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.

Operating principle

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.