The role of the Neutron Moderator (The Moderator) in a thermal fission reactor


Similar to the word moderate (which means to reduce) the moderator (or neutron moderator) is a material or medium usually made out of light water (H2O), heavy water (D2O), beryllium, or solid graphite, which lowers the kinetic energy of fast neutrons by reducing its speed converting it into thermal neutrons. Fast neutrons with energy of about 1 MeV (or speed of about 107 ms-1) are highly unlikely to cause further fissions, due to their low cross section. After the fast neutrons undergo moderation the neutrons become a thermal neutron with energy of about 0.025 eV. The thermal neutrons that are slower have a high chance to initiate further Uranium-235 fissions. The moderator is crucial to sustain a nuclear chain reaction, because there will be no material or medium to moderate the fast neutrons to thermal neutrons. Therefore a controlled fission or a nuclear chain reaction will continue.


The picture above shows the cycle of a thermal fission reactor. The neutrons are always pumped back to the moderator after heating up the water through a Uranium-235 fission. Source: University of KwaZulu Natal


The graph above is a function of a cross-section of a Uranium-235 fission. As the energy of the neutrons increases, the chance of a Uranium-235 decreases. Source: Wikipedia


The material of the moderator determines the cost of the reactor, and the material the reactor processes.

Quote from Wikipedia - "The ideal moderator is of low mass, high scattering cross section, and low absorption cross section."

  • Light water (H2O)
    • Cheap, but absorbs too many neutrons from natural uranium. Therefore the natural uranium needs to be enriched first and increases the cost.
  • Heavy water (D2O)
    • Relatively expensive because they have to around 99.75% pure to be able to process natural uranium.
  • Solid graphite
    • Cheap, light, solid, stable, and easily available, the solid graphite processes natural uranium.
  • Beryllium
    • Expensive, and processes natural uranium.

List of Sources

"Basic Principles." The Virtual Nuclear Tourist. 2006. 1 Apr. 2009 <>.

M. Moodley. "Chapter 6." 6 Nuclear Fission Reactors. 2008. 1+. Chpt6.pdf. 2008. University of KwaZulu Natal. 1 Apr. 2009 <>

M. Moodley. Figure 2. Digital image. Chpt6.pdf. 2008. University of KwaZulu Natal. 1 Apr. 2009 <>

"Neutron moderator -." Wikipedia, the free encyclopedia. 1 Apr. 2009


"Research - Energy - The physics of nuclear reactors." Europa. European Comission. 1 Apr. 2009 <>.