What is the primary purpose of control rods in a nuclear reactor?

The primary purpose of control rods in a nuclear reactor is to absorb neutrons and regulate the fission process. Control rods are made of materials that have a high neutron absorption cross-section, such as boron or cadmium, which allows them to effectively capture neutrons that are produced during fission. By absorbing these neutrons, control rods decrease the number of neutrons available to sustain the chain reaction.

This regulation is critical because it allows operators to control the reactivity within the reactor, maintaining it at a safe and desired level for power generation. By inserting the control rods deeper into the reactor core, reactivity is reduced, and the reactor power output can be decreased; conversely, pulling the control rods out increases reactivity, allowing for a higher power output. This precise control prevents the reactor from becoming too reactive, which could lead to unsafe conditions or overheating.

In contrast, the other options do not accurately represent the primary role of control rods in a nuclear reactor. Enhancing the fission process, cooling the reactor core, or increasing thermal efficiency are not the primary functions of control rods and can be attributed to other systems and components within the reactor design.