Enriched uranium has had the concentration of a particular isotope, uranium 235, or U235, increased above naturally occurring levels. Uranium has at least three isotopes in nature, and U235 is particularly useful as a fuel for nuclear reactors and as the basis for nuclear weapons. Uranium is naturally composed of a very high percentage of U238and a very small percentage of other isotopes, U235 being the next most common, around 0.7%. By separating the isotopes, enriched uranium samples with a higher concentration of U235 isotopes can be created.
Uranium can be enriched in a number of ways and to varying degrees depending on the intended use. U235 is important for nuclear reactor fuel and nuclear weapons because of its ability to undergo fission, or splitting. Each U235 atom that is split releases a tremendous amount of energy, as well as a high-energy atomic particle called a neutron. The neutron then strikes another U235 atom, causing it to split, setting off a chain reaction. When the reaction is slow and controlled, it can be harnessed as is the case with a nuclear reactor. When the reaction is violent and uncontrolled, the result is a nuclear explosion, the process that was the basis for the bombs dropped by the United States on Japan at the end of World War II.
Depending on the intended use, enriched uranium is produced with varying concentrations of U235 atoms. The lowest grade is known as slightly enriched uranium (SEU) and has a concentration of up to approximately 2% U235. The next grade is known as low enriched uranium (LEU) and usually has a concentration of approximately 3% or 4% U235, which is the highest standard concentration for reactor fuel, although it can be as high as 20%.
Highly enriched uranium (HEU), also known as weapons grade uranium, is uranium that has a concentration of approximately 80% U235. Uranium enriched to this level is used in the manufacture of the weapons that make up the nuclear arsenals of many countries, although enrichment levels as low as 20% are sufficient for producing the simplest nuclear weapons. This grade of enriched uranium is highly controlled and regulated by national and international governments and regulatory agencies in order to promote its safe use and to prevent the creation of nuclear weapons by entities that the world's civilizations as a whole deem to be potentially dangerous or irresponsible.