FACTSHEET: Uranium enrichment By Lizzy Bloem
U235, unlike U238, is fissile. Fission of the fissile uranium atom releases energy, used in nuclear energy and nuclear weapons.
Most of the industry needs a higher concentration than 0.7% U235 to operate. Most commercial nuclear reactors need 3-5 % U235, some nuclear weapons up to 90 %. Therefore the fissile U235 is concentrated in a process called enrichment.
In the process, physicists use the names 'natural', 'enriched' and 'depleted' uranium. These names refer to different compositions of uranium. Natural uranium consists of 0.7% U235 and 99.3 % U238. In enriched uranium the concentration of U235 is increased above natural levels, while the concentration of U235 in depleted uranium is decreased. This is the origin of the term 'depleted uranium'. In the US, depleted uranium must to contain officially less than 0.3% U235.
The 'natural' ratio of isotopes is found in ore, yellowcake as well as hex. These substances are therefore called 'natural' uranium. This can be confusing because yellowcake and hex are not found in nature at all. It can also be misleading. It is often stated that DU is only 60% as radioactive as natural uranium. This only holds true for purified uranium. DU is some thousand times more radioactive than the initial ore or uranium mineral (depending on the grade of the ore). One also hears that DU has the same chemical properties as natural uranium. Again, this only holds true for purified uranium. Processed uranium, wether natural or depleted, is far more chemically reactive than the initial 'naturally' occurring uranium mineral.
Two enrichment technologies operate on a commercial scale: one uses gas centrifugation and the other uses gas diffusion through fine screens.
Both of these processes are fed by hex (UF6) gas. Both technologies take advantage of the small difference in mass between U235 and U238. The nucleus of a U235 atom contains 3 neutrons, or 3 mass units, less than a U238 nucleus. Molecules of hex with U235 atoms are about one percent lighter than the rest. This difference in mass is used to separate the hex molecules into two fractions: enriched hex and depleted hex, also named enriched uranium and depleted uranium. The enriched part is processed into uranium dioxide (UO2) for use in reactors and weapons. The depleted part is for the greater part stored in huge cylinder yards.
Gas diffusion consumes large amounts of electricity. The most modern gas centrifuge technologies involve relatively high costs for the equipment required, but energy consumption is a factor 50 less. Therefore the ultracentrifuge or gas centrifuge method is considered nowadays as most economically viable.