Neutron moderator

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In a nuclear reactor, a reactor moderator controls the flow of neutrons. Moderators are often in the form of control rods, which increase nuclear fission when withdrawn and decrease fission when inserted; a SCRAM is an emergency shutdown in which all rods are inserted to their maximums.

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. Moderators and reactor coolants have comparable properties, and, if the reactor design permits, making them of the same material increases safety since their mixing produces no unpredictable effects.


Neutrons produced from nuclear fission have an average energy of 2 MeV, but immediately begin to slow as they interact with nuclei of materials about them. Eventually,"the speed of a neutron is reduced to such an extent that it has approximately the same average kinetic energy as the atoms (or molecules) of the medium in which the neutron is undergoing elastic scattering. This energy, which is only a small fraction of an electron volt at ordinary temperatures (0.025 eV at 20(C), is frequently referred to as the thermal energy, since it depends upon the temperature. Neutrons whose energies have been reduced to values in this region (< 1 eV) are designated thermal neutrons.

"The process of reducing the energy of a neutron to the thermal region by elastic scattering is referred to as thermalization, slowing down, or moderation. The material used for the purpose of thermalizing neutrons is called a moderator. A good moderator reduces the speed of neutrons in a small number of collisions, but does not absorb them to any great extent. Slowing the neutrons in as few collisions as possible is desirable in order to reduce the amount of neutron leakage from the core and also to reduce the number of resonance absorptions in non-fuel materials.[1] The ideal moderating material has properties of

  • large scattering cross section
  • small absorption cross section
  • large energy loss per collision

These can be expressed as the moderator ratio (MR):[2]

MR = (ζ ∑s) ⁄ ∑a


ζ = energy loss per collision
s = scattering cross-section
a = absorption cross-section, a nonscalar value differing for different neutron energies

Other desirable properties include high density, chemical stability, and resistance to radiation damage.

Specific moderating materials


Both as ordinary hydrogen and as deuterium, hydrogen moderates neutrons. Most often, it does so in the form of water or heavy water. Water and heavy water can serve as both moderator and reactor coolant.


Light water is both the most common moderator and reactor coolant in Boiling Water Reactors and Pressurized Water Reactors used for electricity generation and naval nuclear propulsion.

Heavy water

Heavy water reactors, such as CANDU, can operate with low-enriched nuclear fuel, which is advantageous for nonproliferation. Heavy water is also used in reactors used to breed plutonium.



Beryllium reflects rather than absorbs neutrons.


  1. , Neutron Moderation, Reactor Theory (Neutron Characteristics), DOE-HDBK-1019/1-93, page 23
  2. John Bernard, Reactor Physics, Part I, 22.05 Neutron Science and Reactor Physics, MIT Open Courseware