Uranium electrolytic reduction

Excer A process for making uranium tetrafluoride by electrolytic reduction of a uranyl fluoride solution, precipitation of a uranium tetrafluoride hydrate, and ignition of this. 

The first transition metal cation which is unstable in water but which can be generated as a stable entity in HF was U3+ [30]. It was formed by oxidation of the metal by protons in a BF3-HF solution which is non-oxidising and relatively weakly acidic. The UV-vis spectrum of the lilac-colored solution was virtually identical with that observed for an acidified aqueous solution in which the uranium solution was under continuous electrolytic reduction to maintain U(III) as the aquo-cation. 

A wet process is also utilized for the production of uranium(IV) fluoride, namely the EXCER process (Ion Exchange Conversion Electrolytic Reduction). In this process the ion exchange- or extraction-purified uranium(VI) solution is either electrolytically or chemically reduced to uranium(IV), which is precipitated with hydrofluoric acid as uranium(IV) fluoride hydrate (UF4 O.75H2O). This is subsequently dehydrated at 400 to 450°C. 

He, J., Zhang, Q., and Lo, L., "The Separation of Uranium and Plutonium by Electrolytic Reduction in the Purex Process," Paper presented at this Conference (INDE). 

In 1968, an electrolytic reduction process was proposed by A. Schneider and A. L. Ayers (6) to circumvent the above disadvantages. A research program was carried out in the Allied Chemical Corporation s laboratories during the years 1968 to 1972 to develop the process and equipment. The work resulted in the development of the Electropulse Column ( 7) for the continuous (differential) electrolytic uranium-plutonium partition process, which was later scaled up, fabricated, and installed in the Allied-General Nuclear Services reprocessing plant at Barnwell, South Carolina. About the same time, a stagewise electrolytic uranium-plutonium partition process was tested on a mini mixer-settler unit in Germany. (8)... 

Initial experiments were performed to verify and demonstrate the feasibility of the electrolytic reduction method with uranium, followed by experiments with a mixture of uranium and plutonium. Experiments were conducted batchwise in a small electrolytic cell. Basic parameters, such as concentration of solutes and type of holding agents (in the aqueous phase) for removal of any nitrite which would reoxidize the reduced heavy metal, electrode material and geometry, off-gas composition and type of diaphragm, were also determined. These data were valuable in the conceptual design of the first continuously operating column for the electrolytic reduction process. 

The above equation was used for scale-up calculations and design of both the pilot plant and full-scale Electropulse Column. A total of 18 experimental runs for uranium(VI) electrolytic reduction was performed on the 20-cm diameter pilot-scale column. (10) As shown in Figure 4, the predicted reduction efficiency calculated from equation (4) correlated well with the experimental values obtained during these runs. The same good correlation between the predicted and experimental R(u) values was achieved later during cold uranium tests in the full-scale unit (Figure 4). The accuracy of correlation was within the range of 6%. 

The equation provides the means for determining the conditions necessary to obtain a desired uranium concentration in the aqueous effluent stream through the electrolytic reduction process. Any change in variables and parameters included in the equation will change the uranium transfer rate to the aqueous phase. The higher the transfer rate, the higher the reduction efficiency and the content of uranium in the aqueous phase leaving the column (9). This will affect the uranium-plutonium partition with respect to process requirements. 

The Separation of Uranium and Plutonium by Electrolytic Reduction in the Purex Process... 

Hydroxylamine is used for plutonium reduction instead of cathodic reduction as in the Barnwell flow sheet Fig. 10.11, because the plutonium/uranium ratio in this LMFBR fuel is 10 times that in LWR fuel and because electrolytic reduction has not been demonstrated for this high plutonium content. 

The electrolytic reduction of uranium (VI) is by no means a simple process. Kanevskii and Pavlovskaya (253) showed that the concentration of sulphuric acid in the electrolyte is a major factor in determining the disproportionation rate of uranium (V), one of the primary reduction products. These authors found n values equal to 1.0 in electrolytes of low sulphuric acid concentrations and 1.5 at high acid concentrations they concluded that the principal reduction process must involve the step... 

Preparation. Uranium metal may be prepared by several methods the reduction of uranium oxides with carbon In an arc-melting furnace reduction of uranium oxides with magnesium, aluminum, calcium or calcium hydride the reduction of uranium halides with alkali or alkaline-earth metals electrolytic reduction of uranium halides and the themal decomposition of uranium Iodide. 

U(III) species and a second three-electron reduction to give U(0) metal. The first reduction, U(IV)/U(III) couple, is elec-trochemically and chemically irreversible except in hexamethylphosphoramide at 298 K where the authors report full chemical reversibility on the voltammetric timescale. The second reduction process is electrochemically irreversible in all solvents and only in dimethylsulfone at 400 K was an anodic return wave associated with uranium metal stripping noted. Electrodeposition of uranium metal as small dendrites from CS2UCI6 starting material was achieved from molten dimethylsulfone at 400 K with 0.1 M LiCl as supporting electrolyte at a platinum cathode. The deposits of uranium and the absence of U CI3, UCI4, UO2, and UO3 were determined by X-ray diffraction. Faradaic yield was low at 17.8%, but the yield can be increased (55.7%) through use of a mercury pool cathode. 

In the Electropulse Column, plutonium reduction is achieved both through the redox reaction of plutonium(IV) with uranium(IV), produced electrolytically from uranium(VI) in the cathode chamber, and by direct reduction of the plutonium(IV) at the cathode. The basic reactions involved in the electro-lytic method are as follows ... 

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. 

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