The main commercial processes employed for this enrichment involves gaseous uranium in centrifuges. An Australian process based on laser excitation is under development in the USA.
Prior to enrichment, uranium oxide must be converted to a fluoride.
Uranium found in nature consists largely of two isotopes, U-235 and U-238. The production of energy in nuclear reactors is from the ‘fission’ or splitting of the U-235 atoms, a process which releases energy in the form of heat. U-235 is the main fissile isotope of uranium.
Natural uranium contains 0.7% of the U-235 isotope. The remaining 99.3% is mostly the U-238 isotope which does not contribute directly to the fission process (though it does so indirectly by the formation of fissile isotopes of plutonium).
Uranium-235 and U-238 are chemically identical, but differ in their physical properties, particularly their mass. The nucleus of the U-235 atom contains 92 protons and 143 neutrons, giving an atomic mass of 235 units. The U-238 nucleus also has 92 protons but has 146 neutrons – three more than U-235, and therefore has a mass of 238 units.
The difference in mass between U-235 and U-238 allows the isotopes to be separated and makes it possible to increase or “enrich” the percentage of U-235. All present enrichment processes, directly or indirectly, make use of this small mass difference.
Some reactors, for example the Canadian-designed Candu and the British Magnox reactors, use natural uranium as their fuel. Most present day reactors (Light Water Reactors or LWRs) use enriched uranium where the proportion of the U-235 isotope has been increased from 0.7% to about 3% or up to 5%. (For comparison, uranium used for nuclear weapons would have to be enriched in plants specially designed to produce at least 90% U-235.)
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