Nuclear power is generated via a process known as fission in nuclear power plants that are designed to convert the latent heat of radioactive decay into electrical power. The fuel for the reaction is any of a number of radioactive elements, most commonly slightly enriched uranium.
Uranium occurs in two main isotopes, U-235 and U-238. Uranium-238 is only weakly radioactive, so it decays slowly and is the most common isotope found in nature. The more radioactive U-235 decays readily and is a productive source of thermal energy. For energy applications, U-238 is only partially refined to increase its concentration of U-235 to around 3 to 5 percent. This is far below the concentration needed for nuclear weapons, and the resulting fuel is generally safe to transport and use.
Partially enriched uranium fuel is organized into structures called fuel rods. These are submerged in water inside a large containment unit called the reactor vessel. There, the neutron flux of decaying U-235 heats the water, which is then pumped in a closed loop through a heat exchange system. In the heat exchanger, the thermal energy of the closed, or "dirty," loop transfers to an open, "clean" loop. Steam from this loop is used to drive a turbine, generating electricity.