Fissile Vs Fissionable
According to the fissile rule, heavy isotopes with 90 ≤ Z ≤ 100 and 2 × Z – N = 43 ± 2, with few exceptions, are fissile (where N = number of neutrons and Z = number of protons).
"Fissile" is distinct from "fissionable." A nuclide capable of undergoing fission after capturing a neutron is referred to as "fissionable." A fissionable nuclide that can be induced to fission with low energy thermal neutrons is referred to as "fissile." Although the terms were formerly synonymous, fissionable materials include also those (such as uranium-238) that can be fissioned only with 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 a fissile material. 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 a fissionable material but not a fissile material.
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, nuclides that are only fissionable 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. These are materials that sustain an explosive fast fission chain reaction.
Under all definitions above, uranium-238 (U-238) is fissionable, but because it cannot sustain a neutron chain reaction, it is not fissile. Neutrons produced by fission of U-238 inevitably inelastically scatter to an energy below 1 MeV (i.e., a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of U-238, so fission of U-238 does not sustain a nuclear 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.
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