"Fissile" is distinguished from "fissionable". "Fissionable" are any materials that can undergo nuclear fission. "Fissile" nuclei are distinguished by their ability to sustain a chain reaction with low energy neutrons. For example, plutonium-239 is fissile but plutonium-240 can only sustain fast neutron chain reactions and is therefore fissionable. "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 will not sustain a neutron chain reaction. Neutrons produced by fission of e.g. U-235 have an energy of around 2 MeV (i.e. a speed of 20,000 km/s) and thus not readily cause fission of U-238, but neutrons produced by the deuterium-tritium fusion reaction have an energy of 14.1 MeV (i.e. a speed of 52,000 km/s), and can effectively 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.
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. These are materials that sustain an explosive fast fission chain reaction. One nuclide that is fissile under this definition but not the traditional nuclear physics definition is Np-237.
Fissile nuclides used as nuclear fuel
|Thermal neutrons||Epithermal neutrons|
Fissile nuclides in nuclear fuels include:
- Uranium-235 which occurs in natural uranium and enriched uranium
- Plutonium-239 bred from uranium-238 by neutron capture
- Plutonium-241 bred from plutonium-240 by neutron capture. The Pu-240 comes from Pu-239 by the same process.
- Uranium-233 bred from thorium-232 by neutron capture
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 (σF), the cross section for neutron capture with emission of a gamma ray (σγ), and the percentage of non-fissions are in the table at right.
Which nuclides are fissile?
|244Cm||241Pu f||250Cf||243Cmf||10–30 y||137Cs||90Sr||85Kr|
|232U f||238Pu||f is for |
|69–90 y||151Sm nc➔|
|4n||249Cf f||242Amf||141–351||No fission product |
has halflife 102
to 2×105 years
|4n||245Cmf||250Cm||239Pu f||8–24 ky|
|248Cm||242Pu||340–373||Long-lived fission products|
|237Np||4n+2||1–2 my||93Zr||135Cs nc➔|
|232Th||238U||235U f||0.7–12by||fission product yield|
In general, most actinide isotopes with an odd neutron number 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 much more stable. The physical basis for this phenomenon comes from the pairing effect in nuclear binding energy.
To be a useful fuel for nuclear fission chain reactions, the material must:
- Be in the region of the binding energy curve where a fission chain reaction is possible (i.e. above radium)
- Have a high probability of fission on neutron capture
- Release two or more neutrons on average per neutron capture (which means a higher average number of them on each fission, to compensate for nonfissions, and absorptions in the moderator)
- Have a reasonably long half life
- Be available in suitable quantities
The International Atomic Energy Agency used to categorize fissile materials according to their security requirements for transportation:
- Fissile Class I: no controls
- Fissile Class II: limits on amount of materials shipped
- Fissile Class III: special shipping arrangements are needed
but these classes were replaced in the mid 1990s.