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Introduction
Uranium found in nature consists largely of two isotopes, U-235 and U-238. The production of energy in the form of heat in nuclear reactors is from the `fission' or splitting of the U-235 atoms. 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.
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.
Uranium is enriched in U-235 by gaseous diffusion or centrifuge technology. Both of these processes work on the principle of separating the lighter U-235 from the heavier U-238, when in the form of uranium hexafluoride gas.
Some reactors, for example the Canadian-designed Candu and the British Magnox reactors, use natural uranium as their fuel, but most types require uranium enriched to 3 to 5 percent U-235 .
Uranium enrichment requires the material to be in gaseous form. The product of a uranium mine is not directly usable and the uranium oxide must be converted into uranium hexafluoride (UF6) which is a gas at relatively low temperature.
There are conversion plants in Europe, Russia or North America . At a conversion facility, the U3O8 is first refined to uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium. Most is then converted into uranium hexafluoride, ready for the enrichment plant. It is shipped in strong metal containers.
The diffusion process involves forcing uranium hexafluoride gas under pressure through a series of porous membranes. As U-235 molecules are lighter than the U-238 molecules they move faster and have a slightly better chance of passing through the pores in the membrane. The UF6 which diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is slightly depleted in U-235.
This process is repeated many times in a series of diffusion stages called a cascade. The enriched UF6 product is withdrawn from one end of the cascade and the depleted UF6 is removed at the other end. The gas must be processed through some 1400 stages to obtain a product with a concentration of 3% to 4% U-235.
At present the gaseous diffusion process accounts for about 40% of world enrichment capacity. However, because they are old and energy-inefficient , most gaseous diffusion plants are being phased out over the next five years and the focus is on energy-efficient centrifuge enrichment technology which will replace them.
Like the diffusion process, the centrifuge process uses UF6 gas as its feed and makes use of the slight difference in mass between U-235 and U-238. The gas is fed into a series of vacuum tubes, each containing a rotor one to two metres long and 15-20 cm diameter. When the rotors are spun rapidly, at 50,000 to 70,000 rpm the outer wall of the spinning cylinder moves at between 400 and 500 meters per second to give a million times the acceleration of gravity. Centrifugal force causes the heavier molecules with U-238 increase in concentration towards the cylinder's outer edge. There is a corresponding increase in concentration of U-235 molecules near the centre.
The enriched gas forms part of the feed for the next stages while the depleted UF 6 gas goes back to the previous stage. Eventually enriched is drawn from the cascade at the desired concentration and depleted uranium is removed for storage .
In some countries used fuel is reprocessed to recover its uranium and plutonium, and to reduce the final volume of high-level wastes. The plutonium is normally recycled promptly into mixed-oxide (MOX) fuel, by mixing it with depleted uranium. The uranium can be re-enriched, usually through the centrifuge process.
Further Information:Uranium EnrichmentMilitary Warheads as a Source of Nuclear Fuel