UF6 URANIUM FLUORIDE

The following reactions can be considered for the preparation of uranium fluorides (UF4 or UF6) starting from urania (U02) at a temperature of 500 °C ... 

One of the most important examples of the fluorination of oxides is the fluorination of uranium dioxide. Uranium tetrafluoride (UF4) is the intermediate compound which is reduced to uranium metal. The gaseous higher fluoride, uranium hexafluoride (UF6) is used for the separation of uranium isotopes to obtain enriched uranium (i.e., uranium containing a higher proportion of the isotope, U235, than natural uranium). 

Enriched UF6 is processed into U02 powder at fuel fabrication facilities using one of several methods. In one process uranium hexafluoride is vaporized and then absorbed by water to produce uranyl fluoride, U02F2, solution. Ammonium hydroxide is added to this solution and ammonium diuranate is precipitated. Ammonium diuranate is dried, reduced, and milled to make uranium dioxide powder. The powder is pressed into fuel pellets for nuclear reactors. 

Tungsten(VI) fluoride (WF6) and molybdenum(VI) fluoride (MoF6) are available commercially, and can be made by reaction of the metals with fluorine.4 In the case of uranium(VI) fluoride (UF6), a preparation that is claimed5 to be feasible in the laboratory uses uranium metal and chlorine trifluoride uranium(VI) fluoride is prepared6 commercially by the fluorination of uranium(IV) fluoride, itself prepared from an oxide and hydrogen fluoride. 

The enrichment procedure uses the small mass difference between the hexafluorides of uranium-235 and uranium-238 to separate them. The first procedure to be developed converts the uranium into uranium hexafluoride, UF6, which can be vaporized readily. The different effusion rates of the two isotopic fluorides are then used to separate them. From Graham s law of effusion (rate of effusion = l/(molar mass)172 Section 4.11), the rates of effusion of 235UF6 (molar mass, 349.0 g-mol ) and 238UF6 (molar mass, 352.1 g-mol 1) should be in the ratio... 

In spite of its toxicity and extreme reactivity, fluorine is widely used for the manufacture of polymers such as Teflon, (C2F4) . Fluorine is also important in the production of UF6, used in the separation of uranium isotopes for nuclear power plants, and fluoride ion is added to toothpaste in the form of NaF to help prevent tooth decay. 

F Chemical Shifts of the UFnCl(6-n) Series. Uranium hexafluoride, UF6, is perhaps the most studied and best characterized actinide molecule. The 19F NMR chemical shift (taken relative to CFCI3) of this molecule is presented in Table HI for the scalar Pauli and all-electron spin-orbit ZORA approaches. In this table, we have also collected results for the whole series of the related uranium (VI) fluoride chlorides, UFnCl(6-n), n =1 - 6. The 19F NMR chemical shifts of all of these molecules have been measured in CFCI3 solution (39). The solvent has also been used as internal standard. All mixed UFnCl(6-n) species have been characterized virtually definitely in these experiments (39). 

Other important inorganic compounds of fluorine include uranium hexafluoride, UF6, a volatile substance that was used to separate 238U from the fissionable 235U by gaseous diffusion in the Manhattan Project. Sodium fluoride, NaF, is often added to drinking water and to toothpaste in order to reduce the incidence of tooth decay. 

Uranium pentafluoride [13775-07-0], UF5, has been isolated under different conditions, leading to two different modifications, OC and p. The former is a grayish white solid, which is synthesized from the interaction of UF6 [7783-81-5] and HBr or by heating UF4 [10049-14-6] and UF6 to 80—100°C. The yellowish white -modification is also obtained by reacting UF4 and UF6, but at higher temperatures (150—200°C). The two different modifications of UF have both been structurally characterized. The a-form consists of infinite chains of octahedral UF6 units. The p-form has eight-coordinate uranium atoms with the fluorides in a geometry between dodecahedral and square antiprismatic. 

Conversion to Uranium Hexafluoride. The calcined U03 is reduced to U02 with hydrogen, then converted first to UF4 with hydrogen fluoride and finally to UF6 using fluorine. These steps usually are carried out in fluidized-bed reactors, but the conversion to UF4 sometimes is done in a stirred trough reactor.34... 

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