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Control rods are long, slender rods made of materials that are good at absorbing neutrons, such as cadmium, boron, or hafnium. In a nuclear reactor, they are used to control the rate of the nuclear fission reaction by regulating the number or neutrons available to induce new fission events. When the control rods are inserted into a reactor core, they absorb more neutrons, reducing the likelihood of a fission event occurring, thus slowing down or stopping the nuclear chain reaction. Conversely, when the control rods are withdrawn, fewer neutrons are absorbed, and the fission rate increases, leading to an acceleration of the chain reaction. This control mechanism ensures a safe and consistent production of heat by the reactor, which can then be used to generate electricity.

A moderator is a material in a nuclear reactor that serves to slow down neutrons emitted during a fission event. Slowing down these neutrons increases the likelihood of them inducing another fission event, leading to a sustained chain reaction. During a fission event, high-energy, fast-moving neutrons are generated, but these neutrons are less likely to interact with other fissile material, such as uranium-235 or plutonium-239. A moderator, which is generally a light element like hydrogen, slows down these fast neutrons by collision without significant absorption. When neutrons are slowed down, they become thermal neutrons, which have a much higher probability of inducing fission in fissile material. Commonly used moderators include water, heavy water (deuterium oxide), and graphite. In modern-day power reactors, water functions as both a moderator and a coolant, while in some reactor designs, heavy water or graphite is used as a moderator with a separate coolant.