Fissile

Fissile is a term used in nuclear science to describe nuclides that can undergo fission after absorbing a neutron and, under suitable conditions, sustain a nuclear chain reaction. In practice, fissile materials are those that readily fission with thermal (slow) neutrons and release additional neutrons that continue the process. This thermal-fission capability distinguishes fissile from other fissionable materials that may require fast neutrons to fission and may not propagate a chain reaction in a reactor.

Common fissile isotopes include uranium-235, uranium-233, plutonium-239, and plutonium-241. U-235 is the primary fissile fuel in conventional light-water reactors; Pu-239 and Pu-241 form in reactor fuel from neutron capture by U-238. U-233 is bred from thorium and also fissile in reactors.

Production and use: Natural uranium contains about 0.7% U-235; enrichment increases it to levels of about 3-5% for most reactors. Weapons-grade material is highly enriched to over 85-90% U-235. Fissile materials are central to reactor design because the neutron economy determines whether a chain reaction can be sustained (critical, subcritical, or supercritical).

Safety and regulation: Fissile materials are tightly controlled due to radiological hazard and proliferation concerns. Handling requires containment and shielding, with stringent international oversight for production, transport, and use.