Also found in: Dictionary, Thesaurus.
Materials whose ability to release energy derives from specific properties of the atom's nucleus. In general, energy can be released by combining two light nuclei to form a heavier one, a process called nuclear fusion; by splitting a heavy nucleus into two fragments of intermediate mass, a process called nuclear fission; or by spontaneous nuclear decay processes, which are generically referred to as radioactivity. Although the fusion process may significantly contribute to the world's energy production in future centuries and although the production of limited amounts of energy by radioactive decay is a well-established technology for specific applications, the only significant industrial use of nuclear fuel so far utilizes fission. Therefore, the term nuclear fuels generally designates nuclear fission fuels only. See Nuclear fusion, Nuclear power
Large releases of energy through a fission or a fusion reaction are possible because the stability of the nucleus is a function of its size. The binding energy per nucleon provides a measure of the nucleus stability. By selectively combining light nuclei together by a fusion reaction or by fragmenting heavy nuclei by a fission reaction, nuclei with higher binding energies per nucleon can be formed. The result of these two processes is a release of energy. The fissioning of one nucleus of uranium releases as much energy as the oxidation of approximately 5 × 107 atoms of carbon.
Many heavy elements can be made to fission by bombardment with high-energy particles. However, only neutrons can provide a self-sustaining nuclear fission reaction. Upon capture of a neutron by a heavy nucleus, the latter may become unstable and split into two fragments of intermediate mass. This fragmentation is generally accompanied by the emission of one or several neutrons, which can then induce new fissions. Only a few long-lived nuclides have been found to have a high probability of fission: 233U, 235U, and 239Pu. Of these nuclides, only 235U occurs in nature as 1 part in 140 of natural uranium, the remainder being mostly 238U. The other nuclides must be produced artificially: 233U from 232Th, and 239Pu from 238U. The nuclides 233U, 235U, and 239Pu are called fissile materials since they undergo fission with either slow or fast neutrons, while 232Th and 238U are called fertile materials. The latter, however, can also undergo the fission process at low yields with energetic neutrons; therefore, they are also referred to as being fissionable.
The term nuclear fuel applies not only to the fissile materials, but often to the mixtures of fissile and fertile materials as well. Using a mixture of fissile and fertile materials in a reactor allows capture of excess neutrons by the fertile nuclides to form fissile nuclides. Depending on the efficiency of production of fissile elements, the process is called conversion or breeding. Breeding is an extreme case of conversion corresponding to a production of fissile material at least equal to its consumption. See Nuclear fuel cycle, Nuclear reactor