Plutonium oxides thermodynamics

Thermodynamic Functions of the Gases. To apply Eqs. (1-10), the free energies of formation, Ag , for all gaseous species as a function of temperature are required. Tabulated data were fit by a least-squares procedure to derive an analytical equation for AG° of each vapor species. For the plutonium oxide vapor species, the data calculated from spectroscopic data (3 ) were used for 0(g) and 02(g), the JANAF data (.5) were used and for Pu(g), data from the compilation of Oetting et al. (6) were used. The coefficients of the equations for AG° of the gaseous species are included in Table I. 

The uncertainties in the condensed-phase thermodynamic functions arise from (1) the possible existence of a solid-solid phase transition in the temperature range 2160 to 2370 K and (2) the uncertainty in the estimated value of the liquid heat capacity which is on the order of 40%. While these uncertainties affect the partial pressures of plutonium oxides by a factor of 10 at 4000 K, they are not limiting because, at that temperature, the total pressure is due essentially entirely to O2 and 0. 

II. International Atomic Energy Agency The Plutonium-Oxygen and Uranium-Plutonium-Oxygen Systems A Thermochemical Assessment, Report of a Panel on Thermodynamic Properties of Plutonium Oxides, Vienna, Oct. 1966, Tech. Rept. Series No. 79, Vieima, 1967. 

Chikalla et al. (85), and Gardner et al. (86), whose results were reviewed in the IAEA Panel on the Thermodynamics of Plutonium Oxides at Vienna in 1966 (55). There still remain some ambiguities, however, regarding the stable region of temperature and composition of the PuOi.gi and PuOj 52 phases (83a). 

Markin TL, Rand MH (1966) Thermodynamic data for plutonium oxides. In Thermodynamics Proceedings of Symposium 1965, vol 1. Intemational Atomic Energy Agency, Vieima, pp 144-156... 

Markin, T. L. Mclver E. J. (1965). Thermodynamic and phase studies for plutonium and uranium -plutonium oxides with application to compatibility calculations, 3 International Conference on Plutonium 1965, p.p. 845-857... 

Sorensen, O. T. (1976). Thermodynamic Studies at Higher Temperatures of the Phase Relationships of Substoichiometric Plutonium and Uranium/Plutonium Oxides, Plutonium and Other Actinides 1975, pp.123-131... 

C.W. (1958) Thermodynamics and phase relationships for plutonium oxides, in Proceedings (f the United Nations International Conference on the Peaceful Uses of Atomic Energy, vol, 6, United Nations,... 

Local association of the reduced cation and the oxygen vacancy is clearly suggested by the thermodynamics of the hypo-stoichiometric mixed oxides (Ui yPUy)02 x, where the thermodynamic functions do not depend on x and y separately, but rather on a quantity, called plutonium valence , which contains the ratio x/ y73,87) Q sters consisting of this association have been proposed in order to explain the thermodynamics of actinide hypostoichiometric dioxides. 

Electrodeposition could be a pseudo-S-L type process, although definitive proof is lacking. The element to be ionized, uranium [16] or plutonium [17], is coelec-trodeposited with a platinum metal layer, then covered with an additional layer of platinum. The U or Pu is believed to be electrodeposited as an oxide, and platinum is electrodeposited as the metal. Hence there is thought to be a U or Pu oxide buried in the metal matrix. When this deposit is heated, after a sufficient length of time atomic cations of U or Pu begin to sublime from the surface without measurable metal oxide ions. Metal oxide ions should be readily observable if they are present in the matrix. Thermodynamic calculations indicate that the hot platinum matrix will not reduce the U and Pu oxides to the metallic state, and yet the observed species are atomic ions and not oxide molecular ions. 

Bromocomplexes of plutonium are limited in number tri- and tetravalent complexes with organic cations, namely C(C6 5)3P l3 PuBrg(c) (70) and [(C2H5)4N]2 PuBr5(c) (71) have been reported. Also, adducts of the type PuBr. 2L (with L = hexamethylphosphora-mide and tri phenyl phosphine oxide) are known (72.). No thermodynamic information are available on these species. More recently, however, the compound Cs2PuBr5(c) has been prepared (63) and its enthalpy of solution in 1 M HCl was measured as - 75.3 + 0.21 kJ. mol l. This measurement leads to a value of AHS(Cs2PuBr, c) =... 

Americium. The low solubilities and high sorption affinity of thorium and americium severely limit their mobility under environmental conditions. However, because each exists in a single oxidation state—Th(IV) and Am(III)— under environmentally relevant conditions, they are relatively easy to study. In addition, their chemical behaviors provide valuable information about the thermodynamic properties of trivalent and tetravalent species of uranium, neptunium, and plutonium. 

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+... 

Plutonium metal is prepared by calcium reduction of plutonium fluorides or oxides in induction-heated MgO crucibles, under an inert atmosphere of helium or argon. The thermodynamics of plutonium reduction are discussed later in this chapter. 

The thermodynamically favorable reduction of PuOj with calcium has the disadvantage that the CaO coproduct is not molten, so that the resulting plutonium metal and umeacted calcium metal remain finely dispersed throughout the slag. However, the dispersed plutonium can be recovered as a massive metal by preferentially extracting the calcium oxide and unreacted calcium with molten calcium chloride at a temperature above the melting points of plutonium and calcium, leaving consolidated plutonium metal with yield efficiencies in excess of 99.9 percent [W1 ]. 

The quite similar chemical properties of uranium and the transuranium elements are the reason for their presence in the irradiated fuel as thermodynamically stable double oxides. According to the redox potential of the fuel matrix, it has to be assumed that neptunium and plutonium will appear in their most stable oxidation state +4. Due to their ionic radii, which are very similar to that of U(IV), both NpO and Pu02 will form mixed crystals with UO2. The type and the properties of the crystal lattice remain essentially unchanged only the dimensions of the elementary cell decrease from 0.5468 nm (5.468 A) in pure UO2 to 0.5466 nm (5.466 A) at 1% plutonium content. This means that neptunium and plutonium will remain at the positions in the irradiated fuel where they were formed the same behavior is to be expected for the higher transuranium elements as well. 

The most widely employed method for plutonium reprocessing used today in almost all of the world s reprocessing plants is the Purex (plutonium-uranium reduction extraction) process. Tributylphosphate (TBP) is used as the extraction agent for the separation of plutonium from uranium and fission products. In effecting a separation, advantage is taken of differences in the extractability of the various oxidation states and in the thermodynamics and kinetics of oxidation reduction of uranium, plutonium, and impurities. Various methods are in use for the conversion of plutonium nitrate solution, the final product from fuel reprocessing plants, to the metal. The reduction of plutonium halides with calcium proved to be the best method... 

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