Fissile vs fissionable
"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 zero kinetic energy. "Fissile" thus, is more restrictive than "fissionable" — although all fissile materials are fissionable, not all fissionable materials are fissile. Some authorities even restrict the term fissionable to mean only non-fissile materials.
Notably, uranium-238 is fissionable but not fissile. Neutrons produced by fission of e.g. U-235 have an energy of ca. 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 deuterium-tritium fusion have an energy of 14.1 MeV neutrons (1400 TJ/kg, i.e. a speed of 52,000 km/s) and can easily fission uranium-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 uranium-238 in the third stage of the fission-fusion-fission weapons contributes greatly to their yield and fallout. Fast fission of uranium-238 also makes a significant contribution to the power output of some fast breeder reactors.
- 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
- Uranium-233 bred from Thorium-232 by neutron capture
In general, actinide isotopes with an odd number of neutrons are fissile. Most nuclear fuels have odd N (number of protons and neutrons) and even Z (number of protons). Isotopes with an odd number of neutrons and odd number of protons (odd Z, even N) are shortlived because they can beta decay to an isotope with an even number of neutrons and even number of protons. (even Z, even N)
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 cross sections for fission and for capture emitting a gamma ray, and the percentage of nonfissions are:
|Thermal neutrons||Epithermal neutrons|
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 an even higher number on each fission, to compensate for nonfissions)
- Have a reasonably long half life
- Be available in suitable quantities
- 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.