Control rods are made of materials such as carbide or alloys of silver, indium and cadmium, hafnium, or boron.
By absorbing neutrons, a control rod controls that is fissions in the chain reaction that is started when materials like uranium are bombarded by subatomic particles.
To prevent overheating in a nuclear reactor, control rods are made up of a material that absorbs neutrons. They are inserted into the uranium bundle using a mechanism that can raise or lower them. Raising and lowering the control rod allow operators to control the rate of the nuclear reaction.
The rods can also be used to shut the rector down in the event of an accident to change the fuel.