PUREX is the "
de facto" standard aqueous nuclear reprocessing method for the recovery of uraniumand plutoniumfrom used nuclear fuel. It is based on liquid-liquid extraction ion-exchange. For other methods of reprocessing, see nuclear reprocessing.
This process can be used to recover weapon-grade materials as well as
reprocessed uraniumfrom spent nuclear reactorfuel, and as such, its component chemicals are monitored. PUREX is an acronym standing for Plutonium - URanium EXtraction. The PUREX process is a liquid-liquid extraction ion-exchangemethod 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.
organic solventcomposed of 30% tributyl phosphate(TBP) in odorless kerosene(or hydrogenated propylenetrimer) 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 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 polymersis 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 raffinateis very low, the polymeric complex of cadmiumof diethyl phosphate is shown as an example.
Here is the structure of a
lanthanidecomplex of diethyl phosphate. Unlike cadmium the concentration of neodymiumin 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 an
ion exchangeliquid-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
technetiumas pertechnetatethrough an ion pair extraction mechanism. Here is an example of a rheniumversion 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 oxidewhich 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
COGEMA La Hague site
Thorp nuclear fuel reprocessing plantand B205at Sellafield, note that at Sellafield ( Windscale) that a series of other older plants were used in the past.
West Valley Reprocessing Plant
Savannah River Site
Idaho Chemical Processing Plant
* Radiochemical Engineering Development Center,
Nuclear fuel cycle
Nuclear breeder reactor
Spent nuclear fuel shipping cask
Global Nuclear Energy Partnershipannounced February, 2006
*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.
* [http://www.world-nuclear.org/info/inf69.html Processing of Used Nuclear Fuel] , World Nuclear Association
* [http://npc.sarov.ru/english/digest/62002/appendix7.html Reactor-Grade Plutonium and Development of Nuclear Weapons] , Analytical Center for Non-proliferation
* [http://www.euronuclear.org/info/encyclopedia/p/purex-process.htm PUREX Process, European Nuclear Society]
* [http://www.world-nuclear.org/info/inf29.html Mixed Oxide Fuel (MOX)] - World Nuclear Association
* [http://www.globalsecurity.org/wmd/library/report/crs/97-564.htm Disposal Options for Surplus Weapons-Usable Plutonium] - Congressional Research Service Report for Congress
* [http://mfnl.xjtu.edu.cn/gov-doe-ornl/RDF/history/03_FuelReprocessinHistory.html Brief History of Fuel Repreocessing]
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