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Nuclear fission is a reaction in which a large nuclei breaks apart into two smaller nuclei, releasing a great deal of energy. Nuclei can fission on their own spontaneously, but only certain nuclei, like uranium-235 and plutonium-239, can sustain a fission chain reaction. This is because these nuclei release neutrons when they break apart, and these neutrons can slam into other nuclei, causing them to also break apart and release more neutrons. Fission as encountered in the modern world is usually a deliberately produced man-made nuclear reaction induced by a neutron. Uranium-235 is the fuel of choice in all commercial reactors (and even one natural reactor). The uranium fuel is packed into the core and usually surrounded by a moderator, which is a substance that slows down the neutrons so they have a better chance of inducing fission.