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In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the threshold required for fission; therefore uranium-235 is fissile. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is fissionable but not fissile.James J. Duderstadt and Louis J. Hamilton (1976). 9780471223634, John Wiley & Sons, Inc.. ISBN 9780471223634John R. Lamarsh and Anthony John Baratta (Third Edition) (2025). 9780201824988, Prentice Hall. ISBN 9780201824988
An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable) and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting. Under this definition, the only nuclides that are fissionable but not fissile are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction. As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control context, particularly in proposals for a Fissile Material Cutoff Treaty, the term fissile is often used to describe materials that can be used in the fission primary of a nuclear weapon. Fissile Materials and Nuclear Weapons , International Panel on Fissile Materials These are materials that sustain an explosive fast neutron nuclear fission chain reaction.
Under all definitions above, uranium-238 () is fissionable, but not fissile. Neutrons produced by fission of have lower kinetic energy than the original neutron (they behave as in an inelastic scattering), usually below 1 MeV (i.e., a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of , so fission of does not sustain a nuclear chain reaction.
Fast fission of in the secondary stage of a thermonuclear weapon, due to the production of high-energy neutrons from nuclear fusion, contributes greatly to the yield and to nuclear fallout of such weapons. Fast fission of tampers has also been evident in pure fission weapons. The fast fission of also makes a significant contribution to the power output of some fast-neutron reactors.
More generally, nuclides 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; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to neutron capture but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission, and they also have relatively much longer partial half-lives for alpha decay or beta decay decay. Examples of these isotopes are uranium-238 and thorium-232. On the other hand, other than the lightest nuclides, nuclides with an odd number of protons and an odd number of neutrons (odd Z, odd N) are usually short-lived (a notable exception is neptunium-236 with a half-life of 154,000 years) because they readily beta decay to their isobars with an even number of protons and an even number of neutrons (even Z, even N) becoming much more stable. The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing. The relatively short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.
According to the fissility rule proposed by Yigal Ronen, for a heavy element with Z between 90 and 100, an isotope is fissile if and only if } (where N = Neutron number and Z = Atomic number), with a few exceptions.Ronen Y., 2006. A rule for determining fissile isotopes. Nucl. Sci. Eng., 152:3, pages 334-335. [2] This rule holds for all but fourteen nuclides – seven that satisfy the criterion but are nonfissile, and seven that are fissile but do not satisfy the criterion.The fissile rule thus formulated indicates 33 isotopes as likely fissile: Th-225, 227, 229; Pa-228, 230, 232; U-231, 233, 235; Np-234, 236, 238; Pu-237, 239, 241; Am-240, 242, 244; Cm-243, 245, 247; Bk-246, 248, 250; Cf-249, 251, 253; Es-252, 254, 256; Fm-255, 257, 259. Only fourteen (including a long-lived metastable nuclear isomer) have half-lives of at least a year: Th-229, U-233, U-235, Np-236, Pu-239, Pu-241, Am-242m, Cm-243, Cm-245, Cm-247, Bk-248, Cf-249, Cf-251 and Es-252. Of these, only U-235 is naturally occurring. It is possible to breeder reactor U-233 and Pu-239 from more common naturally occurring isotopes (Th-232 and U-238 respectively) by single neutron capture. The others are typically produced in smaller quantities through further neutron absorption.
| +Capture-fission ratios of fissile nuclides !colspan=3 | !! !!colspan=3>Epithermal neutrons |
| 16% | |
| 34% | |
| 40% | |
| 22% |
Fissile in nuclear fuels include:
Fertile nuclides in nuclear fuels include:
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