nonfissile

Nonfissile is a term used in nuclear science to describe isotopes or materials that cannot sustain a chain reaction of nuclear fission under conditions typical of a reactor or explosive device. In common usage, it refers to isotopes that are not fissile with thermal neutrons, meaning they cannot maintain a self-sustaining fission chain reaction in a conventional reactor spectrum. Some materials may be fissionable by fast neutrons but are not considered fissile in the usual sense; such cases are part of the nuanced distinction between fissile, fissionable, and nonfissile.

A key distinction is between fissile and fertile materials. Fissile isotopes, such as uranium-235 and plutonium-239, can sustain a chain reaction with slow (thermal) neutrons. Fertile isotopes, such as uranium-238 and thorium-232, are not fissile themselves but can be converted into fissile materials (for example, U-238 can breed Pu-239, and Th-232 can breed U-233) in a reactor. Consequently, nonfissile materials include both nonfissile nonfertile isotopes and fertile isotopes that do not act as fissile fuels on their own but may contribute to breeding.

In practice, nonfissile materials are common in nuclear systems as structural components, moderators, or shielding, and they influence neutron economy through absorption and scattering. The term is relative to neutron energy and reactor design; some isotopes may be nonfissile in a thermal spectrum but can behave differently under fast-neutron conditions. Overall, nonfissile denotes the absence of the capability to sustain a thermal-neutron–induced fission chain reaction.