Nuclear fuels

 

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.

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.

Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear fuel can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials. The most common fissile nuclear fuels are ²³⁵U and ²³⁹Pu, and the actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons.

Not all nuclear fuels are used in fission chain reactions. For example, ²³⁸Pu and some other elements are used to produce small amounts of nuclear power by radioactive decay in radiothermal generators, and other atomic batteries. Light isotopes such as ³H (tritium) are used as fuel for nuclear fusion. If one looks at binding energy of specific isotopes, there can be an energy gain from fusing most elements with a lower atomic number than iron, and fissioning isotopes with a higher atomic number than iron.