Uranium reduction

Then the fuel elements are dissolved in 7m HNO3 to give a solution containing U and Pu which, in the widely used plutonium-uranium-reduction, or Purex process, are extracted into 20% tributyl phosphate (TBP) in kerosene leaving most of the fission products... 

The only anhydrous trioxide is UO3, a common form of which (y-U03) is obtained by heating U02(N03).6H20 in air at 400°C six other forms are also known.Heating any of these, or indeed any other oxide of uranium, in air at 800-900°C yields U3O8 which contains pentagonal bipyramidal UO7 units and can be used in gravimetric determinations of uranium. Reduction with H2 or H2S leads to a series of intermediate... 

Lovley DR, Roden EE, Phillips EJP, Woodward JC. 1993b. Enzymatic iron and uranium reduction by sulfate-reducing bacteria. Marine Geol 113 41-53. 

Wielinga, B. Bostick, B. Hansel, C.M. Rosenz-weig, R.F. Fendorf S. (2000) Inhibition of bacterially promoted uranium reduction Ferric (hydrjoxides as competitive electron acceptors. Environ. Sd. Tedm. 34 2190-2195... 

The Purex process, ie, plutonium uranium reduction extraction, employs an organic phase consisting of 30 wt % TBP dissolved in a kerosene-type diluent. Purification and separation of U and Pu is achieved because of the extractability of U02+2 and Pu(IV) nitrates by TBP and the relative inextractability of Pu(III) and most fission product nitrates. Plutonium nitrate and U02(N03)2 are extracted into the organic phase by the formation of compounds, eg, Pu(N03)4 -2TBP. The plutonium is reduced to Pu(III) by treatment with ferrous sulfamate, hydrazine, or hydroxylamine and is transferred to the aqueous phase U remains in the organic phase. Further purification is achieved by oxidation of Pu(III) to Pu(IV) and re-extraction with TBP. The plutonium is transferred to an aqueous product. Plutonium recovery from the Purex process is ca 99.9 wt % (128). Decontamination factors are 106 — 10s (97,126,129). A flow sheet of the Purex process is shown in Figure 7. 

Many variants of the Purex (Plutonium Uranium Reduction Extraction) process23S based on TBP extraction have been developed but a basic outline flowsheet is illustrated in Figure 38. This shows the so-called early split flowsheet most commonly used in existing plants. It involves the separation of the uranium and plutonium using two different back-extractant streams during the first solvent extraction cycle. Additional solvent extraction cycles are then carried out independently on the uranium and plutonium streams to effect further purification. An alternative arrangement is the iate split flowsheet used at the Cap La Hague plant in France, and the... 

In the second-generation reprocessing, the applied separation technology has been the PUREX process, an acronym of Plutonium Uranium Reduction Extraction (4) based on a liquid-liquid extraction with tri-n-butyl phosphate (TBP) in //-paraffin diluent, which selectively recovers Pu and U on an industrial scale. 

The control of the actinide metal ion valence state plays a pivotal role in the separation and purification of uranium and plutonium during the processing of spent nuclear fuel. Most commercial plants use the plutonium-uranium reduction extraction process (PUREX) [58], wherein spent fuel rods are initially dissolved in nitric acid. The dissolved U and Pu are subsequently extracted from the nitric solution into a non-aqueous phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent such as dodecane or odourless kerosene (OK). The organic phase is then subjected to solvent extraction techniques to partition the U from the Pu, the extractability of the ions into the TBP/OK phase being strongly dependent upon the valence state of the actinide in question. 

Lovley D. R. and Phillips E. J. P. (1992a) Bioremediation of uranium contamination with enzymatic uranium reduction. Enviroru Sci. Technol. 26, 2228—2234. 

Suzuki and Banfield (1999) classify methods of microbial uranium accumulation as either metabolism dependent or metabolism independent. The former consists of precipitation or complexa-tion with metabohcally produced ligands, processes induced by active cellular pumping of metals, or enzyme-mediated changes in redox state. Examples include precipitation of uranyl phosphates due the activity of enzymes such as phosphatases, formation of chelating agents in response to metal stress, and precipitation of uraninite through enzymatic uranium reduction. 

Resultant data have shown that within the experimental range tested the uranium reduction efficiency R(u) > expressed as... 

With respect to the resultant data from uranium runs on both Electropulse Columns, the following correlation was derived for the uranium reduction efficiency ... 

Verification of the validity of equation (4) was made during subsequent uranium runs on the 2.54-cm diameter Elec-tropulse Column. The calculated uranium reduction efficiency for each run [from equation (4)] was compared with its experimental value. In all cases, the predicted value of the reduction efficiency correlated well with the corresponding experimental value, as shown in Figure 3. 

The 2.54-cm diameter Electropulse Column shown in Figure 1, after completion of uranium runs, was installed at Battelle Memorial Institute (Columbus, Ohio) for uranium-plutonium partition tests. Six electrolytic runs were made under conditions corresponding to partitioning in the first process cycle to determine the effect of uranium reduction efficiency R(u) on t le separation process. The organic feed contained 80 to 83 grams/L of uranium and 0.71 to 0.82 grams/L of plutonium. The nitric acid concentration in the aqueous feed was 2.5 to 2.8 M and in the organic feed 0.2 to 0.3 M. 

As expected, the higher uranium reduction efficiency resulted in a higher percentage of plutonium transferred to the aqueous phase (Figure 5). At a uranium reduction efficiency higher than about 2.4%, over 99% of the plutonium in the organic feed is transferred to the aqueous phase, which indicates an efficient partition process. 

The equation indicates high flexibility for the uranium reduction process control. For example, in a given Electro-pulse Column operating under constant flow conditions and at a given uranium concentration in the organic feed, the reduction efficiency can be affected by either acidity of feed streams, current input, or pulse velocity. Any combination of these variables is also possible. 

With ferrous ion or cathodic reduction, conversion of plutonium from Pu to Pu is so rapid that back extraction of plutonium to the aqueous phase and reduction there to Pu can be carried out simultaneously in a single multistage contactor. With tetravalent uranium, reduction of plutonium is slower, so that additional contactor volume is desirable to complete back extraction. With hydroxylamine, reduction of plutonium is so much slower that it is preferable first to return both uranium and plutonium to the aqueous phase by stripping with dilute nitric acid and then to reduce the plutonium in equipment providing sufficient residence time for reduction to proceed to completion. Finally, the uranium is reextracted by TBP. 

Payne RB, Gentry DA, Rapp-Giles BJ, Casalot L and Wall JD (2002) Uranium reduction by Desulfovibrio desulfuricans strain G20 and a cytochrome c3 mutant. Appl Environ Microbiol 68 3129-3132. 

Duff MC, Hunter DB, Bertsch PM, Amrhein C (1999c) Factors influencing uranium reduction and solubility in evaporation pond sediments. Biogeochemistiy 45 94-114 Duff MC, Hunter DB, Triay ., Bertsch PM, Reed DT, Sutton SR, Shea-McCaithy G, Kitten J, Eng P, Chipera SJ, Vaniman DT (1999a) Mineral associations and average oxidation states of soibed Pu on tuff. Environ Sci Technol 33 2163-2169... 

The carbothermic process provides metallic uranium reduction not only from dioxide (UO2) but also from a much more convenient and accessible oxide, UsOs (Wilhelm ... 

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