What component slows down fast neutrons in a nuclear reactor?

In a nuclear reactor, the component responsible for slowing down fast neutrons is the moderator. The moderator is a material that reduces the kinetic energy of the neutrons produced by fission reactions. Fast neutrons are too energetic to effectively sustain a chain reaction when colliding with fuel nuclei. By slowing them down, the moderator increases the probability of these neutrons being captured by the fuel atoms, such as uranium or plutonium, thereby facilitating further fission reactions.

Common materials used as moderators include water, heavy water, and graphite. These materials are effective because they can interact with neutrons through elastic scattering processes, which gradually reduce the speed of the neutrons without capturing them. In doing so, the moderator plays a crucial role in maintaining a controlled nuclear reaction within the reactor core.