Question

Why is Cadmium rod used as an absorbing material to control the reaction rate of neutrons in a nuclear reactor?

Answer 1

We know that a nuclear reactor has uranium as reactants and in the reaction chamber uranium undergoes nuclear fission and this reaction is self sustaining and can spiral out of control. So to control the reaction rate of nuclear fission or reactor, the control rods that are made if cadmium are used.

Complete answer:
To control the rates of nuclear fission of uranium and plutonium in nuclear reactors, control rods are used. For this control rods have to be capable of absorbing neutrons without fissioning themselves. They are made of cross section to stop neutrons of various energies.
So we use Cadmium rod to be used as an absorbing material to control the reaction rate of neutrons in a nuclear rate of neutrons in a nuclear reactor because Cadmium rod has a high cross sectional area.

Note:
We should note that a nuclear reactor works on the principle of energy generated in the process of controlled nuclear fission. The control rods that are made of Cadmium, Boron etc are pushed into the reaction chamber and they absorb the extra neutron and hence provide control over the rate of fission. The reaction is controlled by controlling the number of neutrons escaping per fission and generally $1.5$ neutrons area allowed to escape to keep the fission controlled.