Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. While uranium-238 has a small probability for spontaneous fission or even induced fission with fast neutrons, uranium-235 and to a lesser degree uranium-233 have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction. This generates the heat in nuclear power reactors, and produces the fissile material for nuclear weapons. Depleted uranium (U-238) is used in kinetic energy penetrators and armor plating.
Uranium was originally used as a catalyst to produce ammonia. From this basic material, nitric acid is produced and ammonium nitrate is manufactured. A large part of our food supply is dependent on this catalytic process due to the widespread use of ammonium nitrate as fertilizer for commercial food crops. Iron catalysts have largely replaced the use of uranium catalysts in ammonia production due to lower cost.