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Nuclear fuel cycle

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A number of process stages in the supply and waste management of nuclear fuel for reactors.

  • Supply:
    The point of departure of nuclear energy utilization is the supply of nuclear reactors with uranium. The uranium content of mined ores amounts typically to 0.2%. The uranium is concentrated in a treatment process. The commercial article "Yellow Cake" results, which contains about 70 to 75% uranium. The uranium contained in the Yellow Cake has a natural isotope composition of 0.7% U-235 and 99.3% U-238. Most nuclear power plants need uranium with a proportion of 3 to 5% of the fissile isotope U-235. Therefore, the uranium must be enriched to U-235. For this purpose the uranium is converted into the chemical compound UF6, which can easily be transferred to the gas phase, since enrichment is only easily possible in the gas phase. Enrichment processes (See 'gas centrifuge' or 'diffusion separation processes') use the slight mass difference of the U-235 and U-238 molecules of UF6 to separate these two components. The product of the enrichment plant is UF6 with a U-235 share of approx. 3 to 5%. In the fuel element factory, the UF6 is converted into UO2. The UO2 powder is used to press pellets which are sintered at temperatures of more than 1700 °C, filled into seamlessly drawn cladding tubes made of a zirconium alloy and sealed gas-tight. Thus, individual fuel rods are obtained which are then grouped into fuel elements. The fuel elements of a pressurized water reactor contain about 340 kg uranium, those of a boiling water reactor about 190 kg uranium.

  • Waste management:
    The service period of fuel elements in the reactor is three to four years. Nuclear energy is converted to electricity by nuclear fission. During this process the percentage of fissile U-235 decreases, and the partly radioactive fission products and considerable quantities of the new, partly fissile nuclear fuel plutonium are generated. All activities relating to treatment, reprocessing and disposal of fuel elements are summarized under the term waste management. Two types of disposal are possible: reprocessing by recovery and reuse of the usable plutonium and uranium portions or direct ultimate waste disposal where the spent fuel elements are disposed of altogether as waste. The fuel elements are stored initially in an interim storage facility where the activity decreases. During subsequent reprocessing, reusable uranium and plutonium are separated from the radioactive fission products. For reuse in the nuclear power plant plutonium and uranium - possibly after enrichment - must be processed into fuel elements. With their use in the nuclear power plant the fuel cycle closes. In the case of the direct ultimate waste disposal, the entire fuel element including the valuable substances uranium and plutonium is disposed of as radioactive waste following an interim storage period to allow the short-lived radionuclides to decay and thus the heat development conditional on the decay to decline. For this purpose the fuel elements are sectioned in a conditioning plant, packed in containers suitable for final storage before being stored in a repository. Both methods - reprocessing and direct ultimate storage - have been thoroughly examined in Germany and the required processes and components have been developed. Radioactive waste must be stored safely for a long period and be kept away from biosphere. Under certain circumstances low and medium active liquid radioactive waste is fixed in cement following previous volume reduction by evaporation. Solid radioactive waste is burnt or compacted for volume reduction. For ultimate storage these products are packed in special barrels or containers. The highly active, heat-generating fission product solutions from reprocessing are vitrified in a well-tested procedure adding glass-forming substances and filled into stainless steel containers. Stable geological formations are used as repositories. In Switzerland and in Sweden granite rock is provided for this purpose and in Germany salt domes were investigated for ultimate waste disposal. Rock salt offers excellent properties for the ultimate disposal of heat-generating radioactive waste since it removes the heat well and has plastic behaviour, i.e. cavities gradually close and the waste is safely embedded.

Nuclear fuel cycle

Nuclear fuel cycle

 

 

 


 

 


 

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