Actinide oxide

Uranium(Vl) oxide has been converted almost quantitatively into uranium(VI) fluoride, in a COj flow system, at 1023 K [1498]  

This process was calculated to be thermodynamically more favourable than either of the following competing reactions over the range 200-1200 K [1498]  

Similar conversions were observed in a static system at 180 C [1866a] under similar conditions, ThOj gave ThF in ca. 80% yield [1866a]. The presence of a small amount of CsF promotes the quantitative formation of UOjFj [1309a]. 

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Oxides. Owing to the importance as nuclear fuel material, actinide oxides have been intensively investigated. These are very compHcated compounds because of the formation of non stoichiometric or polymorphic materials. Actinide oxides are very heat-resistant and Th02 is the highest... 

Reliable data on the thermodynamic and phase relationships of actinide oxide systems are essential for reactor safety analysis. This paper reviews certain aspects of thermodynamic data currently available on the nonstoichiometric Pu-0 system, which may serve as a basis for use in reactor safety analysis. Emphasis is placed on phase relationships, vaporization behavior, oxygen-potential measurements, and evaluation of pertinent thermodynamic quantities. 

There is a scarcity of oxygen-transport data for oxygen-deficient actinide oxide systems. Because of this, our understanding and predictive capabilities of the effect of the defect solid state on the properties of reactor fuel systems, as well as on the chemical state of fission products in these systems, are limited. 

Actin, role in heart excitation and contraction coupling, 5 81 Actinide carbides, 4 689 Actinide carbonate, 25 430-431 Actinide-gallium compounds, 22 355 Actinide oxides, 24 761 Actinide peroxides, 28 410 Actinides, 23 569. See also Actinides and transactinides Actinide series absorption and fluorescence spectra, 2 490... 

Figure 5.9. Lanthanide and actinide chemical properties. A scheme is shown of the oxidation states they present in their various classes of compounds. A rough indication of a greater frequency and a higher relative stability of each state is given by the darker blackening of each box. Notice the overwhelming presence of oxidation state 3, in the lanthanides and heavy actinides, oxidation state 2 in Eu andYb and of several higher oxidation states in U and nearby elements.

Newton, T. W. "The Kinetics of Actinide Oxidation Reduction Reactions" of Erda Critical Review Series TID - 26506, 1975. 

B. Metallothermic Reduction of Actinide Oxides Followed by Distillation... 

The metallothermic reduction of an oxide is a useful preparative method for an actinide metal when macro quantities of the actinide are available. A mixture of the actinide oxide and reductant metal is heated in vacuum at a temperature which allows rapid vaporization of the actinide metal, leaving behind an oxide of the reductant metal and the excess reductant metal, in accord with the following equations ... 

The reductant metal must have the following properties (1) the free energy of formation of the oxide of the reductant has to be more negative than that of the actinide oxide and (2) the vapor pressure of the reductant metal needs to be smaller by several orders of magnitude than that of the actinide metal. This difference in vapor pressure should be at least five orders of magnitude to keep the contamination level of the co-evaporated reductant metal in the product actinide metal below the 10 ppm level. 

Actinide metals with lower vapor pressures (Th, Pa, and U) cannot be obtained by this method since no reductant metal exists which has a sufficiently low vapor pressure and a sufficiently negative free energy of formation of its oxide. For the large-scale production of U, Np, and Pu metals, the calciothermic reduction of the actinide oxide (Section II,A) followed by electrorefining of the metal product is preferred (24). In this process the oxide powder and solid calcium metal are vigorously stirred in a CaCl2 flux which dissolves the by-product CaO. Stirring is necessary to keep the reactants in intimate contact. 

Methods have been developed (75) to prepare actinide metals directly from actinide oxides or oxycompounds by electrolysis in molten salts (e.g., LiCl/KCl eutectic). Indeed, the purest U, Np, and Pu metals have been obtained (19, 24) by oxidation of the less pure metal into a molten salt and reduction to purer metal (electrorefining. Section III,D). 

In Chap. C, the thermodynamic and structural outlook of the bond, which had been the matter of discussion in Part A of this chapter, is further developed, and the model formalism, which takes advantage of the well known Friedel s model for d-transition metals but is inspired by the results of refined band calculations, is presented for metals and compounds. Also, a hint is given of the problems which are related to the nonstoichiometry of actinide oxides, such as clustering of defects. Actinide oxides present an almost purely ionic picture nevertheless, covalency is present in considerable extent, and is important for the defect structure. 

The choice of the starting compound of the actinide (oxide or carbide) and of the reductant is determined by the vapour pressure of the actinide metal. For a given temperature, the vapour pressures of the actinide metals (Fig. 2) span a ratio of more than 10 . As the vapour pressure of La is similar to those of Ac, Cm, Pu, only the more volatile... 

The compounds with elements of the platinum group can be prepared by direct synthesis, which requires the availabiUty of actinide metals. Such intermetallics can, however, also be obtained by coupled reduction of actinide oxides with hydrogen in the presence of finely divided noble metals ... 

The only thoroughly studied grossly overstoichiometric actinide oxide is UO2+X. 

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