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PUREX is the de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid-liquid extraction. For other methods of reprocessing, see nuclear reprocessing.
Additional recommended knowledge
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 Plutonium and Uranium Recovery by EXtraction. 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 phases in the solvent extraction community.
An organic solvent composed of 30% tributyl phosphate (TBP) in odorless kerosene (or hydrogenated 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.
Degradation products of TBP
It is normal to extract both the uranium and plutonium from the majority of the fission products, but it is not possible to get an acceptable separation of the fission products from the actinide products with a single extraction cycle. Irradiation 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 lowers the radioactivity of the mixture, allowing the later extraction cycles to be kept cleaner in terms of degradation products.
Dialkyl hydrogen phosphates are able to form complexes with many metals. These include some polymeric metal complexes. Formation of these coordination polymers is one way in which fine solids 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 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 an 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 pertechnetate 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 triphenylphosphine oxide which have been crystallised 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
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "PUREX". A list of authors is available in Wikipedia.|