Depiction of the principle of neutron moderation.

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Neutron moderator’s principle

To increase the probability that a neutron will cause fission in a ²³⁵U nucleus, fast neutrons produced by fission must be slowed down to thermal energies. Fission neutrons typically have energies in the MeV range, while thermal neutrons at room temperature have an energy of about 0.025 eV. Once slowed down, thermal neutrons have a much greater probability of being absorbed by ²³⁵U and causing fission. A moderator is a material that slows down fast neutrons while absorbing as few of them as possible. In thermal reactors, moderation increases the probability that neutrons will cause further fissions and thus helps sustain the chain reaction.

Neutrons lose the greatest fraction of their kinetic energy in elastic collisions with nuclei of similar mass, particularly hydrogen nuclei (protons). Effective moderators contain relatively light nuclei and have a low probability of absorbing neutrons. Common moderator materials include light water, heavy water and graphite.

3D model of water molecules. Each light water molecule (H₂O) contains one oxygen atom and two hydrogen atoms. In heavy water (D₂O), the two ordinary hydrogen atoms are replaced by deuterium atoms.

Light water

Light water is used as a moderator in most power reactors fuelled with enriched uranium. It is readily available, does not require special handling and can also serve as the reactor coolant.

Heavy Water

A molecule of heavy water (D₂O) contains two deuterium atoms instead of the two ordinary hydrogen atoms found in light water. The nucleus of deuterium contains one proton and one neutron. Heavy water must be specially produced, but it absorbs far fewer neutrons than light water. This makes it suitable as a moderator in reactors fuelled with natural or low-enriched uranium.

Model of the crystal lattice of graphite, a non-metallic mineral of very low hardness.

Graphite

Because graphite has a low neutron absorption cross-section and good moderating properties, it can be used in reactors fuelled with natural or low-enriched uranium. Since graphite is a solid, graphite-moderated reactors use a separate fluid as the coolant. At sufficiently high temperatures, graphite can oxidise in the presence of air, which must be taken into account in reactor safety. Nevertheless, graphite continues to be used as a moderator in several reactor types, including modern high-temperature gas-cooled reactors.

Without Moderator

Fast reactors are designed to maintain a fast neutron spectrum and therefore do not use a moderator in the reactor core.