Concentrations of U-235 vs. U-238 in enriched uranium.

Nuclear fission was first demonstrated by Fermi in 1934, nearly 40 years after the discovery of radioactive decay by the French physicist and chemist Henri Bequerel. Fission occurs when a heavy element such as uranium becomes unstable and subsequently splits into lighter elements; in the case of nuclear reactors, the uranium is bombarded with neutrons which cause the instability. Antineutrinos and energetic photons are two by-products, as are free neutrons. The free neutrons cause a chain reaction, as more uranium atoms are made unstable.

Fermi's experiment at the University of Chicago, and another experiment in Washington state, became the basis for the Manhattan Project during World War II. Nuclear power plants were being looked into by many countries the end of the 1950s. For a fission chain reaction to occur, the specific isotope uranium-235 must be extracted from the more common uranium-238.

Simple nuclear fuel cycle of uranium

Safeguards are in place which restrict the number of free neutrons that are able to bombard the fuel source so that the chain reaction does not get out of hand. Energy is extracted by the fission reactions via the creation of large amounts of heat; this heat is used to boil water and power a steam generator. Though the efficiency is low, the heat produced is many orders of magnitude higher than the heat produced by burning coal, which makes nuclear reactors a powerful energy source.