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Control rods play a crucial role in controlling the nuclear reactions occurring inside the reactor. They are used to regulate the rate of fission chain reactions, by absorbing excessive neutrons produced during fission, thereby maintaining the reactor's power output and ensuring its safety. The ability to adjust the position of control rods helps to maintain a steady output of power and prevent the reactor from becoming too hot or experiencing a meltdown.

Control rods are usually constructed from materials with a high neutron absorption capacity, and the common substances used include: 1. Cadmium 2. Boron 3. Hafnium 4. Silver-indium-cadmium alloy These materials have high atomic numbers and large neutron capture cross-sections, which means they are highly effective in absorbing neutrons.

The substances used for control rods are chosen based on the following properties: 1. High neutron-absorption capacity: A good control rod material should be able to absorb a large number of neutrons to effectively control the reactor's fission rate. 2. Mechanical strength and resistance to radiation damage: Control rod materials should be able to withstand the harsh environment of a nuclear reactor, including extreme temperatures, pressures, and radiation exposure. 3. Low neutron-induced swelling: Control rod materials that experience minimal swelling when exposed to neutron bombardment are more desirable, as swelling can cause distortion or jamming of the control mechanism. 4. Chemical stability: Control rod materials should not chemically react with the coolant or other reactor components, as this could adversely impact their neutron-absorption capabilities or compromise the integrity of the reactor. 5. Low corrosion rate: Materials that corrode slowly ensure long life and reduced maintenance of control rods, minimizing reactor downtime and potential safety concerns. Based on these factors, the mentioned substances (Cadmium, Boron, Hafnium, and Silver-indium-cadmium alloy) are selected to construct control rods that ensure efficient control and overall safety of a nuclear reactor.