Uranium photochemical reduction

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

Although the redox potentials for aqueous solution indicate that uranium(IV) should reduce plutonium(IV), anions and other complexing agents can change the potentials sufficiently that uranium(IV) and plutonium(IV) can coexist in solution (25). Since one of the products of photochemical reduction of uranyl by TBP is dibutyl phosphate (DBP), which complexes plutonium(IV) strongly, experiments were done to test the photochemically produced urani-um(IV) solutions as plutonium(IV) reductants (26). Bench-scale stationary tests showed these solutions to be equivalent to hydroxylamine nitrate solutions stabilized with hydrazine (27). 

The photochemical reduction of a solution containing both uranium(VI) and plutonium(IV) is also of interest for reprocessing applications. Early experiments (12a) showed a significant reduction of plutonium(IV) by light in Purex-type process solutions. Since the quantum yield for plutonium redox reactions is about one-tenth that for uranyl reduction (7b,c) the most likely path of plutonium(IV) reduction in these experiments appears to have been by uranium(IV) or uranium(V) generated by photochemical reduction of uranyl by other components of the solutions. Further experiments in this area would be useful. 

In the following procedures, /3-uranium pentafluoride is conveniently prepared by the photochemical reduction of uranium hexafluoride, in a manner similar to an earlier smaller-scale preparation.9 Pentaethoxyuranium is prepared directly from 0-UFs and sodium ethoxide in ethanol. The preparation of hexafluoro-uranium salts from /3-UFs in nonaqueous solvents is described in a procedure that avoids the use of hydrofluoric acid common to previous methods.10,11... 

The uranium(v) ion, U02, is extraordinarily unstable towards disproportionation and has a transitory existence under most conditions, although evidence for its occurrence can be obtained polarographically. It is also an intermediate in photochemical reductions of uranyl ions in presence of sucrose and similar substances. The ion is most stable in the pH range 2.0-4.0 where the disproportionation reaction to give U4+ and U02+ is negligibly slow. By contrast, reduction of U02+ in dimethyl sulfoxide gives U02 in concentrations sufficiently high to allow the spectrum to be obtained and disproportionation occurs with a half-life of about an hour.42 As noted above, Uv can be stabilized in HF solutions as UF, as well as in concentrated Cl- and C03 solutions.42... 

Uranium (IV) can also be reduced to U(III) photochemically. Irradiation of a solution of UCI4 in methanol at 248 nm gave up to 85% reduction with a quantum yield of 0.17. The U(III) produced could be stabilized as its (18-crown-6) complex (24a). 

Goldstein, M Barker, J. J. Gangwer, T. A Photochemical Technique for Reduction of Uranium and Subsequently Plutonium in the Purex Process , BNL-22443 (1976). 

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