinwiki, Mardin8985, Monkbot, Tom-seim, Orduin, Fhutch21 and Anonymous: 62

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