Parabolic mirrors in a solar power plant. (Source: © luchschenF / stock.adobe.com)

A medium that slows down fast neutrons arising from fission reactions, turning them into thermal neutrons that have a higher probability of engaging in another fission reaction. Neutrons best lose energy through collisions with the nuclei of light atoms, so materials like water, heavy water, graphite, or beryllium are used as moderators. The most commonly used moderator is ordinary water, which serves also as a coolant.