A particularly efficient type of nuclear reactor that is able to utilize the tremendous energy latent in 238U. This cannot be exploited in conventional (thermal) nuclear reactors, which are fueled with enriched uranium or plutonium, for these eventually become depleted and must be replaced. The fuel used for the breeder reactor is a mixture of nonfissionable 238U and 239Pu sealed in long, thin hexagonal metal tubes, which are in turn contained in cans called subassemblies. These constitute the reactor core. Around it are placed several layers of U238, also in subassemblies. When criticality is reached, the unmoderated neutron flux from the core permeates the entire system and thus “breeds” fissionable 239Pu in the surrounding 238U. The amount of fissionable material thus made available is about 100 times as great as that obtainable with a conventional reactor, since all the energy potential of the 238U can be released. Twenty pounds of uranium has the potential of delivering approximately 52 × 106 kWh of electricity; only a small fraction of this would be extractable without breeding.The breeder utilizes fast neutrons that are much more efficient than the slow (thermal) neutrons used in conventional reactors. Liquid sodium is the coolant in breeder reactors, as it has no retarding effect on the neutrons: 2.9 neutrons per fission are produced in the breeder compared with only 2.4 in water-moderated reactors. This excess of neutrons makes it possible for the fast breeder to produce more fuel than it consumes. Breeders have been operating on a commercial scale in several European countries for some years. The NRC authorized construction of the Clinch River breeder, but funding was canceled as a result of opposition by environmentalist and antinuclear groups. The only operating breeder reactor in the U.S. is Argonne's EBR-11 in Idaho.
