Distinction between Fissionable, Fissile and Fertile

• Fissile materials are a subset of fissionable materials.
• Fissionable material consists of isotopes that are capable of undergoing nuclear fission after capturing either fast neutron (high energy neutron – let say >1 MeV) or thermal neutron (low energy neutron – let say 0.025 eV).   Typical fissionable materials: ²³⁸U, ²⁴⁰Pu, but also ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu
• Fissile material consists of fissionable isotopes capable of undergoing nuclear fission only after capturing a thermal neutron. ²³⁸U is not a fissile isotope because a thermal neutron cannot fission 238U. ²³⁸U does not also meet the alternative requirement to fissile materials. ²³⁸U cannot sustain a nuclear fission chain reaction because neutrons produced by the fission of ²³⁸U have lower energies than the original neutron (usually below the threshold energy of 1 MeV). Typical fissile materials: ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu.
• Fertile material consists of isotopes that are not fissionable by thermal neutrons but can be converted into fissile isotopes (after neutron absorption and subsequent nuclear decay). Typical fertile materials: ²³⁸U, ²³²Th.

See also: Neutron cross-section

Comparison of cross-sections

Source: JANIS (Java-based nuclear information software)  http://www.oecd-nea.org/janis/

Fissionable – Non-Fissile

Fissionable – Fissile

Fertile

See above:

Glossary