Editing Control rod

{{Short description|Device used to regulate the power of a nuclear reactor}}
[[File:PWR control rod assemby.jpg|thumb|Control rod assembly for a pressurized water reactor, above fuel element]]
'''Control rods''' are used in [[nuclear reactor]]s to control the rate of fission of the nuclear fuel – [[uranium]] or [[plutonium]].  Their compositions include [[chemical element]]s such as [[boron]], [[cadmium]], [[silver]], [[hafnium]], or [[indium]], that are capable of absorbing many [[neutron]]s without themselves decaying. These elements have different [[neutron capture]] [[neutron cross section|cross sections]] for neutrons of various [[neutron temperature|energies]]. [[Boiling water reactor]]s (BWR), [[pressurized water reactor]]s (PWR), and [[heavy-water reactor]]s (HWR) operate with [[thermal neutron]]s, while [[breeder reactor]]s operate with [[fast neutron]]s. Each reactor design can use different control rod materials based on the energy spectrum of its neutrons. Control rods have been used in nuclear aircraft engines like [[Project Pluto]] as a method of control.

==Operating principle==
[[File:Nuclear Reactor Uranium Pile (30502443888).jpg|thumb|upright=1.25|1943 Reactor diagram using boron control rods]]
Control rods are inserted into the [[nuclear reactor core|core of a nuclear reactor]] and adjusted in order to [[process control|control]] the [[reaction rate|rate]] of the [[nuclear chain reaction]] and, thereby, the [[thermal power]] output of the reactor, the rate of [[steam]] production, and the [[electrical power]] output of the power station.

The number of control rods inserted, and the distance to which they are inserted, strongly influence the [[reactivity of a nuclear reactor|''reactivity'']] of the reactor. When reactivity (as [[effective neutron multiplication factor]]) is above 1, the rate of the [[nuclear chain reaction]] increases exponentially over time. When reactivity is below 1, the rate of the reaction decreases exponentially over time. When all control rods are fully inserted, they keep reactivity barely above 0, which quickly slows a running reactor to a stop and keeps it stopped (in [[shutdown (nuclear reactor)|shutdown]]). If all control rods are fully removed, reactivity is significantly above 1, and the reactor quickly runs hotter and hotter, until some other factor (such as [[Fuel temperature coefficient of reactivity|temperature reactivity feedback]]) slows the reaction rate. Maintaining a constant power output requires keeping the long-term average neutron multiplication factor close to 1.

A new reactor is assembled with its control rods fully inserted. Control rods are partially removed from the core to allow the [[nuclear chain reaction]] to start up and increase to the desired power level. [[Neutron flux]] can be measured, and is roughly proportional to reaction rate and power level. To increase power output, some control rods are pulled out a small distance for a while. To decrease power output, some control rods are pushed in a small distance for a while. Several other factors affect the reactivity; to compensate for them, an automatic control system adjusts the control rods small amounts in or out, as-needed in some reactors. Each control rod influences some part of the reactor more than others; calculated adjustments to fuel distribution can be made to maintain similar reaction rates and temperatures in different parts of the core.

Typical [[shutdown (nuclear reactor)|shutdown]] time for modern reactors such as the [[European Pressurized Reactor]] or [[Advanced CANDU reactor]] is two seconds for 90% reduction, limited by [[decay heat]].
{{anchor|control rod drive mechanism}}
Control rods are usually used in control rod assemblies (typically 20 rods for a commercial PWR assembly) and inserted into guide tubes within the fuel elements. 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.

==Materials==
[[File:Neutroncrosssectionboron.png|thumb|upright=1.25|The absorption cross section for <sup>10</sup>B (top) and <sup>11</sup>B (bottom) as a function of energy]]

Chemical elements with usefully high neutron capture cross-sections include [[silver]], [[indium]], and [[cadmium]]. Other candidate elements include [[boron]], [[cobalt]], [[hafnium]], [[samarium]], [[europium]], [[gadolinium]], [[terbium]], [[dysprosium]], [[holmium]], [[erbium]], [[thulium]], [[ytterbium]], and [[lutetium]].<ref>[https://www.nndc.bnl.gov/sigma/ ytterbium (n.gamma) data with Japanese or Russian database]</ref> Alloys or compounds may also be used, such as high-[[boron steel]],{{efn|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}} silver-indium-cadmium alloy, [[boron carbide]], [[zirconium diboride]], [[titanium diboride]], [[hafnium diboride]], gadolinium nitrate,{{efn|injected into D<sub>2</sub>O moderator of [[Advanced CANDU reactor]]}} gadolinium titanate, [[dysprosium titanate]], and boron carbide–europium hexaboride composite.<ref>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. https://doi.org/10.1111/jace.15376 </ref>

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. The tubes can be made of stainless steel or other "neutron window" materials such as zirconium, chromium, [[silicon carbide]], or cubic {{chem|11|B}}{{chem|15|N}} (cubic [[boron nitride]]).<ref>{{cite web |title=Boron Use and Control in PWRs and FHRs |author1=Anthony Monterrosa |author2=Anagha Iyengar |author3=Alan Huynh |author4=Chanddeep Madaan |year=2012 |url=http://fhr.nuc.berkeley.edu/wp-content/uploads/2014/10/12-007_Boron_Use_in_PWRs_and_FHRs.pdf}}</ref>

The burnup of "[[burnable poison]]" [[isotope]]s also limits lifespan of a control rod. They may be reduced by using an element such as hafnium, a "non-burnable poison" which captures multiple neutrons before losing effectiveness, or by not using neutron absorbers for trimming. For example, in [[pebble bed reactor]]s or in possible new type [[lithium-7]]-moderated and -cooled reactors that use fuel and absorber pebbles.

Some [[rare-earth element]]s are excellent neutron absorbers and are more common than silver (reserves of about 500,000t). For example, ytterbium (reserves about one 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) (Yb<sub>0.40</sub>Y<sub>0.27</sub>Lu<sub>0.12</sub>Er<sub>0.12</sub>Dy<sub>0.05</sub>Tm<sub>0.04</sub>Ho<sub>0.01</sub>)PO<sub>4</sub>,<ref>Harvey M. Buck, Mark A. Cooper, Petr Cerny, Joel D. Grice, Frank C. Hawthorne: ''Xenotime-(Yb), YbPO<sub>4</sub>,a new mineral species from the Shatford Lake pegmatite group, southeastern Manitoba, Canada.'' In: ''Canadian Mineralogist.'' 1999, 37, S.&nbsp;1303–1306 ([http://www.minsocam.org/msa/ammin/TOC/Abstracts/2000_Abstracts/Sept00_Abstracts/Jambor_p1321_00.pdf Abstract in American Mineralogist, S. 1324]; PDF</ref> or keiviite (Yb) (Yb<sub>1.43</sub>Lu<sub>0.23</sub>Er<sub>0.17</sub>Tm<sub>0.08</sub>Y<sub>0.05</sub>Dy<sub>0.03</sub>Ho<sub>0.02</sub>)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>, lowering the cost.<ref>A. V. Voloshin, Ya. A. Pakhomovsky, F. N. Tyusheva: ''Keiviite Yb<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>, A new ytterbium silicate from amazonitic pegmatites of the Kola Peninsula.'' In: ''Mineralog. Zhurnal.'' 1983, 5-5, S.&nbsp;94–99 ([http://www.minsocam.org/ammin/AM69/AM69_1190.pdf Abstract in American Mineralogist, S.&nbsp;1191]; PDF; 853&nbsp;kB).</ref> [[Xenon]] is also a strong neutron absorber as a gas, and can be used for controlling and (emergency) stopping [[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 use as a [[gamma ray]] source. 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 one second.

Silver-indium-cadmium alloys, generally 80% Ag, 15% In, and 5% Cd, are a common control rod material for [[pressurized water reactor]]s.<ref>{{cite tech report |last1=Bowsher|first1=B. R.|last2=Jenkins|first2=R. A.|last3=Nichols|first3=A. L.|last4=Rowe|first4=N. A.|last5=Simpson|first5=J. a. H.|date=1986-01-01|title=Silver-indium-cadmium control rod behaviour during a severe reactor accident|url=http://inis.iaea.org/Search/search.aspx?orig_q=RN:19051554|publisher=UKAEA Atomic Energy Establishment}}</ref> 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.<ref>{{cite web|title=CONTROL MATERIALS|url=http://web.mit.edu/nrl/Training/Absorber/absorber.htm|website=web.mit.edu|access-date=2015-06-02|archive-date=2016-03-04|archive-url=https://web.archive.org/web/20160304051247/http://web.mit.edu/nrl/Training/Absorber/absorber.htm|url-status=dead}}</ref> Although indium is less rare than silver, it is more expensive.

Boron is another common neutron absorber. Due to the different cross sections of <sup>10</sup>B and <sup>11</sup>B, materials containing boron enriched in <sup>10</sup>B 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. <sup>10</sup>B/<sup>11</sup>B separation is done commercially with [[gas centrifuge]]s over BF<sub>3</sub>, but can also be done over BH<sub>3</sub> 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.<ref>{{cite web |url=http://web.mit.edu/nrl/Training/Absorber/absorber.htm |title=Control Materials |publisher=Web.mit.edu |access-date=2010-08-14 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304051247/http://web.mit.edu/nrl/Training/Absorber/absorber.htm |url-status=dead }}</ref> 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.<ref name="PATENT1">{{cite web|title=Hafnium alloys as neutron absorbers|work=Free Patents Online|url=http://www.patentgenius.com/patent/5330589.html|access-date=September 25, 2008|archive-url=https://web.archive.org/web/20081012050058/http://www.patentgenius.com/patent/5330589.html|archive-date=October 12, 2008}}</ref> 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&nbsp;°C and density higher than that of uranium dioxide for sinking, unmelted, through [[corium (nuclear reactor)|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.<ref name="DYSP">{{cite web|title=Dysprosium (Z=66)|work=Everything-Science.com web forum|url=http://www.everything-science.com/sci/Forum/Itemid,82/topic,4866.0/prev_next,prev|access-date=September 25, 2008}}</ref> 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.<ref name="PATENT2">{{cite web|title=Method for making neutron absorber material|work=Free Patents Online|url=http://www.freepatentsonline.com/6669893.html|access-date=September 25, 2008}}</ref>

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.<ref>{{cite web|url=http://www.patent-de.com/20100401/DE102008049595A1.html |title=Infrarotabsorbierende Druckfarben - Dokument DE102008049595A1 |publisher=Patent-de.com |date=2008-09-30 |access-date=2014-04-22}}</ref> Less absorptive compounds of boron similar to titanium, but inexpensive, such as [[molybdenum]] as Mo<sub>2</sub>B<sub>5</sub>. Since they all swell with boron, in practice other compounds are better, such as carbides, or compounds with two or more neutron-absorbing elements together. It is important that [[tungsten]], and probably also other elements such as [[tantalum]],<ref>{{cite web|url=https://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15270&mf=3&mt=102&nsub=10 |title=Sigma Plots |publisher=Nndc.bnl.gov |access-date=2014-04-22}}</ref> have much the same high capture qualities as [[hafnium]],<ref>{{cite web|url=https://www.nndc.bnl.gov/sigma/ |title=Sigma Periodic Table Browse |publisher=Nndc.bnl.gov |date=2007-01-25 |access-date=2014-04-22}}</ref> 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 greater capture of uranium, and others over [[metastability in nuclear decay#Metastable isomers|metastable]] conditions such as for [[isotopes of uranium|isotope <sup>235m</sup>U]], which has a half-life of approximately 26 minutes.

===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,<ref name="EAGLE">{{cite web|title=Enriched boric acid for pressurized water reactors |work=EaglePicher Corporation |url=http://www.epcorp.com/NR/rdonlyres/71174EA7-B374-4934-8596-B51D105C4F30/0/w_c_01.pdf |access-date=September 25, 2008 |archive-url=https://web.archive.org/web/20071129121350/http://www.epcorp.com/NR/rdonlyres/71174EA7-B374-4934-8596-B51D105C4F30/0/w_c_01.pdf |archive-date=November 29, 2007}}</ref> 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/[[neutron moderator|moderator]], increasing power).

==Safety==
In most reactor designs, as a [[nuclear safety|safety measure]], control rods are attached to the lifting machinery by [[electromagnet]]s, 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 [[scram]]ming.

===Criticality accident prevention===
Mismanagement or control rod failure have often been blamed for [[nuclear accident]]s, including the [[SL-1]] explosion and the [[Chernobyl disaster]].
''Homogeneous'' neutron absorbers have often been used to manage [[criticality accident]]s which involve aqueous solutions of [[fissile]] [[metal]]s. 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 [[Advanced Gas-cooled Reactor|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]] <sup>10</sup>B is responsible for the majority of the neutron absorption. Boron-containing materials can also be used as neutron shielding, to reduce the [[neutron activation|activation]] of material close to a reactor core.

==See also==
*[[Nuclear power]]
*[[Nuclear reactor]]
*[[Nuclear safety]]
*[[Wigner effect]]

==Notes==
{{Notelist}}

==References==
{{Reflist|30em}}

==External links==
{{Sister project links|Nuclear power}}
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==Further reading==
*{{cite tech report |title=Behavior of control rods during core degradation: pressurization of silver-indium-cadmium control rods|last=Powers|first=D.A.|date=August 1, 1985|publisher=[[Office of Scientific and Technical Information]], [[United States Department of Energy]]|osti = 6332291}}
*{{cite tech report |title=Silver-indium-cadmium control rod behavior and aerosol formation in severe reactor accidents|last=Petti|first=D.A.|date=March 1, 1987|publisher=Office of Scientific and Technical Information, United States Department of Energy|osti = 6380030}}
*{{cite web|url=https://inis.iaea.org/search/search.aspx?orig_q=RN:43018415|title=Experiments on silver-indium-cadmium control rod failure during severe accident sequences|last1=Steinbrueck|first1=M.|last2=Stegmaier|first2=U.|date=May 6, 2010|publisher=[[Karlsruhe Institute of Technology]]|access-date=May 29, 2017}}

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[[Category:Alloys]]
[[Category:Nuclear power plant components]]
[[Category:Nuclear reactor safety]]
[[Category:Pressurized water reactors]]