Final answer:
Nuclear power plants primarily use uranium-235 to generate energy through the process of nuclear fission. U-235 must be enriched for use in nuclear reactors, and it is a nonrenewable resource. Plutonium-239 can also be used in reactors, primarily in breeder reactors, which can create additional fissile material.
Step-by-step explanation:
Nuclear power plants use the radioactive element uranium-235 to create energy. U-235 is a fissionable isotope that can sustain a chain reaction within a nuclear reactor. This reaction releases heat and radiation, which is then used to generate electricity. Uranium is a nonrenewable energy source found in rocks worldwide, but it is relatively rare. In power plants, uranium must be enriched to increase the concentration of U-235, typically to about 5% of uranium mass to facilitate the fission process without the possibility of an explosion.
When a uranium nucleus emits an alpha particle, it transforms into a different element while conserving charge. In this case, the resulting nucleus is thorium.
There are a few fissionable isotopes suitable for a fission reactor, including uranium-233, uranium-235, and plutonium-239. Of these, U-235 is naturally occurring and is the isotope most commonly used in commercial nuclear reactors, such as the pressurized water reactor design. Breeder reactors can create additional fissile material, such as plutonium-239, offering a way to extend the use of available nuclear fuel.