Plutonium vapor pressure

Figure 1. Temperature dependence of plutonium vapor pressure over intermetallics as measured by target collection.

Marcon and Poiteau (595) also have given the equation for plutonium vapor pressure over PuN(s) in the temperature range 1400 to 2000"C. Their data in the lower temperature range are a little higher than those by Kent and Leary [143). Besides these, data on nitrogen decomposition pressures have been reported by several authors (141, 399). 

Thermodynamic. The thermodynamic properties of elemental plutonium have been reviewed (35,40,41,43—46). Thermodynamic properties of sohd and Hquid Pu, and of the transitions between the known phases, are given in Table 5. There are inconsistencies among some of the vapor pressure measurements of Hquid Pu (40,41,43,44). 

A good deal was learned about plutonium metal, including the determination of its density by both capillary displacement and x-ray diffraction methods, its melting point and vapor pressure. 

Vapor pressures of phases in these systems were measured by the Knudsen effusion technique. Use of mass spectrometer-target collection apparatus to perform thermodynamic studies is discussed. The prominent sublimation reactions for these phases below 2000 K was shown to involve formation of elemental plutonium vapor. Thermodynamic properties determined in this study were correlated with corresponding values obtained from theoretical predictions and from previous measurements on analogous intermetallics. 

It should be emphasized that a survey of the vapor pressure measurements of plutonium-bearing species above bivariant Pu02-x(s) revealed that in general these measurements suffer from a lack of knowledge of the composition of the condensed phase. 

Measurements of oxygen potentials and vapor pressure of plutonium-bearing species above a wide range of Pu-0 compositions, including the difficult region close to stoichiometric plutonia, should be made at high temperatures, particularly above 3000°C, for purpose of safety analysis. 

Vapor Pressures and Vapor Compositions in Equilibrium with Hypostoichiometric Plutonium Dioxide at High Temperatures... 

Vapor pressures and vapor compositions in equilibrium with a hypostoichiometric plutonium dioxide condensed phase have been calculated for the temperature range 1500 I H 4000 K. Thermodynamic functions for the condensed phase and for each of the gaseous species were combined with an oxygen-potential model, which we extended from the solid into the liquid region to obtain the partial pressures of O2, 0, Pu, PuO and Pu02 as functions of temperature and of condensed phase composition. The calculated oxygen pressures increase rapidly as stoichiometry is approached. At least part of this increase is a consequence of the exclusion of Pu +... 

One of the most Important thermophysical properties of reactor fuel In reactor safety analysis Is vapor pressure, for which data are needed for temperatures above 3000 K. We have recently completed an analysis of the vapor pressure and vapor composition In equilibrium with the hypostolchiometric uranium dioxide condensed phase (1 ), and we present here a similar analysis for the plutonium/oxygen (Pu/0) system. 

Green, D. W. Fink, J. K. Leibowitz, L. "Vapor Pressures- and Vapor Compositions in Equilibrium with Hypostoichiometric Uranium-Plutonium Dioxide at High Temperatures," presented at the 8th European Conference on Thermophysical Properties, Baden-Baden, September 27 - October 1, 1982 to be published in High Temperatures-High Pressures. 

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

Proceeding from thorium to plutonium along the actinide series, the vapor pressure of the corresponding iodides decreases and the thermal stability of the iodides increases. The melting point of U metal is below 1475 K and for Np and Pu metals it is below 975 K. The thermal stabilities of the iodides of U, Np, and Pu below the melting points of the respective metals are too great to permit the preparation of these metals by the van Arkel-De Boer process. 

With tantalum effusion cells, the results of Ackermann et al. (16) are virtually identical with the earlier work of Phipps et al. (19) (1593-2063 K) and yielded total vapor pressure values of plutonium-bearing species which were considerably higher (by approximately a factor of ten) than those obtained with the less reducing tungsten (as well as Re and Ir) cells. Ackermann et al. (16) correlated the vapor pressure results obtained with tantalum effusion cells with the univariant diphasic system consisting of Pu203(s)-PuOi,51(s) as the condensed phases and PuO(g) as the primary gaseous species. 

Weinstock et al. (87) have made an exceedingly thorough study of the vapor pressure of neptunium and plutonium hexafluorides (Table X). The vapor pressures of NpF6 and PuF6 are compared to the UF6 vapor pressures... 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

The vapor pressure of plutonium has been measured on two occasions with excellent agreement (Phipps et al, 1956 Mulford, 1966a). The more... 

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