Describe the nature and purpose of these components of a nuclear reactor: (a) control rods; (b) moderator; (c) reflector.

Control rods are essential components in a nuclear reactor responsible for controlling the fission reaction. These rods are usually made from materials that can absorb neutrons, such as boron, cadmium, or hafnium.
By inserting and removing control rods in the reactor core, operators can adjust the rate of the nuclear reaction. This is because the control rods capture neutrons that would otherwise contribute to the fission process. When control rods are fully inserted into the reactor core, they absorb more neutrons and slow down the reaction. Conversely, when they are withdrawn, fewer neutrons are absorbed, and the reaction rate increases.
Control rods play a vital role in maintaining the reactor's stability and safety. They can be used to shut down the reactor completely in an emergency or to reduce its power output during normal operations. The precise adjustment of control rods ensures that the fission process stays under control and operates at the desired level of power.

The moderator is a substance located within the reactor core that reduces the speed of fast neutrons produced during the fission process. Fast neutrons are not as efficient in sustaining the chain reaction necessary for continuous energy production. As such, it is crucial to slow down these neutrons to create what are known as thermal neutrons, which are more likely to cause further fission in fuel nuclei.
Common materials used as moderators include ordinary water (H2O), heavy water (D2O), which contains a higher proportion of the hydrogen isotope deuterium, graphite, and certain metal hydrides. The choice of moderator influences the efficiency and type of reactor. For instance, light water reactors commonly use ordinary water, while certain types of reactors, like the Canadian CANDU reactors, utilize heavy water.
The moderation process significantly influences the reactor's ability to sustain a stable and efficient chain reaction, making the moderator a critical component for the reactor's overall performance.

The reflector is typically placed around the reactor core, serving to direct escaping neutrons back into the core. Reflectors help in maintaining the neutron economy of the reactor, ensuring that more neutrons participate in the fission process rather than escaping and being lost.
Reflectors are often made from the same materials as the moderators, like water or graphite, but can also include other materials such as beryllium. The use of appropriate reflector materials enhances the efficiency of the reactor by reducing the number of external neutrons and reflecting them back into the core. This boosts the overall reactivity and efficiency of the reactor.
Having a reflector means fewer neutrons are wasted, facilitating a more economically viable and effective reactor operation. It contributes to sustaining the chain reaction more robustly and uniformly throughout the reactor core, playing a crucial role in the reactor's fuel utilization and overall performance.