nuclear reactor, device for producing NUCLEAR ENERGY by controlled nuclear reactions. It can be used for either research or power production. The reactor is so constructed that the fission of atomic nuclei produces a self-sustaining nuclear chain reaction, in which the produced neutrons are able to split other nuclei. A fission reactor consists basically of (1) a fuel, usually uranium or plutonium, enclosed in shielding; (2) a moderator--a substance such as graphite, beryllium, or heavy water--that slows down the neutrons so that they may be more easily captured by the fissionable atoms; and (3) a cooling system that extracts the heat energy produced. The fuel is sometimes enriched--i.e., its concentration of fissionable isotopes is artificially increased--to increase the frequency of neutron capture. The breeder reactor is a special type of reactor that produces more fissionable atoms than it consumes by using surplus neutrons to transmute certain nonfissionable atoms into fissionable atoms.
The design of fusion reactors is still in an experimental stage because of the problems involved in containing the plasma fuel and attaining the high temperatures needed to initiate the reaction.