Editing Control rod (section)

==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).