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Nuclear fission is a process in which the nucleus of a heavy atom (such as uranium or plutonium) is split into two smaller nuclei, releasing a large amount of energy in the form of radiation and kinetic energy of the fragments. This process can be initiated by bombarding the heavy nucleus with neutrons. When a neutron is absorbed by the nucleus, it becomes unstable and subsequently splits, releasing even more neutrons.

A chain reaction occurs when the neutrons released from one fission event go on to cause further fission events in other nuclei, in a self-sustaining process. In a nuclear reactor, the fuel is enriched with fissile isotopes (such as uranium-235) that can undergo fission when they capture a neutron. In order to maintain a controlled chain reaction, the number of neutrons available to initiate further fission events should be kept at a constant level.

Neutrons are critical to the nuclear fission process since they initiate the chain reaction by being absorbed by the fissile material. The number of neutrons available for fission must be regulated to ensure a stable and controlled reaction. If too many neutrons are present, the chain reaction can become rapid and uncontrollable, potentially leading to a catastrophic event such as a nuclear meltdown.

Materials that have a high affinity for capturing and absorbing neutrons can be used to control the fission chain reaction. These materials, like cadmium or boron, act as a neutron "sink," effectively reducing the number of neutrons available for initiating further fission events in the reactor core. By adjusting the amount of neutron-absorbing material in the reactor, the rate of the chain reaction can be controlled.

Control rods are devices made of neutron-absorbing materials that can be inserted into or withdrawn from a nuclear reactor core to regulate the fission chain reaction. By adjusting the position of the control rods, the number of neutrons available for fission can be altered, effectively controlling the rate of the chain reaction. Fully inserting the control rods can even stop the reaction by absorbing a significant portion of the neutrons, thus preventing further fission events. In summary, materials that absorb neutrons are used to control nuclear fission chain reactions by regulating the number of available neutrons for initiating further fission events. This is achieved through the use of control rods, which can be adjusted in the reactor core to maintain a stable and controlled chain reaction, or stop it if necessary.