Nuclear fission occurs when an atom of a heavy element (such as uranium) breaks into two or more atoms of lighter elements (such as barium and krypton). Fission of elements heavier than iron will release energy, while fission of elements lighter than iron will always consume energy. Efficient production of energy using nuclear fission requires heavy, fissile isotopes (such as uranium-235 or plutonium-239) that will decay relatively easily to liberate a relatively large amount of energy.
If enough fissile atoms are in close proximity, particles released by their normal decay can trigger the immediate decay of neighboring atoms, creating a chain reaction. In a nuclear reactor, such a chain reaction is created, and the heat released is used to generate electricity. Essentially, heat from the nuclear reaction boils water into steam, the steam pressure spins a turbine, and the spinning turbine runs an electrical generator.
Types of nuclear reactors include:
- Light water reactors
- Heavy water reactors
- "Pebble bed" reactors