Control Rod
A movable rod of neutron-absorbing material used to regulate the rate of a nuclear chain reaction.
Unit
Dimensionless
In depth
Inserting the rods absorbs neutrons and lowers the multiplication factor, while withdrawing them raises it. The delayed neutrons released seconds after fission make the reactor response slow enough for mechanical control to work at all.
Examples in the real world
Boron carbide and cadmium rods; emergency shutdown drops them fully into the core within seconds.