Uranium is the element used as fuel in most nuclear power plants. Commercial reactors generally use uranium-235, a naturally occurring fissile isotope, usually after the uranium has been processed and modestly enriched.
Inside a reactor, uranium-235 nuclei absorb neutrons and split in fission. This releases heat and additional neutrons, sustaining a controlled chain reaction. The heat turns water into steam, and the steam drives turbines that generate electricity—much like in a fossil-fuel power station, but with fission providing the heat.
Natural uranium contains about 0.72% uranium-235, with most of the remainder being uranium-238. Many light-water reactors enrich the uranium-235 concentration to roughly 3–5%. Uranium-238 can also absorb neutrons and eventually become plutonium-239, which contributes to the reactor's energy production.
Thorium is often discussed as a possible alternative fuel, while plutonium is used in some mixed-oxide fuels. Neither, however, is the standard fuel in most of the world's commercial nuclear reactors.