Moderator (nuclear)

In nuclear engineering, a moderator is a material that restricts the free flow of various particles, such as neutrons, or of radiation, such as X-rays. In general, materials with a low-Z (i.e., low atomic number, such as beryllium or hydrogen) restrict the flow of particles while high-Z materials (e.g., lead, uranium) block radiation).

Moderators may be in various physical forms. In nuclear reactors, neutron moderators may be control rods of boron, or cadmium, which are be moved in or out of the main reactor area to discourage or encourage the flow of neutrons. Graphite moderates neutrons as well. Water and heavy water can serve as both moderator and reactor coolant. Solutions containing high concentrations of neutron absorbers can be used to flood a reactor and do an emergency shutdown.

Beryllium reflectors redirect neutrons away from them.