A control rod is a rod made of chemical elements capable of absorbing many neutrons without fashioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium. Chemical elements with a sufficiently high capture cross-section for neutrons include silver, indium and cadmium. Because these elements have different capture cross-section for neutrons of varying energies, the composition of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control.