As one of the mostvolatile compounds of uranium, uranium hexafluoride is relatively convenient to process and is used in both of the mainuranium enrichment methods, namelygaseous diffusion and thegas centrifuge method. Since thetriple point ofUF6, which is 64 °C (147 °F; 337 K) and 152 kPa (22 psi; 1.5 atm),[13] is close to ambient conditions, phase transitions can be achieved with littlethermodynamic work.
Fluorine has only a single naturally occurring stable isotope, soisotopologues ofUF6 differ in their molecular weight based solely on the uraniumisotope present.[14] This difference is the basis for the physical separation of isotopes in enrichment.
All the other uranium fluorides are nonvolatile solids that arecoordination polymers.
The conversion factor for the238U isotopologue ofUF6 ("hex") to "U mass" is 0.676.[15]
Gaseous diffusion requires about 60 times as much energy as the gas centrifuge process: gaseous diffusion-produced nuclear fuel produces 25 times more energy than is used in the diffusion process, while centrifuge-produced fuel produces 1,500 times more energy than is used in the centrifuge process.
In addition to its use in enrichment, uranium hexafluoride has been used in an advanced reprocessing method (fluoride volatility), which was developed in theCzech Republic. In this process,spent nuclear fuel is treated with fluorine gas to transform the oxides or elemental metals into a mixture of fluorides. This mixture is then distilled to separate the different classes of material. Somefission products form nonvolatile fluorides which remain as solids and can then either be prepared for storage as nuclear waste or further processed either bysolvation-based methods orelectrochemically.
Uranium enrichment produces large quantities ofdepleted uranium hexafluoride (DUF6 or D-UF6) as a waste product. The long-term storage of D-UF6 presents environmental, health, and safety risks because of its chemical instability. WhenUF6 is exposed to moist air, it reacts with the water in the air to produceUO2F2 (uranyl fluoride) and HF (hydrogen fluoride) both of which are highly corrosive and toxic. In 2005, about 686,000 tonnes of D-UF6 was housed in 57,122 storage cylinders located nearPortsmouth, Ohio;Oak Ridge, Tennessee; andPaducah, Kentucky.[16][17] Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades.[18]
There have been several accidents involving uranium hexafluoride in the US, including a cylinder-filling accident and material release at theSequoyah Fuels Corporation in 1986 where an estimated 29,500 pounds of gaseousUF6 escaped.[19][20] The US government has been converting DUF6 to soliduranium oxides for disposal.[21] Such disposal of the entire DUF6 stockpile could cost anywhere from $15 million to $450 million.[22]
Ruptured 14-tonUF6 shipping cylinder. 1 fatality, dozens injured. ~29500 lb of material released. Sequoyah Fuels Corporation 1986.
DUF6 storage yard from afar
DUF6 cylinders: painted (left) and corroded (right)
^J. H. Levy; John C. Taylor; Paul W. Wilson (1976). "Structure of Fluorides. Part XII. Single-Crystal Neutron Diffraction Study of Uranium Hexafluoride at 293 K".J. Chem. Soc., Dalton Trans. (3):219–224.doi:10.1039/DT9760000219.
^J. H. Levy, J. C. Taylor and A. B. Waugh (1983). "Neutron Powder Structural Studies of UF6, MoF6 and WF6 at 77 K".Journal of Fluorine Chemistry.23:29–36.doi:10.1016/S0022-1139(00)81276-2.
^J. C. Taylor, P. W. Wilson, J. W. Kelly: „The structures of fluorides. I. Deviations from ideal symmetry in the structure of crystalline UF6: a neutron diffraction analysis",Acta Crystallogr.,1973,B29, p. 7–12;doi:10.1107/S0567740873001895.
^G. H. Olah; J. Welch (1978). "Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions".J. Am. Chem. Soc.100 (17):5396–5402.doi:10.1021/ja00485a024.
^J. A. Berry; R. T. Poole; A. Prescott; D. W. A. Sharp; J. M. Winfield (1976). "The oxidising and fluoride ion acceptor properties of uranium hexafluoride in acetonitrile".J. Chem. Soc., Dalton Trans. (3):272–274.doi:10.1039/DT9760000272.
^S. M. Walker; P. S. Halasyamani; S. Allen; D. O'Hare (1999). "From Molecules to Frameworks: Variable Dimensionality in the UO2(CH3COO)2·2H2O/HF(aq)/Piperazine System. Syntheses, Structures, and Characterization of Zero-Dimensional (C4N2H12)UO2F4·3H2O, One-Dimensional (C4N2H12)2U2F12·H2O, Two-Dimensional (C4N2H12)2(U2O4F5)4·11H2O, and Three-Dimensional (C4N2H12)U2O4F6".J. Am. Chem. Soc.121 (45):10513–10521.doi:10.1021/ja992145f.
Gmelin, Leopold, ed. (1955) [1817].Gmelins Handbuch der anorganischen Chemie (in German). Vol. System Nr. 55, Uran, Teil A. Weinheim: Verlag Chemie, Gmelin-Institut für Anorganische. pp. 121–123.
Gmelin, Leopold (ed.).Gmelins Handbuch der anorganischen Chemie (in German). Vol. System Nr. 55, Uran, Teil C. Weinheim: Verlag Chemie, Gmelin-Institut für Anorganische. pp. 71–163.
Grenthe, Ingmar; Drożdżynński, Janusz; Fujino, Takeo; Buck, Edgar C.; Albrecht-Schmitt, Thomas E.; Wolf, Stephen F. (2006)."Uranium"(PDF). In Lester R. Morss; Norman M. Edelstein; Jean Fuger (eds.).The Chemistry of the Actinide and Transactinide Elements. Dordrecht: Springer. pp. 253–698.doi:10.1007/1-4020-3598-5_5.ISBN1-4020-3555-1. Archived fromthe original(PDF) on 2012-01-18.
US 2535572, "Preparation of UF6", issued 1950-12-26
US 5723837, "Uranium Hexafluoride Purification", issued 1998-03-03
