Plutonium photochemistry

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,, ]) ... 

Other reasons for investigating plutonium photochemistry in the mid-seventies included the widely known uranyl photochemistry and the similarities of the actinyl species, the exciting possibilities of isotope separation or enrichment, the potential for chemical separation or interference in separation processes for nuclear fuel reprocessing, the possible photoredox effects on plutonium in the environment, and the desire to expand the fundamental knowledge of plutonium chemistry. 

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. 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

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. 

Visible and UV spectrophotometric techniques are most convenient for studying the polymer and various oxidation states of plutonium. The spectra of the plutonium states and the procedure for resolution of the concentrations were previously described (9 ). Changes in the relative concentrations of the oxidation states and of the polymer generally are determined from corresponding changes in the spectra and a comparison of the changes to standard spectra of the various states. These techniques have been used exclusively for studying the photochemistry of aqueous plutonium. 

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. 

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. 

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