Plutonium oxidation-reduction reactions

T. W. Newton, The Kinetics of the Oxidation Reduction Reactions of Uranium, Neptunium, Plutonium, andMmericium inMqueous Solution, TlD-26506, U.S. Energy, Research, and Development Administration (ERDA) Technical Information Center, Washington, D.C., 1975. 

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

A primary goal of chemical separation processes in the nuclear industry is to recover actinide isotopes contained in mixtures of fission products. To separate the actinide cations, advantage can be taken of their general chemical properties [18]. The different oxidation states of the actinide ions lead to ions of charges from +1 (e.g., NpOj) to +4 (e.g., Pu" " ) (see Fig. 12.1), which allows the design of processes based on oxidation reduction reactions. In the Purex process, for example, uranium is separated from plutonium by reducing extractable Pu(IV) to nonextractable Pu(III). Under these conditions, U(VI) (as U02 ) and also U(IV) (as if present, remain in the... 

F. P.L. Boxall, C. Oxidation-reduction reactions of simple hydroxamic acids and plutonium(IV) ions in nitric acid, Radiochim. Acta 96 (2008) 333-343. 

Newton, T. W and F. B. Baker Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium. In if. F. Gould (Ed.), Lanthanide/Actinide Chemistry, Advances in Chemistry Series, Vol. 71, p. 268. Washington American Chemical Society 1967. 

Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium... 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

Plutonium can exist simultaneously in four oxidation states in acidic, aqueous solutions. All four oxidation state species [Pu(III), Pu(IV), Pu(V), and Pu(VI)] can be present in solution to give thermodynamically stable systems (17). At environmental acidities, Pu(III) is probably unstable to oxidation (20) thus, the three higher oxidation states are more likely to E encountered, with Pu(IV) being the most stable (17). Because of the relatively slow oxidation-reduction reaction (Hue to the Pu-0 bond) occurring between Pu4+ and Pu022+, these two species should be the easiest to assay for by radiochemical methods. The presence of Pu02+... 

A typical oxidation-reduction reaction of the type met in processing plutonium is the reduction of Pu to Pu by ferrous ion ... 

Plutonium found in soils may undergo the same oxidation/reduction reactions described for surface waters in places where soil contacts water. In addition to oxidation/reduction reactions, plutonium can react with other ions in soil to form complexes. These complexes may then be absorbed by roots and move within plants however, the relative uptake by plants is low. In plants, the complex can be degraded but the elemental plutonium will remain. 

R. E. Connick, "Oxidation States, Potentials, Eqxailibria, and Oxidation-Reduction Reactions of Plutonium."in The Actinide Elements, Natl. Nucl. Energy Series, Div. 

TABLE rV-6. Oxidation-Reduction Reactions of Plutonium Ions... 

Newton, T. W. (1975) The Kinetics of Oxidation-Reduction Reactions of Uramum, Neptunium, Plutonium and Americium in Aqueous Solution, TID-26506. 

Cerium(IV) is a strong oxidising agent and, as such, it has a number of chemical applications as an oxidant. In studying the hydrolysis of cerium(IV), a number of studies have utilised oxidation-reduction reactions in determining the relevant stability constants. As a tetravalent cation with a relatively small ionic radius, the hydrolysis of cerium(IV) occurs at very low pH. Consequently, data have been provided that show that the metal ion is extensively hydrolysed even at a pH of around 0 (i.e. Imoll" H" ) (Baes and Mesmer, 1976). Cerium(IV) is used as an analogue for plutonium(IV) in nuclear fuel manufacturing studies. 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

When the Plutonium Project was established early in 1942, for the purpose of producing plutonium via the nuclear chain reaction in uranium in sufficient quantities for its use as a nuclear explosive, we were given the challenge of developing a chemical method for separating and isolating it from the uranium and fission products. We had already conceived the principle of the oxidation-reduction cycle, which became the basis for such a separations process. This principle applied to any process involving the use of a substance which carried plutonium in one of its oxidation states but not in another. By use of this... 

The use of this direct oxide reduction process is replacing fluoride reduction as it eliminates neutron exposure to operating personnel (alpha particles from plutonium decay have sufficient energy to eject neutrons from fluorine by the a,n reaction) and eliminates reduction residues which require subsequent recovery. 

Direct Oxide Reduction. In DOR, plutonia is reduced with calcium metal to form plutonium metal and calcium oxide.2 3 The reaction takes place in a CaCl2 solvent which dissolves the calcium oxide and allows the plutonium metal to coalesce in the bottom of the crucible. 

Calcinm glectrowinning in piutniiium production. A potential application of inorganic membranes in radioactive waste treatment is in the industrially practiced direct oxide reduction process. In this process plutonium oxide is calciothermit y reduced to plutonium in the presence of calcium chloride according to the following reaction ... 

The fuel is dissolved during reduction of the mixed oxides by calcium. This oxide reduction operation is done in the presence of a CaCl2-CaF2 salt and a Cu-Mg alloy. The FP-2 elements and the CaO reaction product are taken up by the salt and the reduced uranium, plutonium, FP-3, and FP-4 elements are taken up by the alloy. Uranium is present in excess of its solubility limit and precipitates as the UCu5 intermetallic compound. 

Both plutonium trifluoride and tetrafluoride can be prepared by hydro-fluorination of plutonium dioxide. The oxidation-reduction relations of plutonium (III) and (IV) are such that it is readily possible to specify which of the fluorides shall be the product. If the reaction is carried out under reducing conditions, then the product is PuF3, and under oxidizing conditions, PuF4 ... 

The chemical transformation reactions plutonium undergoes in the environment are primarily oxidation and reduction reactions. There are five oxidation states found in the environment. These are plutonium(lll), plutonium(IV), plutonium(V), plutonium(VI), and plutonium(VII). The last, plutonium(VII), is not commonly found and it is only found under very alkaline conditions. The dominant oxidation state of plutonium in the environment is plutonium(IV) (Wildung et al. 1987). 

Hydrazine is also used in conjunction with HNA to impede hydroxylamine oxidation by nitrous acid, always present in nitric acid media, which increases the HNA availability for the plutonium reduction. Both hydrazine and HNA interact with nitrous acid (HNO2), but the hydrazine oxidation is much faster. On the other hand, the use of HAN in conjunction with nitric acid introduces the possibility of an autocatalytic reaction resulting in the overpressurization of the system or explosion in a reprocessing facility as pointed out in Barney s report (Barney, 1998). The main function of HNA is to reduce plutonium from the tetravalent state to the trivalent state and, thus, separate the plutonium from the uranium, which is retained in the hexavalent oxidation state and, hence, in the organic phase. The reduction reaction by HNA is the result of two irreversible reactions (Equations 14.10 and 14.11) ... 

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