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PUREX is an acronym standing for Plutonium - URanium EXtraction — 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 ion-exchange. For other methods of reprocessing, see nuclear reprocessing.

Overview

The spent nuclear fuel to which this process is applied consists primarily of certain very high atomic-weight (actinoid or "actinide") elements (e.g., uranium) along with smaller amounts of material composed of lighter atoms, notably the so-called fission products.

The actinoid elements in this case consist primarily of the largely unconsumed remains of the original fuel (typically U-238 and other isotopes of uranium). In addition there are smaller quantities of other actinoids, created when one isotope is transmuted into another by a reaction involving neutron capture. Plutonium-239 is the leading example. Another term sometimes seen in relation to this secondary material (and other material produced similarly) is activation products.

In response to the PUREX process' ability to extract nuclear weapons materials from the spent fuel, trade in the relevant chemicals is monitored.

In brief, the PUREX process is a liquid-liquid extraction ion-exchange method used to reprocess spent nuclear fuel, in order to extract primarily uranium and plutonium, independent of each other, from the other constituents.

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. The unavoidable irradiation (by the material being processed) 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 in the left image. The right one is the structure of a lanthanide complex of diethyl phosphate. Unlike cadmium the concentration of neodymium in these mixtures formed from fuel is very high.

Image:cadmiumdobadz.png|This complex is formed from cadmium ions and diethyl phosphate ionsImage:neodyniumdobadz.png|This complex is formed from neodymium ions and diethyl phosphate

Below 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.
This complex is formed from uranyl ions, two nitrates, two dibutyl phosphates and two molecules of tributyl phosphate


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.

Image:uraniumtccomplex2.jpg|This complex is formed from a uranyl ion and three molecules of triphenylphosphine oxide. The anions are in the first coordination sphere of the metalImage:uraniumtccomplex.jpg|This complex is formed from a neptunyl ion and four molecules of triphenylphosphine oxide. The anions are separated from the metal centre

Pollution

The PUREX Plant at the Hanford Sitemarker was responsible for producing 'copious volumes of liquid wastes', resulting in the radioactive contamination of groundwater.

List of nuclear reprocessing sites



See also



References & Notes



Further reading

  • OECD Nuclear Energy Agency, The Economics of the Nuclear Fuel Cycle, Paris, 1994
  • I. Hensing and W Schultz, Economic Comparison of Nuclear Fuel Cycle Options, Energiewirtschaftlichen Instituts, Cologne, 1995.
  • Cogema, Reprocessing-Recycling: the Industrial Stakes, presentation to the Konrad-Adenauer-Stiftung, Bonn, 9 May 1995.
  • OECD Nuclear Energy Agency, Plutonium Fuel: An Assessment, Paris, 1989.
  • National Research Council, "Nuclear Wastes: Technologies for Separation and Transmutation", National Academy Press, Washington D.C. 1996.


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