What is "control rod"

n. Any of a number of steel tubes, containing boron or other neutron absorber, that is inserted into the core of a nuclear reactor in order to control its rate of reaction

n. a steel or aluminum rod that can be moved up or down to control the rate of the nuclear reaction

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.