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Concept
Control rod
Summary
Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing many neutrons without themselves decaying. These elements have different neutron capture cross sections for neutrons of various energies. Boiling water reactors (BWR), pressurized water reactors (PWR), and heavy-water reactors (HWR) 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. Control rods have been used in nuclear aircraft engines like Project Pluto as a method of control.
Control rods are inserted into the core of a nuclear reactor and adjusted in order to control the rate of the nuclear chain reaction and, thereby, the thermal power output of the reactor, the rate of steam production, and the electrical power output of the power station.
The number of control rods inserted, and the distance to which they are inserted, strongly influence the reactivity of the reactor. When reactivity (as effective neutron multiplication factor) is above 1, the rate of the nuclear chain reaction increases exponentially over time. When reactivity is below 1, the rate of the reaction decreases exponentially over time. When all control rods are fully inserted, they keep reactivity barely above 0, which quickly slows a running reactor to a stop and keeps it stopped (in shutdown). If all control rods are fully removed, reactivity is significantly above 1, and the reactor quickly runs hotter and hotter, until some other factor (such as temperature reactivity feedback) slows the reaction rate. Maintaining a constant power output requires keeping the long-term average neutron multiplication factor close to 1.
A new reactor is assembled with its control rods fully inserted. Control rods are partially removed from the core to allow the nuclear chain reaction to start up and increase to the desired power level.
Official source
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