Scram
Encyclopedia
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 farm
s 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.
s 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 neutron
s, 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
).
, 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.
decreases as the fission products decay.
. The actual axe man at the first chain-reaction was Norman Hilberry. In a letter to Dr. Raymond Murray (January 21, 1981), Hilberry wrote:
The US Nuclear Regulatory Commission gives contradictory information. Their glossary supports this etymology of SCRAM, stating:
However, a May 17, 2011, entry on their official blog from NRC historian Tom Welleck argues that this account is effectively an urban legend
and arose many years after the event.
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.
Leona Marshall Libby, who was present that day at the Chicago Pile, recalled that the term was coined by Volney Wilson:
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
– though the term has been extended to cover shutdowns of other complex operations, such as server farm
Server farm
A server farm or server cluster is a collection of computer servers usually maintained by an enterprise to accomplish server needs far beyond the capability of one machine. Server farms often have backup servers, which can take over the function of primary servers in the event of a primary server...
s and even large model railroads (see Tech Model Railroad Club
Tech Model Railroad Club
The Tech Model Railroad Club is a student organization at the Massachusetts Institute of Technology , and one of the most celebrated model railroad clubs in the world, because of its historic role as a wellspring of hacker culture...
). 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.
Mechanisms
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 rodControl rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s 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
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
directly into the reactor coolant. Neutron poisons
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
are water-based solutions
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
that contain chemicals that absorb neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s, such as common household borax
Borax
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...
, sodium polyborate
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
, boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
, or gadolinium nitrate
Gadolinium nitrate
Gadolinium nitrate is an inorganic compound of gadolinium. It is used as a water-soluble neutron poison in nuclear reactors. Gadolinium nitrate, like all nitrates, is an oxidizing agent.-Use:...
, 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
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
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
Nuclear marine propulsion
Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships...
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
Nuclear navy
Nuclear navy, or nuclear powered navy consists of ships powered by relatively small onboard nuclear reactors known as naval reactors. The concept was revolutionary for naval warfare when first proposed, as it meant that these vessels did not need to stop for fuel like their conventional...
).
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 velocityNeutron temperature
The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is...
, so they are likely to escape into the moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
before being captured
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
. On average, it takes about 13 μs for the neutrons to be slowed by the moderator enough to facilitate
Neutron cross-section
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power...
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
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
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
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 by decay heatDecay heat
Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...
decreases as the fission products decay.
Etymology
Scram is usually cited as being an acronym for safety control rod axe man, however the term is probably a backronymBackronym
A backronym or bacronym is a phrase constructed purposely, such that an acronym can be formed to a specific desired word. Backronyms may be invented with serious or humorous intent, or may be a type of false or folk etymology....
. 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.
However, a May 17, 2011, entry on their official blog from NRC historian Tom Welleck argues that this account is effectively an urban legend
Urban legend
An urban legend, urban myth, urban tale, or contemporary legend, is a form of modern folklore consisting of stories that may or may not have been believed by their tellers to be true...
and arose many years after the event.
Articles from Oak Ridge National Laboratory
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
(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
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
at Chicago Pile 1, under Stagg Field
Stagg Field
Amos Alonzo Stagg Field is the name of two different football fields for the University of Chicago. The earliest Stagg Field is probably best remembered for its role in a landmark scientific achievement by Enrico Fermi during the Manhattan Project. The site of the first nuclear reaction received...
at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
, and later a research physicist at ORNL, reportedly said that Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...
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
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
solution into the reactor if reactor period entered into the sub-optimal range.
Leona Marshall Libby, who was present that day at the Chicago Pile, recalled 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.
See also
- Nuclear safety systemsNuclear safety systemsThe three primary objectives of nuclear reactor safety systems as defined by the Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition, and prevent the release of radioactive material during events and accidents...
- Nuclear powerNuclear powerNuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
- Nuclear reactor
- The China SyndromeThe China SyndromeThe China Syndrome is a 1979 American thriller film that tells the story of a reporter and cameraman who discover safety coverups at a nuclear power plant. It stars Jane Fonda, Jack Lemmon, Michael Douglas, Scott Brady, James Hampton, Peter Donat, Richard Herd, and Wilford Brimley.The film was...
(1979 film) - Scram (video game)
- Iodine pitIodine pitIodine pit, also called iodine hole and xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the core of a nuclear reactor. The main isotope responsible is xenon-135, mainly produced by natural decay of iodine-135. Iodine-135 is a weak neutron...
- Sergei PremininSergei PremininSergei Preminin was a Russian sailor who sacrificed his life on the submarine K-219, when he manually prevented an impending meltdown by means of a reactor SCRAM.-Biography:Preminin's father was an electrician and his mother worked in a flax factory...
, Russian sailor who was posthumously awarded the tile of hero of the Russian FederationHero of the Russian FederationHero of the Russian Federation is a Russian decoration and the highest honorary title that can be bestowed on a citizen by the Russian Federation. The President of the Russian Federation is the main conferring authority of the medal, which is bestowed on those committing actions or deeds that...
for performing a SCRAM onboard a submarine in 1986
External links
- NRC Glossary: Scram
- " Scram Switch" entry in The Jargon File