page_title: Control rod
section_title: Materials
hierarchical_section_title: Control rod / Materials
caption_attribution_description: English: Neutron absroption cross sections as a function of neutron energy for B-10 and B-11. Black is B-10 and blue is B-11.
context_page_description: Control rods are used in nuclear reactors to control the fission rate of uranium or plutonium. Their compositions includes chemical elements such as boron, cadmium, silver, or indium, that are capable of absorbing many neutrons without themselves fissioning. These elements have different neutron capture cross sections for neutrons of various energies. Boiling water reactors, pressurized water reactors, and heavy-water reactors operate with thermal neutrons, while breeder reactors operate with fast neutrons. Each reactor design can use different control rod materials based on the energy spectrum of its neutrons.
context_section_description: 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. Alloys or compounds may also be used, such as high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium nitrate, gadolinium titanate, dysprosium titanate, and boron carbide–europium hexaboride composite.
  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 ¹¹
  B¹⁵
  N (cubic boron nitride).
  The burnup of "burnable poison" isotopes 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 reactors or in possible new type lithium-7-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, or keiviite (Yb) (Yb1.43Lu0.23Er0.17Tm0.08Y0.05Dy0.03Ho0.02)₂Si₂O₇, lowering the cost. 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 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. 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. 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. 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 molybd
caption: The absorption cross section for 10B (top) and 11B (bottom) as a function of energy
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