Photochemical plutonium

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

The authoritative documents on plutonium 0 >2) do not include photo-chemical reactions of plutonium in aqueous systems. The first papers in Western world literature on studies that were dedicated to aqueous plutonium photochemistry appeared in 1976 (3, 4 ), even though photochemical changes in oxidation states were indicated as early as 1952 (5,, ]) ... 

Burger and coworkers (5) in 1952 reported that some distribution coefficients for Pu022+ in organic-aqueous systems at lighted conditions were different from those observed for dark conditions, and those authors believed that some Pu022+ had been photochemically reduced. That reduction was confirmed by others (6) in 1965, and in 1969 a report suggested that most aqueous plutonium reactions were affected by light (7 ). 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

The possible application of aqueous plutonium photochemistry to nuclear fuel reprocessing probably has been the best-received justification for investigating this subject. The necessary controls of and changes in Pu oxidation states could possibly be improved by plutonium photochemical reactions that were comparable to the uranyl photochemistry. 

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. 

After observing the photochemical reduction of Pu(VI) and Pu(IV), it seems obvious that reaction (3) should be light-sensitive. However, it is not obvious how photons would affect the equilibrium concentrations of the plutonium species. The experimental results [3,4] are very interesting and are described below, but a complete explanation is yet to be developed. 

Later experiments (4 ) were designed to determine a cell e.m.f. for the plutonium disproportionation system with a particular light source. Concentration quotients for the light and dark conditions, Qg and Qj, were determined, and an energy difference of 1.65 kcal (32 mV) was calculated by the relation -RTln C /Qd This reversible photochemical shift may be the only single-element system known at this time and certainly is the simplest such system. Even though the radioactive properties could prevent development and utilization of a plutonium photoconversion system, these studies certainly suggest that similar nonradioactive and more acceptable systems could be discovered and developed. 

In view of the results obtained by Bell and Friedman (136) it is possible that sunlight could bring about a photochemical solubilisation of Pu(IV) polymers. These workers obtained 13 % degradation of polymeric plutonium in the presence of organic materials in one hour when the polymer (0.0093m Pu in 0.47m HC104) was irradiated at 260—280 nm. This degradation rate was four times faster than that observed over a similar period of time in the absence of the UV source. 

Phosphoric Acid Phosphorus Photoallergens Photochemical Oxidants Phthalate Ester Plasticizers Physical Hazards Picloram Picric Acid Piperazine Piperonyl Butoxide "Plants, Poisonous" Platinum (Pt) Plutonium (Pu) Poinsettia Poisoning Emergencies in Humans Pokeweed Pollutant Release and Transfer Registries (PRTRs) Pollution Prevention Act "Pollution, Air" "Pollution, Air Indoor" "Pollution, Soil" "Pollution, Water" Polybrominated Biphenyls (PBBs) Polybrominated Diphenyl Ethers (PBDEs) Polychlorinated Biphenyls (PCBs)... 

The first attention given to actinide photochemistry was for the purpose of identifying any photochemical activity which might alter the efficiency of the extraction or exchange processes. Subsequently, the identification of photochemically active species of uranium and plutonium gave some indication that the photoreactions could be turned to a useful end and, perhaps, offer a cleaner way to separate actinides from each other and from the other elements accompanying them in nuclear fuel elements. 

Other reactions observed for plutonium have been (1) the direct photochemical reduction of Pu(IV) using reductants such as formic acid, ethanol and hydrazine (8,9,10) in perchloric acid ... 

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

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

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. 

Toth, L. M., Friedman, H. A., and Bell, J. T., "Photochemical Separation of Actinides in the Purex Process," Paper presented at the Plutonium Fuel Cycle Mtg., Bal Harbour, FL., 1977. 

The photochemical effects on four aqueous plutonium perchlorate solutions have been reported. The photochemical reductions of PuOl to PUO2 and of Pu to Pu were observed. A reversible photochemical shift in the equilibrium of the Pu disproportionation was also recorded and the equilibrium coefficient found to increase by a factor of three when the sample was irradiated. Light was found to increase the rate and extent of depolymerization of plutonium(iv) polymer. The preparation of the new plutonium-palladium phase Pu3Pds has been reported and the structure found to resemble that of Ga5Zr3. 

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