What is the role of control rods in a nuclear reactor?

Control rods play a crucial role in the operation of a nuclear reactor by enhancing neutron absorption. They are made of materials that can effectively capture neutrons, such as boron or cadmium. When inserted into the reactor core, control rods absorb excess neutrons, thereby reducing the number of neutrons available to sustain the fission chain reaction. This absorption helps to regulate the reactor's power output and maintain a controlled and safe reaction rate.

By adjusting the position of the control rods, operators can manage the reactor's reactivity; lowering the rods decreases reactivity and power output, while withdrawing them allows more neutrons to cause fission, thereby increasing power output. This capability is essential for achieving stable operation, responding to changes in demand, and ensuring the safety of the reactor.

The other options do not accurately describe the function of control rods within the reactor system, as they do not involve increasing reactor volume, facilitating fuel enrichment, or directly managing radioactive waste, which are handled through other processes and systems in nuclear reactor operations.