Actinide metals electrorefining

Actinide metals with lower vapor pressures (Th, Pa, and U) cannot be obtained by this method since no reductant metal exists which has a sufficiently low vapor pressure and a sufficiently negative free energy of formation of its oxide. For the large-scale production of U, Np, and Pu metals, the calciothermic reduction of the actinide oxide (Section II,A) followed by electrorefining of the metal product is preferred (24). In this process the oxide powder and solid calcium metal are vigorously stirred in a CaCl2 flux which dissolves the by-product CaO. Stirring is necessary to keep the reactants in intimate contact. 

Methods have been developed (75) to prepare actinide metals directly from actinide oxides or oxycompounds by electrolysis in molten salts (e.g., LiCl/KCl eutectic). Indeed, the purest U, Np, and Pu metals have been obtained (19, 24) by oxidation of the less pure metal into a molten salt and reduction to purer metal (electrorefining. Section III,D). 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Preparation Methods. Actinide metal preparation is based on methods known or developed to yield high purity material by metallothermic reduction or thermal dissociation of prepurified compounds. Electrolytic reduction is possible from molten salts, but not from aqueous solutions. Further purification of the metals can be achieved by electrorefining, selective evaporation or chemical vapour transport. 

When the actinide products are removed from electrorefiners, the electrorefiner salts cover the metal. The cathodes are processed to distill adhering salt and to consolidate the actinide metals. These salts are recycled to the electrorefiner for further use (Westphal and Mariani, 2000 Westphal et al., 2002). In the case of the liquid cadmium cathode, the cadmium is... 

Electrorefining was envisaged by ANL in the late 1990s for the treatment of U-Al fuels [10]. However, the potential of U and A1 are too close to each other in molten chloride to reach a separation by this method. Some experimental studies on the An extraction with an immiscible and distillable metal, carried out by Atomic Energy of Canada in 1956, showed that Pu and Am, dissolved in liquid Al, can be transferred into liquid Bi after contact at 800 C [11], Bi was then successfully evaporated under vacuum at 750 C. Two major drawbacks were noticed (i) the volatility of Am that contaminates the Bi condensate and (ii) the solubility of Al in Bi which contaminates the final actinide metallic product. 

Figure 3 shows a flowsheet for plutonium processing at Rocky Flats. Impure plutonium metal is sent through a molten salt extraction (MSE) process to remove americium. The purified plutonium metal is sent to the foundry. Plutonium metal that does not meet foundry requirements is processed further, either through an aqueous or electrorefining process. The waste chloride salt from MSE is dissolved then the actinides are precipitated with carbonate and redissolved in 7f1 HN03 and finally, the plutonium is recovered by an anion exchange process. 

Also with the same Institute, metal fuel recycling is being researched using electrorefining techniques. We expect to have a first sample of actinide electrodeposited by the end of 1995 ... 

Recycling of refractory metals from wastes is an important issue today since they are in relatively low amount in the earth s crust and it should really be a substantial economy of expensive raw materials and also of energy. Molten salt electrolysis is proved to be appropriate to this function which can be assimilated in the case of metallic wastes to electrorefining. Today, waste treatment of other transition metals like actinides or lanthanides is a reality in the nuclear field, while other strategic elements such as silicon are expected to be recovered with a molten salt technology. 

Figure 6.12.1 Representation of the principle of electrorefining. MAs minor actinides REs rare earths AMs alkaline metals AEMs alkaline earth metals...

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