In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy.^[1] 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.

Fissile vs fissionable

According to the Ronen Fissile rule,^[2] for a heavy element with 90 ≤ Z ≤ 100, its isotopes with 2 × Z − N = 43 ± 2, with few exceptions, are fissile (where N = number of neutrons and Z = number of protons).^{[3][4][note 1]}

The term fissile is distinct from fissionable. A nuclide capable of undergoing fission (even with a low probability) after capturing a neutron of high or low energy^[5] is referred to as fissionable. A fissionable nuclide that can be induced to fission with low-energy thermal neutrons with a high probability is referred to as fissile.^[6] Fissionable materials include also those (such as uranium-238) for which fission can be induced only by high-energy neutrons. As a result, fissile materials (such as uranium-235) are a subset of fissionable materials.

Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy 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.^[7][8]

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.^[9] These are materials that sustain an explosive fast neutron nuclear fission chain reaction.

Under all definitions above, uranium-238 (²³⁸
U
) is fissionable, but not fissile. Neutrons produced by fission of ²³⁸
U
have lower energies 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 ²³⁸
U
, so fission of ²³⁸
U
does not sustain a nuclear chain reaction.

Fast fission of ²³⁸
U
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 fallout of such weapons. Fast fission of ²³⁸
U
tampers has also been evident in pure fission weapons.^[10] The fast fission of ²³⁸
U
also makes a significant contribution to the power output of some fast-neutron reactors.

Fissile nuclides