moderator

In a nuclear reactor, a moderator serves to slow free neutrons produced in the reactor core during the fission process. Similar to billiard balls, neutrons collide with the nuclei of the moderator and thus give off part of their kinetic energy. Since the largest energy transfers occur in the elastic collisions of two equally heavy particles, light atoms such as hydrogen (in the form of water or polyethylene) and carbon (in the form of graphite) are mostly used as moderator. The slowed neutrons are better suitable for both continuing the fission in the reactor core and carrying out scientific experiments in a research reactor.