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{{short description|Emergency shutdown of a nuclear reactor}}
{{About|nuclear reactors}}

[[File:EBR-I - SCRAM button.jpg|thumb|SCRAM button at the [[Experimental Breeder Reactor I|Experimental Breeder Reactor&nbsp;I]] in Idaho. Sometimes the switch will have a flip cover to prevent inadvertent operation.]]

A '''scram''' or '''SCRAM''' is an emergency shutdown of a [[nuclear reactor]] effected by terminating the [[Nuclear fission|fission]] reaction. It is also the name that is given to the manually operated [[kill switch]] that initiates the shutdown. In commercial reactor operations, this type of shutdown is often referred to as a "scram" at [[boiling water reactor]]s and a "reactor ''trip''" at [[pressurized water reactor]]s. In many cases, a scram is part of the routine shutdown procedure which serves to test the emergency shutdown system.

==Etymology==
[[File:Szilard and Hilberry.jpg|right|thumb|upright|[[Norman Hilberry]] (left) and [[Leó Szilárd]] at Stagg Field, site of the first self-sustaining nuclear chain-reaction]]

There is no definitive origin for the term. United States [[Nuclear Regulatory Commission]] historian Tom Wellock notes that ''[[wikt:scram#Verb|scram]]'' is English-language slang for leaving quickly and urgently (as in scrambling to get away), and he cites this as the original and most likely accurate basis for the use of ''scram'' in the technical context.<ref name="nrc-gateway.gov">{{cite web |last=Wellock |first=Tom |date=17 May 2011 |title=Putting the Axe to the 'Scram' Myth |url=https://www.nrc.gov/docs/ml2015/ml20154k751.pdf |access-date=26 May 2015 |publisher=[[United States Nuclear Regulatory Commission]]}}</ref>

Scram is sometimes cited as being an acronym for '''safety control rod axe man''' or '''safety cut rope axe man'''. This was supposedly coined by [[Enrico Fermi]] when he oversaw the construction of the [[Chicago Pile-1|world's first nuclear reactor]]. The [[Nuclear reactor core|core]], which was built under the spectator seating at the University of Chicago's [[Stagg Field]], had an actual [[control rod]] tied to a rope with a man with an axe standing next to it; cutting the rope would mean the rods would fall by gravity into the reactor core, shutting the reactor down.<ref name="ornl.gov">{{cite journal |last=Blackburn |first=Edwin |date=September 2000 |title="Scram!" - Reactor veteran recalls account of the birth of a key word in the nuclear vernacular |url=https://web.ornl.gov/info/reporter/no19/scram.htm |journal=ORNL Reporter |publisher=[[Oak Ridge National Laboratory]] |volume=19 |access-date=25 October 2014}}</ref> The axe man at the first [[chain reaction]] was [[Norman Hilberry]]. In a letter to Raymond Murray (January 21, 1981), Hilberry wrote:

<blockquote>When I showed up on the balcony on that [[timeline of the Manhattan Project#1942|December 2, 1942]] afternoon, I was ushered to the balcony rail, handed a well sharpened fireman's axe 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 [[Walter Zinn|Zinn's]] construction crew called me Mr. Scram. I asked him, "How come?" And then the story.</blockquote>

[[Leona Marshall Libby]], who was present that day at the Chicago Pile, recalled<ref name="libby">''The Uranium People'', Crane, Rusak & Co., 1979</ref> that the term was coined by [[Volney Wilson]] who led the team that designed the control rod circuitry:

<blockquote>The 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.</blockquote>

[[File:Control Rod Circuitry for the Chicago Pile, The World's First Nuclear Reactor.png|right|thumb|Control rod and SCRAM circuitry for the Chicago Pile-1]] Other witnesses that day agreed with Libby's crediting "scram" to Wilson. Wellock wrote that Warren Nyer, a student who worked on assembling the pile, also attributed the word to Wilson: "The word arose in a discussion Dr. Wilson, who was head of the instrumentation and controls group, was having with several members of his group," Nyer wrote. "The group had decided to have a big button to push to drive in both the control rods and the safety rod. What to label it? 'What do we do after we punch the button?,' someone asked. 'Scram out of here!,' Wilson said. Bill Overbeck, another member of that group said, 'OK I'll label it SCRAM.{{' "}}<ref>Tom Wellock, [https://public-blog.nrc-gateway.gov/2016/02/18/refresh-putting-the-axe-to-the-scram-myth/#comments "Putting the Axe to the Scram Myth"], U.S. Nuclear Regulatory Commission Blog, February 18, 2016. {{PD-notice}}</ref>

The earliest references to "scram" among the Chicago Pile team were also associated with Wilson's shutdown circuitry and not Hilberry. In a 1952 [[United States Atomic Energy Commission|U.S. Atomic Energy Commission]] (AEC) report by Fermi, the AEC declassified information on the Chicago Pile. The report includes a section written by Wilson's team shortly after the Chicago Pile achieved a self-sustaining chain reaction on December 2, 1942. It includes a wiring schematic of the rod control circuitry with a clearly labeled "SCRAM" line (see image on the right and pages 37 and 48).<ref>E. Fermi, ''Experimental Production of a Divergent Chain Reaction, AECD-3269'' (Oak Ridge, TN: U.S. Atomic Energy Commission, January 4, 1952), https://www.osti.gov/biblio/4414200</ref>

The Russian name, AZ-5 ({{lang|ru|АЗ-5}}, in [[Cyrillic script|Cyrillic]]), is an abbreviation for {{lang|ru|аварийная защита 5-й категории}} ({{lang|ru-Latn|avariynaya zashhchita 5-y kategorii}}), which translates to "emergency protection of the 5th category" in English.<ref>{{cite web |title=Глава 6. О нажатии АЗ-5 |trans-title=Chapter 6. About pressing AZ-5 |url=https://zaotvet.info/library/glava_6_o_nazhatii_az_5 |work=Za otvyetstvennuyu vlast'! |script-work=ru:За ответственную власть!}}</ref>

==Mechanisms==
{{More citations needed|section|date=October 2022}}
In any reactor, a scram is achieved by inserting large amounts of negative [[reactivity (nuclear)|reactivity]] mass into the midst of the fissile material, to immediately terminate the fission reaction. In [[light-water reactor]]s, 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 [[pressurized water reactor]]s the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. A scram is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, rapidly halting the nuclear reaction by absorbing liberated neutrons. Another design uses electromagnets to hold the rods suspended, with any cut to the electric current resulting in an immediate and automatic control rod insertion.

In [[boiling water reactor]]s, 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. 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.

[[File:NS Savannah SCRAM MD1.jpg|thumb|right|SCRAM button in the control room of the [[NS Savannah|NS ''Savannah'']] ]]
Liquid neutron absorbers ([[Neutron poison|neutron poisons]]) are also used in rapid shutdown systems for heavy and light water reactors. Following a scram, if the reactor (or section(s) thereof) are not below the shutdown margin (that is, they could return to a critical state due to insertion of positive reactivity from cooling, poison decay, or other uncontrolled conditions), the operators can inject solutions containing neutron poisons directly into the reactor coolant.

Neutron poison solutions are [[aqueous solution|water-based solutions]] that contain chemicals that absorb neutrons, such as [[borax|common household borax]], [[boric acid|sodium polyborate]], [[boric acid]], or [[gadolinium nitrate]], causing a decrease in [[effective neutron multiplication factor|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 [[boiling water reactor safety systems#Standby liquid control system (SLCS)|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 (chemistry)|precipitation]] of solid boron compounds on fuel cladding,<ref>{{cite book |last= Shultis |first= J. Kenneth |author2=Richard E. Faw |title= Fundamentals of Nuclear Science and Engineering |publisher= Marcel Dekker |year= 2002 |isbn= 0-8247-0834-2}}</ref> 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.

In a fusion reactor, there are multiple options for a so-called "plasma scram" system, including complete fuel cut-off and cryogenic impurity pellet bombardment.<ref>{{Citation |last=Villar Colome |first=J. |title=Plasma scram in ITER L-mode ignited plasmas |date=1995 |work=Controlled fusion and plasma physics |pages=129–132 |url=https://inis.iaea.org/records/jjesp-ggd27 |access-date=2025-06-23 |language=English |last2=Johner |first2=J. |last3=Ane |first3=J. M.}}</ref><ref>{{Cite journal |last=Pesetti |first=A. |last2=Marini |first2=A. |last3=Raucci |first3=M. |last4=Giambartolomei |first4=G. |last5=Olcese |first5=M. |last6=Sarkar |first6=B. |last7=Aquaro |first7=D. |date=2021-10-01 |title=Large scale experimental facility for performance assessment of the vacuum vessel pressure suppression system of ITER |url=https://www.sciencedirect.com/science/article/pii/S0920379621002994 |journal=Fusion Engineering and Design |volume=171 |article-number=112523 |doi=10.1016/j.fusengdes.2021.112523 |issn=0920-3796|url-access=subscription }}</ref><ref>{{Cite journal |last=Yang |first=Yu |last2=Maruyama |first2=So |last3=Kiss |first3=Gabor |last4=Ciattaglia |first4=Sergio |last5=Putvinski |first5=Sergei |last6=Yoshino |first6=Ryuji |last7=Li |first7=Wei |last8=Jiang |first8=Tao |last9=Li |first9=Bo |last10=Varoutis |first10=Stylianos |last11=Day |first11=Christian |date=2013-10-01 |title=Concept design of fusion power shutdown system for ITER |url=https://www.sciencedirect.com/science/article/pii/S0920379613000653 |journal=Fusion Engineering and Design |series=Proceedings of the 27th Symposium On Fusion Technology (SOFT-27); Liège, Belgium, September 24-28, 2012 |volume=88 |issue=6 |pages=824–826 |doi=10.1016/j.fusengdes.2013.01.055 |issn=0920-3796|url-access=subscription }}</ref><ref>{{Cite journal |last=Yang |first=Yu |last2=Maruyama |first2=So |last3=Kiss |first3=Gabor |last4=Ciattaglia |first4=Sergio |last5=Putvinski |first5=Sergei |last6=Yoshino |first6=Ryuji |last7=Li |first7=Wei |last8=Jiang |first8=Tao |last9=Li |first9=Bo |last10=Varoutis |first10=Stylianos |last11=Day |first11=Christian |date=October 2013 |title=Preliminary Design of the Fusion Power Shutdown System Unit for ITER |journal=Fusion Science and Technology |volume=77 |issue=3 |pages=228–234 |doi=10.1080/15361055.2021.1874764 |issn=1536-1055}}</ref>

==Reactor response==
Most neutrons in a reactor are [[prompt neutrons]]; that is, neutrons produced directly by a fission reaction. These neutrons move at a [[neutron temperature|high velocity]], so they are likely to escape into the [[neutron moderator|moderator]] before being [[neutron capture|captured]]. On average, it takes about 13 μs for the neutrons to be slowed by the moderator enough to [[neutron cross-section|facilitate]] a sustained reaction, which allows the insertion of neutron absorbers to affect the reactor quickly.<ref name="dude">{{cite book |last= Duderstadt |first= James J. |author2=Louis J. Hamilton |title= Nuclear Reactor Analysis |url= https://archive.org/details/nuclearreactoran00dude |url-access= limited |publisher= Wiley-Interscience |year= 1976 |isbn= 0-471-22363-8 |pages= [https://archive.org/details/nuclearreactoran00dude/page/n267 245]}}</ref>

As a result, once the reactor has been scrammed, the reactor power will drop significantly almost instantaneously. 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.<ref name="dude"/>

Due to flaws in its original control rod design, scramming an [[RBMK]] reactor could raise reactivity to dangerous levels before lowering it. This was noticed when it caused a power surge at the startup of [[Ignalina Nuclear Power Plant]] Unit number 1, in 1983. On April 26, 1986, the [[Chernobyl disaster]] happened due to a fatally flawed shutdown system, after the AZ-5 shutdown system was initiated after a core overheat. RBMK reactors were subsequently either retrofitted to account for the flaw, or decommissioned.

===Decay heat===
{{Further|Decay heat}}
Not all of the heat in a nuclear reactor is generated by the chain reaction that a scram is designed to stop. For a reactor that is scrammed 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 that cannot be stopped. 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, but it is large enough that failure to remove decay heat may cause the reactor core temperature to rise to dangerous levels and has caused [[nuclear accidents]], including the nuclear accidents at [[Three Mile Island accident|Three Mile Island]] and [[Fukushima I nuclear accidents|Fukushima I]].

==See also==
{{Portal|Nuclear technology}}
*{{annotated link|Iodine pit}}
*{{annotated link|Kill switch}}
*{{annotated link|Nuclear reactor}}
*{{annotated link|Nuclear reactor safety system}}
*{{annotated link|Sergei Preminin}}, posthumously awarded the title of [[Hero of the Russian Federation]] for performing a manual SCRAM on board the submarine ''[[Soviet submarine K-219|K-219]]'' in 1986

==References==
{{Reflist}}

==External links==
{{Wiktionary|SCRAM}}
{{Commons category}}
*NRC Glossary: [https://www.nrc.gov/reading-rm/basic-ref/glossary/scram.html Scram]
*"[http://catb.org/jargon/html/S/scram-switch.html Scram Switch]" entry in ''[[The Jargon File]]''

{{Nuclear technology}}
{{Authority control}}

[[Category:Nuclear safety and security]]
[[Category:Safety switches]]