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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:

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.

Fissile nuclides

Fissile nuclides in nuclear fuels include:

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
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%

Nuclear fuel

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

Legal controls

The International Atomic Energy Agency used to categorize fissile materials according to their security requirements for transportation:[1][2]

  • 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.[3]


  1. Safe Transport of Radioactive Materials, International Atomic Energy Agency, 1964
  2. 10CFR71, 49CFR173.403
  3. 49CFR & 10CFR71 changes

See also

it:Materiale fissile he:חומר בקיע sl:Cepljivi material fi:Fissiili