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. Because these elements have different capture cross sections for neutrons of varying energies, the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control.

Light water reactors (BWR, PWR) operate with "thermal" neutrons, breeder reactors with "fast" neutrons. Control rods are usually combined into control rod assemblies — typically 20 rods for a commercial Pressurized Water Reactor (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 control the neutron flux — increase or decrease the number of neutrons which will split further uranium atoms.

This in turn affects the thermal power of the reactor, the amount of steam produced, and hence the electricity generated. Control rods often stand vertically within the core. In pressurised water reactors (PWR), they are inserted from above, the control rod drive mechanisms being mounted on the reactor pressure vessel head.

Due to the necessity of a steam dryer above the core of a boiling water reactor (BWR) this design requires insertion of the control rods from underneath the core. 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 by which they are inserted can be varied to control the reactivity of the reactor.

Chemical elements with a sufficiently high capture cross section for neutrons include silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, dysprosium, gadolinium, samarium, erbium, and europium, or their alloys and compounds, e.g. high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium titanate, and dysprosium titanate. The choice of materials is influenced by the energy of neutrons in the reactor, their resistance to neutron-induced swelling, and the required mechanical and lifetime properties.

The rods may have the form of stainless steel tubes filled with neutron absorbing pellets or powder. The swelling of the material in the neutron flux can cause deformation of the rod, leading to its premature replacement. The burn up of the absorbing isotopes is another limiting lifetime factor.

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