regulating rod[′reg·yə‚lād·iŋ ‚räd]
a rod that is made of a substance that easily absorbs neutrons and is used to regulate the rate of nuclear fission in a nuclear reactor.
The neutron balance, or reactivity, of the reactor, that is, the ratio between the number of neutrons released and the number of neutrons absorbed during fission per unit time, is controlled by means of a regulating rod. The introduction of a regulating rod into the core results in a decrease in the reactivity and power of the reactor and even in the complete termination of a chain reaction. The removal of a rod from the core results in an increase in the reactor’s reactivity and a corresponding increase in its power. The position of a rod is also varied to compensate for operational changes in the reactivity of a nuclear reactor that result from, for example, a temperature change, a reduction in nuclear fuel, or an increase in the number of fragments of atomic nuclei that absorb neutrons.
Boron, cadmium, and rare earth elements are primarily used in rod manufacturing. Regulating rods are set in motion usually by electric or hydraulic drive. When an emergency shutdown of a chain reaction is needed, a regulating rod is simply released and allowed to fall freely into the reactor core.
S. A. SKVORTSOV