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OVERVIEW OF PUREX This process can be used to recover Weapon-grade materials as well as Reprocessed Uranium from spent Nuclear Reactor fuel, and as such, its component chemicals are monitored. PUREX is an acronym standing for '''P'''lutonium and '''U'''ranium '''R'''ecovery by '''EX'''traction. The PUREX process is a Liquid-liquid Extraction method used to reprocess spent nuclear fuel, in order to extract uranium and plutonium, independent of each other, from the Fission products. THE CHEMICAL PROCESS The irradiated fuel is first dissolved into Nitric Acid . After the dissolution step it is normal to remove the fine insoluble solids because otherwise they will disturb the solvent extraction process by altering the liquid liquid interface. It is known that the presence of a fine solid can stabilize an Emulsion . Emulsions are often referred to as Third Phase s in the solvent extraction community. An Organic Solvent composed of 30% Tributyl Phosphate (TBP) in odorless Kerosene (or Hydrogenate d Propylene trimer) is used to recover the uranium and plutonium; the fission products remain in the aqueous nitric phase. Once separated from the Fission Products , further processing allows separation of the heavier plutonium from the uranium. The PUREX extraction process uses a 'solvation' liquid-liquid extraction process in which a complex is formed between the tributyl phosphate and the extracted actinides. The extraction is favoured by a high nitric acid concentration and the back extraction (stripping) is favoured by a low nitric acid concentration. For the plutonium back extraction it is possible to use redox stripping in which the oxidation state of the plutonium is lowered by the action of a reducing agent. THE ORGANIC SOLUBLE COMPLEX The nature of the organic soluble uranium Complex has been the subject of some Research , a series of complexes of uranium with nitrate and trialkyl phosphates and Phosphine oxides have been characterised. ''Reference J.H. Burns, ''Inorganic Chemistry'', 1983, 22, 1174.'' Degradation products of TBP It is normal to extract both the uranium and plutonium from the majority of the Fission Products but it is the case that it is not possible to get an acceptable separation of the fission products from the actinide products with a single extraction cycle. The irradation of the tributyl phosphate / hydrocarbon mixture produces dibutyl hydrogen phosphate. This degradation product is able to act as an extraction agent for many metals, hence leading to the contamination of the product by fission products. Hence it is normal to use more than one extraction cycle, the first cycle is to lower the radioactivity level of the mixture so allowing the later extraction cycles to be kept cleaner (in terms of degradation products). The dialkyl hydrogen phosphates are able to form complexes with many metals, these include some Polymer ic metal complexes. The formation of these Coordination Polymers is one way in which fine solid can be formed in the process. While the cadmium concentration in both the fuel dissolution liquor and the PUREX Raffinate is very low, the polymeric complex of Cadmium of diethyl phosphate is shown as an example. Here is the strucutre of a Lanthanide complex of diethyl phosphate, unlike cadmium the concentration of Neodynium in these mixtures formed from fuel is very high. Here is a mixed tributyl phosphate dibutyl phosphate complex of uranium, because the dibutyl phosphate ligands are acidic it will now be possible to extract uranium by a Ion Exchange liquid-liquid extraction mechanism rather than only by a solvation mechanism. This will potentially make the stripping of uranium with dilute nitric acid less effective. Extraction of technetium In addition the uranium(VI) tributyl phosphate system is able to extract Technetium as pertectinate through an ion pair extraction mechanism. Here is an example of a Rhenium version of a uranium / technetium complex which is thought to be responsible for the extraction of technetium into the organic phase. Here are two pictures of actinyl complexes of Triphenyl Phosphine oxide which have been crystalised with Perrhenate . With the less highly charged neptunyl ion it is also possible to form a complex. ''Reference G.H. John,I. May,M.J. Sarsfield,H.M. Steele,D. Collison,M. Helliwell and J.D. McKinney, ''Dalton Trans.'', 2004, 734.'' LIST OF NUCLEAR REPROCESSING SITES
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