b
Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. It is achieved by removing the excess neutrons from the reactor core. The only way to remove the excess neutrons through some elements which posses strong neutron absorption capacity. One such element is boron. By regulating the movement or insertion of boron rods into the reactor core while fission process is in progress, the excess neutrons produced during the chain reaction are absorbed by boron which rods regulate the availability of neutron.