Nuclear fission chain reaction involving uranium-235.
Nuclear fission is a process in which a heavy atomic nucleus splits into two more-or-less equal fragments accompanied by the release of large amounts of energy. A chain reaction results when the neutrons released during fission cause other nearby nuclei to break apart. Fission may occur spontaneously or may be induced by capture of bombarding particles. In addition to the fission fragments, neutrons and gamma rays are usually produced during fission.
Fissile material is any isotope that is capable of undergoing nuclear fission. Sometimes the term is restricted to apply only to isotopes that are capable of undergoing fission upon impact with a slow neutron (e.g. uranium-233, uranium-235, and plutonium-239.