The Seperation Of Uranium Essay

1132 words - 5 pages

1. Mining and millingUranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. The mined uranium ore is sent to a mill which is usually located close to the mine. At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid:UO3 + 2H+ ====> UO22+ + H2OUO22+ + 3SO42- ====> UO2(SO4)34-to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate. The UO2 is oxidised to UO3.With some ores, carbonate leaching is used to form a soluble uranyl tricarbonate ion: UO2(CO3)34-. This can then be precipitated with an alkali, eg as sodium or magnesium diuranate. Alkaline leaching is not undertaken in Australia at present.2. ConversionBecause uranium needs to be in the form of a gas before it can be enriched, the U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant in Europe or North America. The mixed uranium oxide concentrate U3O8 received by the refinery is dissolved in nitric acid. The resulting solution of uranium nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined (heated strongly) to produce pure UO3.3. EnrichmentMost nuclear reactors require uranium to be enriched from its natural isotopic composition of 0.7% U-235 (most of the rest being U-238) to 3.5-4% U-235. The enrichment process removes 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level and then passes to the next stage. The other stream is depleted in U-235 and is called 'tails'. It is mostly U-238.So little U-235 remains in the tails (usually less than 0.3%) that it is of no further use for energy, though such 'depleted uranium' is used in metal form in yacht keels, as counterweights, and as radiation shielding, since it is 1.7 times denser than lead.The first enrichment plants used the gaseous diffusion process, but more modern plants mostly use the centrifuge process (bottom page 3) this has the advantage of using much less power per unit of enrichment and can be built in smaller, more economic units. Research is being conducted into laser enrichment, which appears to be a promising new technology.A small number of reactors, notably the Canadian and early British gas-cooled reactors, do not require uranium to be enriched.4. Fuel fabricationEnriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy or stainless steel, to form fuel rods. The rods are then sealed and assembled in...

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