Fissile material

Fissile material contains nuclei that can sustain a fission chain reaction, including when triggered by low-energy neutrons. Examples include uranium-235 and plutonium-239.

Nuclear fission: A nuclear reaction in which a heavy atomic nucleus splits into lighter nuclei, releasing energy and usually neutrons. Fission releases the neutrons that can continue a reaction in fissile material.
Isotope: One of several forms of an element whose atoms have the same number of protons but different numbers of neutrons. Fissility depends on the nucleus's isotope, not merely its chemical element.
Nuclear reactor: A system that initiates, controls, and sustains a nuclear chain reaction, usually to produce heat or neutrons. Reactors use controlled fission of fissile material to generate heat or neutron flux.
Fertile material: Nuclear material that can be converted into fissile material through neutron capture and subsequent radioactive decay. Fertile isotopes can breed fissile nuclei but do not themselves sustain low-energy fission chains.
Nuclear proliferation: The spread of nuclear weapons, weapons technology, or the capacity to produce them to additional states or groups. Access to fissile material is a central constraint on producing nuclear weapons.
Neutron: A subatomic particle with no electric charge, found in atomic nuclei and produced in many nuclear reactions. Neutrons trigger fission and propagate the chain reaction.
Uranium-235: A naturally occurring uranium isotope that readily undergoes fission after absorbing a low-energy neutron. It is the principal naturally occurring fissile isotope.
Nuclear fuel: Material used in a nuclear reactor to produce energy through fission or fusion reactions. Fissile isotopes are the chain-reaction-sustaining constituents of many fission fuels.
Breeder reactor: A nuclear reactor designed to produce more fissile material than it consumes. It converts fertile material into fissile isotopes while generating power.
Uranium enrichment: A process that increases the proportion of uranium-235 relative to uranium-238. Enrichment raises uranium's fissile-isotope concentration for reactor fuel or other purposes.

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