The purpose of the moderator is to slow down fission neutrons with MeV energies by collision process to energies of less than 2 eV so that they will have a sufficiently high probability of fission producing capture by another fissile atom. Approximately 2.5 high energy neutrons are emitted per fission. One of these is essential to maintain the chain reaction by being absorbed in another fissile atom. If more than 1.5 are lost from the surface or by absorption in the moderator, structural materials or U238, to produce plutonium, the chain reaction will cease. For this reason the moderator must have a low cross-section and all materials must be of ultra high purity.
Moderators are usually heavy water or graphite.
The reactor also contains a circulated coolant, such as carbon dioxide in a gas cooled reactor, or water in a liquid cooled reactor. Once constructed a power reactor is expected to operate entirely without internal maintenance, except for the changing of fuel elements. It is thus essential that there is complete chemical stability between the various component parts, i.e. coolant, moderator, structural elements, containment vessel and fuel element cans. This chemical compatibility must be total and be the case not only at the elevated temperatures of several hundred degrees at which power reactors operate, but also be the case at intense levels of radiation. Here a knowledge of radiation chemistry is essential.