Control rod

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning. Because these elements have different capture cross sections for neutrons of varying energies, the composition of the control rods must be designed for the reactor's neutron spectrum. Boiling water reactors (BWR), pressurized water reactors (PWR) and heavy water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons.

PWR control rod assemby
Control rod assembly for a pressurized water reactor, above fuel element

Operating principle

Nuclear Reactor Uranium Pile (30502443888)
1943 Reactor diagram using boron control rods

Control rods are usually used in control rod assemblies (typically 20 rods for a commercial PWR assembly) and inserted into guide tubes within a fuel element. A control rod is removed from or inserted into the central core of a nuclear reactor in order to increase or decrease the neutron flux, which describes the number of neutrons that split further uranium atoms. This in turn affects the thermal power, the amount of steam produced and hence the electricity generated.

Control rods often stand vertically within the core. In PWRs they are inserted from above, with the control rod drive mechanisms mounted on the reactor pressure vessel head. In BWRs, due to the necessity of a steam dryer above the core, this design requires insertion of the control rods from beneath. The control rods are partially removed from the core to allow a chain reaction to occur. The number of control rods inserted and the distance to which they are inserted can be varied to control activity. Typical shutdown time for modern reactors such as the European Pressurized Reactor or Advanced CANDU reactor is 2 seconds for 90% reduction, limited by decay heat.


Chemical elements with a sufficiently high neutron capture cross-section include silver, indium and cadmium. Other candidate elements include boron, cobalt, hafnium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.[1] Alloys or compounds may also be used, such as high-boron steel,[2] silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium nitrate,[3] gadolinium titanate, dysprosium titanate and boron carbide - europium hexaboride composite.[4]

The material choice is influenced by the neutron energy in the reactor, their resistance to neutron-induced swelling and the required mechanical and lifespan properties. The rods may have the form of tubes filled with neutron-absorbing pellets or powder. They can be made out of stainless steel or other neutron window materials such as zirconium, chromium, silicon carbide or cubic 11
(cubic boron nitride).[5]

The burn up of the absorbing isotopes is another limiting lifespan factor. They may be reduced by capturing long isotope rows of the same element or by not using neutron absorbers for trimming. For example, in pebble bed reactors or in possible new type 7lithium-moderated and -cooled reactors that use fuel and absorber pebbles.

Some rare earth elements are excellent neutron absorbers and are less rare than silver (reserves of about 500,000t). For example, ytterbium (reserves about 1 M tons) and yttrium, 400 times more common, with middle capturing values, can be found and used together without separation inside minerals like xenotime (Yb) (Yb0.40Y0.27Lu0.12Er0.12Dy0.05Tm0.04Ho0.01)PO4,[6] or keiviite (Yb) (Yb1.43Lu0.23Er0.17Tm0.08Y0.05Dy0.03Ho0.02)2Si2O7, lowering the cost.[7] Xenon is also a strong neutron absorber as a gas and can be used for controlling and (emergency) stopping of helium-cooled reactors, but does not function in cases of pressure loss, or as a burning protection gas together with argon around the vessel part especially in case of core catching reactors or if filled with sodium or lithium. Fission-produced xenon can be used after waiting for caesium to precipitate, when practically no radioactivity is left. Cobalt 59 is also used as an absorber for winning of cobalt 60 for x-ray production. Control rods can also be constructed as thick turnable rods with a tungsten reflector and absorber side turned to stop by a spring in less than 1 second.

Silver-indium-cadmium alloys, generally 80% Ag, 15% In and 5% Cd, are a common control rod material for pressurized water reactors.[8] The somewhat different energy absorption regions of the materials make the alloy an excellent neutron absorber. It has good mechanical strength and can be easily fabricated. It must be encased in stainless steel to prevent corrosion in hot water.[9] Also, although indium is less rare than silver, it is more expensive.

Boron is another common neutron absorber. Due to the different cross sections of 10B and 11B, materials containing boron enriched in 10B by isotopic separation are frequently used. The wide absorption spectrum of boron also makes it suitable as a neutron shield. The mechanical properties of boron in its elementary form are unsuitable, and therefore alloys or compounds have to be used instead. Common choices are high-boron steel and boron carbide. The latter is used as a control rod material in both PWRs and BWRs. 10B/11B separation is done commercially with gas centrifuges over BF3, but can also be done over BH3 from borane production or directly with an energy optimized melting centrifuge, using the heat of freshly separated boron for preheating.

Hafnium has excellent properties for reactors using water for both moderation and cooling. It has good mechanical strength, can be easily fabricated, and is resistant to corrosion in hot water.[10] Hafnium can be alloyed with other elements, e.g. with tin and oxygen to increase tensile and creep strength, with iron, chromium and niobium for corrosion resistance, and with molybdenum for wear resistance, hardness and machineability. Such alloys are designated as Hafaloy, Hafaloy-M, Hafaloy-N, and Hafaloy-NM.[11] The high cost and low availability of hafnium limit its use in civilian reactors, although it is used in some US Navy reactors. Hafnium carbide can also be used as an insoluble material with a high melting point of 3890 °C and density higher than that of uranium dioxide for sinking unmelted through corium.

Dysprosium titanate was undergoing evaluation for pressurized water control rods. Dysprosium titanate is a promising replacement for Ag-In-Cd alloys because it has a much higher melting point, does not tend to react with cladding materials, is easy to produce, does not produce radioactive waste, does not swell and does not outgas. It was developed in Russia and is recommended by some for VVER and RBMK reactors.[12] A disadvantage is less titanium and oxide absorption, that other neutron absorbing elements do not react with the already high-melting point cladding materials and that just using the unseparated content with dysprosium inside of minerals like Keiviit Yb inside chromium, SiC or c11B15N tubes deliver superior price and absorption without swelling and outgassing.

Hafnium diboride is another such material. It can be used alone or in a sintered mixture of hafnium and boron carbide powders.[13]

Many other compounds of rare earth elements can be used, such as samarium with boron-like europium and samarium boride, which is already used in the colour industry.[14] Less absorptive compounds of boron similar to titanium, but inexpensive, such as molybdenum as Mo2B5 Since they all swell with boron, in practice other compounds are better, such as carbides etc. or compounds with two or more neutron absorber elements together. It is important that tungsten, and probably also other elements like tantalum,[15] have much the same high capture qualities as hafnium,[16] but with the opposite effect. This is not explainable by neutron reflection alone. An obvious explanation is resonance gamma rays increasing the fission and breeding ratio versus causing more capture of uranium etc. over metastable conditions like for isotope 235mU, which offers a half-life of about 26 min.

Additional means of reactivity regulation

Other means of controlling reactivity include (for PWR) a soluble neutron absorber (boric acid) added to the reactor coolant, allowing the complete extraction of the control rods during stationary power operation, ensuring an even power and flux distribution over the entire core. This chemical shim, along with the use of burnable neutron poisons within the fuel pellets, is used to assist regulation of the core's long term reactivity,[17] while the control rods are used for rapid reactor power changes (e.g. shutdown and start up). Operators of BWRs use the coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps (an increase in coolant flow through the core improves the removal of steam bubbles, thus increasing the density of the coolant/moderator, increasing power).


In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods fall automatically, under gravity, all the way into the pile to stop the reaction. A notable exception to this fail-safe mode of operation is the BWR, which requires hydraulic insertion in the event of an emergency shut-down, using water from a special tank under high pressure. Quickly shutting down a reactor in this way is called scramming.

Criticality accident prevention

Either mismanagement or control rod failure have often been cited as the cause for nuclear accidents, including the SL-1 explosion and the Chernobyl disaster.

The absorption cross section for 10B (top) and 11B (bottom) as a function of energy

Homogeneous neutron absorbers have often been used to manage criticality accidents which involve aqueous solutions of fissile metals. In several such accidents, either borax (sodium borate) or a cadmium compound has been added to the system. The cadmium can be added as a metal to nitric acid solutions of fissile material; the corrosion of the cadmium in the acid will then generate cadmium nitrate in situ.

In carbon dioxide-cooled reactors such as the AGR, if the solid control rods fail to arrest the nuclear reaction, nitrogen gas can be injected into the primary coolant cycle. This is because nitrogen has a larger absorption cross-section for neutrons than carbon or oxygen; hence, the core then becomes less reactive.

As the neutron energy increases, the neutron cross section of most isotopes decreases. The boron isotope 10B is responsible for the majority of the neutron absorption. Boron-containing materials can be used as neutron shields to reduce the activation of objects close to a reactor core.

See also


  1. ^ ytterbium (n.gamma) datas with Japanese or Russian database
  2. ^ limited to use only in research reactors due to increased swelling from helium and lithium due to neutron absorption of boron in the (n, alpha) reaction
  3. ^ injected into D2O moderator of Advanced CANDU reactor
  4. ^ Sairam K, Vishwanadh B, Sonber JK, et al. Competition between densification and microstructure development during spark plasma sintering of B4C–Eu2O3. J Am Ceram Soc. 2017;00:1–11.
  5. ^ Anthony Monterrosa; Anagha Iyengar; Alan Huynh; Chanddeep Madaan (2012). "Boron Use and Control in PWRs and FHRs" (PDF).
  6. ^ Harvey M. Buck, Mark A. Cooper, Petr Cerny, Joel D. Grice, Frank C. Hawthorne: Xenotime-(Yb), YbPO4,a new mineral species from the Shatford Lake pegmatite group, southeastern Manitoba, Canada. In: Canadian Mineralogist. 1999, 37, S. 1303–1306 (Abstract in American Mineralogist, S. 1324; PDF
  7. ^ A. V. Voloshin, Ya. A. Pakhomovsky, F. N. Tyusheva: Keiviite Yb2Si2O7, A new ytterbium silicate from amazonitic pegmatites of the Kola Peninsula. In: Mineralog. Zhurnal. 1983, 5-5, S. 94–99 (Abstract in American Mineralogist, S. 1191; PDF; 853 kB).
  8. ^ Bowsher, B. R.; Jenkins, R. A.; Nichols, A. L.; Rowe, N. A.; Simpson, J. a. H. (1986-01-01). "Silver-indium-cadmium control rod behaviour during a severe reactor accident". UKAEA Atomic Energy Establishment.
  9. ^ "CONTROL MATERIALS". Retrieved 2015-06-02.
  10. ^ "Control Materials". Retrieved 2010-08-14.
  11. ^ "Hafnium alloys as neutron absorbers". Free Patents Online. Retrieved September 25, 2008.
  12. ^ "Dysprosium (Z=66)". web forum. Retrieved September 25, 2008.
  13. ^ "Method for making neutron absorber material". Free Patents Online. Retrieved September 25, 2008.
  14. ^ "Infrarotabsorbierende Druckfarben - Dokument DE102008049595A1". 2008-09-30. Retrieved 2014-04-22.
  15. ^ "Sigma Plots". Retrieved 2014-04-22.
  16. ^ "Sigma Periodic Table Browse". 2007-01-25. Retrieved 2014-04-22.
  17. ^ "Enriched boric acid for pressurized water reactors" (PDF). EaglePicher Corporation. Archived from the original (PDF) on November 29, 2007. Retrieved September 25, 2008.

Further reading

Advanced boiling water reactor

The advanced boiling water reactor (ABWR) is a Generation III boiling water reactor. The ABWR is currently offered by GE Hitachi Nuclear Energy (GEH) and Toshiba. The ABWR generates electrical power by using steam to power a turbine connected to a generator; the steam is boiled from water using heat generated by fission reactions within nuclear fuel. Kashiwazaki-Kariwa unit 6 is considered the first Generation III reactor in the world.

Boiling water reactors (BWRs) are the second most common form of light water reactor with a direct cycle design that uses fewer large steam supply components than the pressurized water reactor (PWR), which employs an indirect cycle. The ABWR is the present state of the art in boiling water reactors, and is the first Generation III reactor design to be fully built, with several reactors complete and operating. The first reactors were built on time and under budget in Japan, with others under construction there and in Taiwan. ABWRs were on order in the United States, including two reactors at the South Texas Project site (although the project is currently halted). The projects in both Taiwan and US are both reported over-budgeted.

The standard ABWR plant design has a net electrical output of about 1.35 GW, generated from about 3926 MW of thermal power.


Annihilus () is a fictional supervillain appearing in American comic books published by Marvel Comics, primarily as an adversary to the Fantastic Four and first appeared in Fantastic Four Annual #6, published in November 1968. He was created by writer Stan Lee and artist Jack Kirby. He was responsible for the attacks in the "Annihilation" comic book event.

Annihilus has at various times been the ruler of the Negative Zone, controlling its inhabitants via his powerful Cosmic Control Rod. He first encountered the Fantastic Four after Reed Richards discovered how to travel to the Negative Zone from Earth. Over the years he clashed with the Fantastic Four on many occasions, often with the group foiling his plans to invade Earth. He is often the partner of Blastaar, who started out as a rival to Annihilus' rule of the Negative Zone before becoming allies.

He would later lead an enormous fleet of space ships from the Negative Zone into the main universe, setting off the Annihilation Wave by destroying or subjugating many planets. The armada was opposed by a number of cosmic heroes such as Star-Lord, Drax the Destroyer, and Silver Surfer, and was ultimately stopped by the cosmic entity Galactus, killing Annihilus in the process, but he was later reborn as an infant in the aftermath of the "Annihilation" storyline.

Annihilus has appeared in a number of Marvel media, including several Fantastic Four shows, The Super Hero Squad Show, The Avengers: Earth's Mightiest Heroes, Hulk and the Agents of S.M.A.S.H. and Ultimate Spider-Man. In 2009, Annihilus was ranked as IGN's 94th Greatest Comic Book Villain of All Time.

Boiling water reactor

A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR), which is also a type of light water nuclear reactor. The main difference between a BWR and PWR is that in a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine. In a PWR, the reactor core heats water, which does not boil. This hot water then exchanges heat with a lower pressure water system, which turns to steam and drives the turbine. The BWR was developed by the Argonne National Laboratory and General Electric (GE) in the mid-1950s. The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor.

Chernobyl disaster

The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophic nuclear accident. It occurred on 25–26 April 1986 in the No. 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near the now-abandoned town of Pripyat, in northern Ukrainian Soviet Socialist Republic, Soviet Union, approximately 104 km (65 mi) north of Kiev.The event occurred during a late-night safety test which simulated a station blackout power-failure, in the course of which safety systems were intentionally turned off. A combination of inherent reactor design flaws and the reactor operators arranging the core in a manner contrary to the checklist for the test, eventually resulted in uncontrolled reaction conditions. Water flashed into steam generating a destructive steam explosion and a subsequent open-air graphite fire. This fire produced considerable updrafts for about nine days. These lofted plumes of fission products into the atmosphere. The estimated radioactive inventory that was released during this very hot fire phase approximately equaled in magnitude the airborne fission products released in the initial destructive explosion.

This radioactive material precipitated onto parts of the western USSR and other European countries.

During the accident, steam-blast effects caused two deaths within the facility: one immediately after the explosion, and the other compounded by a lethal dose of radiation.

Over the coming days and weeks, 134 servicemen were hospitalized with acute radiation sickness (ARS), of which 28 firemen and employees died in the days-to-months afterward.

Additionally, approximately fourteen radiation induced cancer deaths among this group of 134 hospitalized survivors were to follow within the next ten years (1996).

Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011.

It will take further time and investigation to definitively determine the elevated relative risk of cancer among the surviving employees, those that were initially hospitalized with ARS, and the population at large.The Chernobyl accident is considered the most disastrous nuclear power plant accident in history, both in terms of cost and casualties.

It is one of only two nuclear energy accidents classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in Japan in 2011. The struggle to safeguard against scenarios which were perceived as having the potential for greater catastrophe, together with later decontamination efforts of the surroundings, ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.The remains of the No. 4 reactor building were enclosed in a large cover which was named the "Object Shelter", often known as "The Sarcophagus". The purpose of the structure was to reduce the spread of the remaining radioactive dust and debris from the wreckage and the protection of the wreckage from further weathering.

The sarcophagus was finished in December 1986 at a time when what was left of the reactor was entering the cold shut-down phase.

The enclosure was not intended as a radiation shield, but was built quickly as occupational safety for the crews of the other undamaged reactors at the power station, with No. 3 continuing to produce electricity up into 2000.The accident motivated safety upgrades on all remaining Soviet-designed reactors in the RBMK (Chernobyl No. 4) family, of which eleven continued to power electric grids as of 2013.

Davis–Besse Nuclear Power Station

Davis–Besse Nuclear Power Station is a nuclear power plant northeast of Oak Harbor in Ottawa County, Ohio, United States, approximately 25 miles east of the city of Toledo. It has a single pressurized water reactor. As of 2011, it is operated by the FirstEnergy Nuclear Operating Company subsidiary of FirstEnergy Corp.

On March 5, 2002, maintenance workers discovered that corrosion had eaten a football-sized hole into the reactor vessel head of the Davis–Besse plant. Although the corrosion did not lead to an accident, this was considered to be a serious nuclear safety incident. The Nuclear Regulatory Commission kept Davis–Besse shut down until March 2004, so that FirstEnergy was able to perform all the necessary maintenance for safe operations. The NRC imposed its largest fine ever—more than $5 million—against FirstEnergy for the actions that led to the corrosion. The company paid an additional $28 million in fines under a settlement with the U.S. Department of Justice.According to the NRC, Davis–Besse has been the source of two of the top five most dangerous nuclear incidents in the United States since 1979.Davis–Besse is expected to close in 2020. Plans have been updated indicating possible shut down by May 31, 2020.

Emery Worldwide Flight 17

Emery Worldwide Flight 17 was a regularly scheduled domestic cargo flight, flying from Reno to Dayton with an intermediate stopover at Rancho Cordova. On February 16, 2000, the DC-8 crashed onto a salvage yard shortly after taking off from Sacramento Mather Airport, killing all three crew members on board. An investigation by the National Transportation Safety Board (NTSB) revealed that during the aircraft's rotation, a control rod to the right elevator control tab detached, causing a loss of pitch control. The crew attempted unsuccessfully to return to Mather airport. The NTSB further found that an incorrect maintenance procedure, which was implemented by Emery Worldwide, introduced an incorrect torque-loading on the bolts which were supposed to connect the control rod.Fifteen recommendations were issued by the NTSB. One of the recommendations was to evaluate every DC-8 on U.S soil to prevent further crashes that could be caused by the disconnection of the right elevator tab. The Federal Aviation Administration subsequently found more than 100 maintenance violations in the airline including one that caused another accident on April 26, 2001. Emery Worldwide later grounded its entire fleet for good on August 13, 2001.


General Electric's BWR product line of Boiling Water Reactors represents the designs of a large percentage of the commercial fission reactors around the world.

George Weil

George Leon Weil (September 18, 1907 – July 1, 1995) was an American physicist. On December 2, 1942, he removed the control rod from the Chicago Pile-1 nuclear reactor, initiating the first man-made, self-sustaining nuclear chain reaction.

High Flux Isotope Reactor

The High Flux Isotope Reactor (or HFIR) is a nuclear research reactor located at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, United States. Operating at 85 MW, HFIR is one of the highest flux reactor-based sources of neutrons for condensed matter physics research in the United States, and it provides one of the highest steady-state neutron fluxes of any research reactor in the world. The thermal and cold neutrons produced by HFIR are used to study physics, chemistry, materials science, engineering, and biology. The intense neutron flux, constant power density, and constant-length fuel cycles are used by more than 500 researchers each year for neutron scattering research into the fundamental properties of condensed matter. HFIR has approximately 600 users each year for both scattering and in-core research.

The neutron scattering research facilities at HFIR contain a world-class collection of instruments used for fundamental and applied research on the structure and dynamics of matter. The reactor is also used for medical, industrial, and research isotope production; research on severe neutron damage to materials; and neutron activation to examine trace elements in the environment. Additionally, the building houses a gamma irradiation facility that uses spent fuel assemblies and is capable of accommodating high gamma dose experiments.

With projected regular operations, the next major shutdown for a beryllium reflector replacement will not be necessary until approximately 2023. This outage provides an opportunity to install a cold source in radial beam tube HB-2, which would provide an unparalleled flux of cold neutrons feeding instruments in a new guide hall. With or without this additional capability, HFIR is projected to continue operating through 2040 and beyond.

In November 2007 ORNL officials announced that time-of-flight tests on a newly installed cold source (which uses liquid helium and hydrogen to slow the movement of neutrons) showed better performance than design predictions, equaling or surpassing the previous world record set by the research reactor at the Institut Laue–Langevin in Grenoble, France.

Idaho Falls, Idaho

Idaho Falls is the county seat of Bonneville County, Idaho, United States, and the state's largest city outside the Boise metropolitan area. As of the 2010 census, the population of Idaho Falls was 56,813 (2016 estimate: 60,211), with a metro population of 133,265.Idaho Falls serves as the commercial, cultural, and healthcare hub for eastern Idaho, as well as parts of western Wyoming and southern Montana. It is served by the Idaho Falls Regional Airport and is home to the College of Eastern Idaho, Museum of Idaho, and the Idaho Falls Chukars minor league baseball team. It is the principal city of the Idaho Falls Metropolitan Statistical Area and the Idaho Falls-Blackfoot, Idaho Combined Statistical Area.

Iodine pit

The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the reactor core. The main isotope responsible is 135Xe, mainly produced by natural decay of 135I. 135I is a weak neutron absorber, while 135Xe is the strongest known neutron absorber. When 135Xe builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons which provide the nuclear reaction.

The presence of 135I and 135Xe in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod positions.

The buildup of short-lived fission products acting as nuclear poisons is called reactor poisoning, or xenon poisoning. Buildup of stable or long-lived neutron poisons is called reactor slagging.

Load following power plant

A load following power plant, regarded as producing mid-merit or mid-priced electricity, is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load following plants are typically in-between base load and peaking power plants in efficiency, speed of start up and shut down, construction cost, cost of electricity and capacity factor.

Nuclear reactor physics

Nuclear reactor physics is the branch of science that deals with the study and application of chain reaction to induce a controlled rate of fission in a nuclear reactor for the production of energy.

Most nuclear reactors use a chain reaction to induce a controlled rate of nuclear fission in fissile material, releasing both energy and free neutrons. A reactor consists of an assembly of nuclear fuel (a reactor core), usually surrounded by a neutron moderator such as regular water, heavy water, graphite, or zirconium hydride, and fitted with mechanisms such as control rods that control the rate of the reaction.

The physics of nuclear fission has several quirks that affect the design and behavior of nuclear reactors. This article presents a general overview of the physics of nuclear reactors and their behavior.

Piqua Nuclear Generating Station

The Piqua Nuclear Power Facility was a nuclear power plant which operated just outside the southern city limits of Piqua, Ohio in the United States. The plant contained a 45.5-megawatt (thermal) organically cooled and moderated nuclear reactor (terphenyl, a biphenyl like oil). The Piqua facility was built and operated between 1963 and 1966 as a demonstration project by the Atomic Energy Commission. The facility ceased operation in 1966. It was dismantled between 1967 and 1969, and the radioactive coolant and most other radioactive materials were removed. The remaining radioactive structural components of the reactor were entombed in the reactor vessel under sand and concrete.

Prompt criticality

In nuclear engineering, prompt criticality is said to be reached during a nuclear fission event if one or more of the immediate or prompt neutrons released by an atom in the event causes an additional fission event resulting in a rapid, exponential increase in the number of fission events. Prompt criticality is a special case of supercriticality.


The RBMK (Russian: Реактор Большой Мощности Канальный Reaktor Bolshoy Moshchnosti Kanalnyy, “High Power Channel-type Reactor”) is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union.

The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the RBMK reactor design, such as the active removal of decay heat, the positive void coefficient properties, the graphite-tipped control rods and instability at low power levels, contributed to the 1986 Chernobyl disaster, in which an RBMK experienced a meltdown during a mishandled test, and radioactivity was released over a large portion of Europe. The disaster prompted worldwide calls for the reactors to be completely decommissioned; however, there is still considerable reliance on RBMK facilities for power in Russia. The imperfections in the design of RBMK-1000 reactors were eliminated soon after the Chernobyl accident and a dozen reactors have since been operating without any serious incidents for over twenty years. While nine RBMK blocks under construction were cancelled after the Chernobyl disaster, and the last of three remaining RBMK blocks at the Chernobyl Nuclear Power Plant was finally shut down in 2000, as of December 2017 there were still 11 RBMK reactors, and four small EGP-6 graphite moderated light water reactors operating in Russia, though all have been retrofitted with a number of safety updates.

The only difference between RBMK-1000 and RBMK-1500 reactors is that the RBMK-1500 is cooled with less water and thus, more of the water turns into steam. It also uses less uranium. The only reactors of this type and power output are the ones at Ignalina Nuclear Power Plant. The RBMKP-2400 is rectangular instead of cylindrical, and it was intended to be made in sections at a factory for assembly in situ. It was designed to have a power output of 2400 MWe. No reactor with this power output has ever been built, with the most powerful one currently being as of 2018 the 1750 MWe EPR.


The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor in the United States which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators. The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the reactor core. The event is the only reactor accident in the U.S. which resulted in immediate fatalities. The accident released about 80 curies (3.0 TBq) of iodine-131, which was not considered significant due to its location in the remote high desert of eastern Idaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.The facility, located at the National Reactor Testing Station (NRTS) approximately forty miles (65 km) west of Idaho Falls, Idaho, was part of the Army Nuclear Power Program and was known as the Argonne Low Power Reactor (ALPR) during its design and build phase. It was intended to provide electrical power and heat for small, remote military facilities, such as radar sites near the Arctic Circle, and those in the DEW Line. The design power was 3 MW (thermal), but some 4.7 MW tests were performed in the months prior to the accident. Operating power was 200 kW electrical and 400 kW thermal for space heating.During the accident the core power level reached nearly 20 GW in just four milliseconds, precipitating the steam explosion.


A scram or SCRAM is an emergency shutdown of a nuclear reactor. It is a type of kill switch. In commercial reactor operations, this type of shutdown is often referred to as a "SCRAM" at boiling water reactors (BWR), a "reactor trip" at pressurized water reactors (PWR) and EPIS at a CANDU (CANDU). In many cases, a SCRAM is part of the routine shutdown procedure as well.

The etymology of the term is a matter of debate. United States Nuclear Regulatory Commission historian Tom Wellock notes that scram is English language slang for leaving quickly and urgently, and cites this as the original and mostly likely accurate basis for the use of scram in the technical context. A persistent alternative explanation posits that scram is an acronym for 'safety control rod axe man', which was supposedly coined by Enrico Fermi when the world's first nuclear reactor was built under the spectator seating at the University of Chicago's Stagg Field. It could also stand for "Safety Control Rods Activation Mechanism" or "Control Rods Actuator Mechanism". Both of these are probably backronyms from the original, non-technical usage.

Shutdown (nuclear reactor)

In a nuclear reactor, shutdown refers to the state of the reactor when it is subcritical by at least a margin defined in the reactor's technical specifications. Further requirements for being shut down may include having the reactor control key be secured and having no fuel movements or control systems maintenance in progress.

The shutdown margin is defined in terms of reactivity, frequently in units of delta-k/k (where k is taken to mean k-effective, the effective multiplication factor) or occasionally in dollars (the dollar is a unit equal to the change in reactivity needed to go from critical to prompt critical). Shutdown margin can refer either to the margin by which the reactor is subcritical when all control rods are inserted or to the margin by which the reactor would be shut down in the event of a scram. Hence, care must be taken to define shutdown margin in the most conservative way in the reactor's technical specifications; a typical research reactor will specify the margin when in the cold condition, without xenon. Under this specification, the shutdown margin can be simply calculated as the sum of the control rod worths minus the core excess.

Minimum shutdown margin can be calculated in the same way as shutdown margin, except that the negative reactivity of the most reactive control rod and non-scramable rods is ignored. This definition allows the reactor to be designed so that it remains safely shut down even if that most reactive control rod becomes stuck out of the core.

A reactor is in cold shutdown when, in addition, its coolant system is at atmospheric pressure and at a temperature below 100 °C (210 °F). This temperature is low enough that the water cooling the fuel in a light water reactor does not boil even when the reactor coolant system is depressurized.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.