Uranium vapor pressure, high temperature

Uranium Isotope Separation. The successful industrial separation of uranium isotopes results from (a) UFg having a substantial vapor pressure at temperatures below 100°C, and (b) fluorine being monoiso-topic. However UFg is a toxic, highly corrosive gas which decomposes on contact with water. As a result its use, on an industrial scale, gives rise to plant problems in selecting materials for construction and containment. 

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. 

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

Uranium hexafluoride is probably the most interesting of the uranium fluorides. Under ordinary conditions, it is a dense, white solid with a vapor pressure of about 120 hull ai room temperature. It can readily be sublimed or distilled, and it is by far the most volatile uranium compound known. Despite its high molecular weight, gaseous UFg is almost a perfect gas, and many of the properties of the vapor can be predicted from kinetic theory. 

Since the vapor pressure is generally high at higher temperature and vice versa, a transport process would naturally be considered to be caused by the evaporation taking place at the higher temperature side w ith condensation at the lower temperature zones. For example, in UOj+j, the vapor pressures of uranium oxide gases rise with temperature, as seen in Fig. 28, which would induce transport by evaporation-condensation. In such cases, as mentioned already, the composition of solid and gas phases are not necessarily the same, and the compositions of the solid in both high and low temperature ends varies as the process proceeds. 

While other materials have been used as feed to uranium-enrichment processes, the most widely used volatile compound of uranium is the hexafluoride. At room temperature, UFe is a colorless solid with a density of 5.1 g/cm. It sublimes at atmospheric pressure, and at room temperature has a vapor pressure of 100 torr. The main disadvantage of working with UFe is its high chemical reactivity. It reacts vigorously with water, but is not very reactive with dry air. UF5 reacts with most metals however, nickel, copper, and aluminum are resistant. This holds only for pure UFg the presence of even small amounts of HF increases the rate of attack on even the resistant metals. 

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