Neutron moderation is a process used in nuclear reactors to slow down fast neutrons produced during fission. Fast neutrons are less likely to cause further fission reactions in heavy nuclei like uranium-235 or plutonium-239. By slowing them down, the likelihood of these neutrons interacting with the fuel increases, enhancing the reactor's efficiency. Moderators, such as water, graphite, or heavy water, are materials that facilitate this slowing down process. They work by colliding with fast neutrons, transferring some of their energy and reducing the neutrons' speed. This makes the neutrons more effective in sustaining a controlled nuclear chain reaction.