Uranium vapor pressure

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

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. 

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. 

The pentafluorides MoFs (yellow), ReFs (green), OsFs (blue), and UFS (pale yellow-green) are extremely moisture sensitive and must be handled and stored in a dry box. The samples can be stored in Kel-F bottles. With the exception of UF5 these compounds have sufficient vapor pressure so that they can be sublimed. Uranium pentafluoride, on the other hand, is a nonvolatile solid at room temperature. The melting points for these compounds are MoFs, 65° ReFs, 47° OsFs, 70°. The infrared spectra (Nujol mull) show the following broad bands MoFs, 740, 693, 653, 520 cm"1 ReFs, 720, 691, 660, 530 cm 1 OsFs, 710, 690, 655, 530 cm 1 UFS, 620, 565, 510, 405 cm"1. More detailed spectroscopic and powder diffraction data have been summarized elsewhere.5 7... 

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. 

A laboratory preparation is the reaction of uranium metal and CIF3.20 The hexafluoride forms colorless crystals (mp 64.1°C) with a vapor pressure of 115 mm Hg at 25°C. It is a powerful fluorinating agent (e.g., it converts CS2 to SF4) and is rapidly hydrolyzed by water. The hexafluoride is also used to make UF4 and UF5 by the following reactions ... 

Uranium(IV) chloride forms dark green octahedral crystals of tetragonal symmetry (m.p. 590°). The vapor pressure of the solid (350 to 505°) is given by the expression... 

Uranium(III) chloride, as obtained by this procedure, is a dark purple, crystalline compound. Other procedures may yield products with varying colors. Uranium(III) chloride has a hexagonal lattice and is isomorphous with cerium(III) chloride and lanthanum bromide. The compound melts at 842° and has a density of 5.51. The vapor pressure (600 to 1000°) is given by the expression... 

B7. Brooks, A. A., and P. Wood Vapor Pressure Tables for Liquid Uranium Hexafluoride, Report K-722, Nov. 21, 1957. 

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. 

The volatility of U(OMe)6 has recently attracted attention for laser-induced uranium isotope separation with a CO2 laser. At 330°C, U(OMe)e has a vapor pressure of IVmTorr and A/f biimation = 96 13 kJmol and AS° b,i ation = 318 17 JK mol . From IR and Raman spectroscopic studies of the O and 0 labell methoxide, a good vibrational analysis has been performed, allowing the assignments of the U— O stretching frequencies 505.0cm" ... 

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. 

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. 

However, if the molecules are cooled, the population of thermally excited vib-rot states falls drastically and the spectrum simplifies. Thus, at 77 K, 69% of UFs is in its lowest vibrational state, and this increases to 85% at 55 K. However, the vapor pressure is untenably low at such temperatures (7 x 10 Pa at 77 K), and equilibrium cooling is out of the question. Still by cooling in a nonequilibrium expansion nozzle, uranium vapor concentration can be kept at a useful level and LIS is possible. None of this changes the fact that molecular LIS was abandoned in favor of AVLIS. 

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. 

Uranium hexafluoride (UF ), also called hex, is probably the best known and most widely investigated compound of uranium mainly because it is the only uranium compound with significant vapor pressure at ambient temperatures and therefore an essential raw material for most commercial isotope enrichment processes. UFg is a white monoclinic crystalline solid that sublimes directly to a gas (reaches atmospheric pressure at 56.5°C), but when heated in a closed vessel will melt at 64.05°C, which is the triple point where the solid, liquid, and gas phases coexist, as shown in Figure 1.8. This is probably one of the most weU-recognized phase diagrams in the chemical literature. 

---