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In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.
All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate. That is, they can all be used to fuel:
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"Fissile" is distinguished from "fissionable". "Fissionable" are any materials with atoms that can undergo nuclear fission. "Fissile" is defined to be materials that are fissionable by neutrons with low kinetic energy. "Fissile" thus, is more restrictive than "fissionable" — although all fissile materials are fissionable, not all fissionable materials are fissile. A few writers[who?] even restrict the term fissionable to include only fissile materials.
Notably, uranium-238 is fissionable but not fissile. Neutrons produced by fission of e.g. U-235 have an energy of around 1 MeV (100 TJ/kg, i.e. a speed of 14,000 km/s) and do not cause fission of U-238, but neutrons produced by the deuterium-tritium fusion reaction have an energy of 14.1 MeV (1400 TJ/kg, i.e. a speed of 52,000 km/s), and they can easily fission U-238 and other non-fissile actinides. The neutrons produced by this fission are again not fast enough to produce new fissions, so U-238 does not sustain a chain reaction.
Fast fission of U-238 in the secondary stage of a nuclear weapon contributes greatly to yield and to fallout. The fast fission of U-238 also makes a significant contribution to the power output of some fast neutron reactors.
Fissile nuclides in nuclear fuels include:
In general, most actinide isotopes with an odd number of neutrons are fissile. Most nuclear fuels have an odd atomic mass number (N = the total number of protons and neutrons), and an even atomic number (Z = the number of protons). This implies an odd number of neutrons.
More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others - and hence, less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else just to absorb the neutron. They are also less likely to undergo spontaneous fission, and have long half-lives for alpha or beta decay. Examples of these elements are U-238 and thorium-232. On the other hand, isotopes with an odd number of neutrons and odd number of protons (odd Z, even N) are short-lived because they readily decay by beta-particle emission to an isotope with an even number of neutrons and an even number of protons - (even Z, even N) - becoming a lot more stable.
Fissile nuclides do not have a 100% chance of fissioning on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture cross sections for fission, the cross section for neutron capture with emission of a gamma ray, and the percentage of non-fissions are:
| Thermal neutrons | Epithermal neutrons | |||||
|---|---|---|---|---|---|---|
| σF | σγ | % | σF | σγ | % | |
| 585 | 99 | 14.5% | 235U | 275 | 140 | 34% |
| 750 | 271 | 26.5% | 239Pu | 300 | 200 | 40% |
| 1010 | 361 | 26.3% | 241Pu | 570 | 160 | 22% |
| 531 | 46 | 8.0% | 233U | 760 | 140 | 16% |
To be a useful fuel for nuclear fission chain reactions, the material must:
The International Atomic Energy Agency used to categorize fissile materials according to their security requirements for transportation:[1][2]
but these classes were replaced in the mid 1990s.[3]