•The best moderator is something that is
the same size
as the neutron itself - hydrogen.
•However, hydrogen does absorb neutrons as well.
•The deuterium isotope of hydrogen, at
twice the mass
of hydrogen, is almost as good a slowing down agent but it has a very low absorption cross section -> better moderator
•D2O + natural fuel = CANDU
•H2O
+ enriched fuel = PWR
•Other possible moderators include graphite and gases such as carbon dioxide and helium.
• A good
moderator = high scattering cross section + low absorption cross section + slows the neutron in the least number of collisions.