Development of new methods for the reduction of UF6

Methods for the preparation of UF₅ are discussed with respect to the formation of β-UF₅. The reduction of UF₆ by HBr in liquid HF /1/ can be used to synthesize pure β-UF₅ even if greater amounts are required.Details of the direct photolysis of UF₆ without scavenger /2/ are presented. In an advanced version a simplified photo-reactor is used which consists of a stainless steel vessel with a diameter of 100 mm and a volume of about 3 liters. UV-light of a 1000 W super high pressure mercury lamp is used to photolyze about 50 g of high purity UF₆ within 12 h, giving pure β-UF₅ in about 90 % yield without intermediately removing the F₂ formed.Two simple new methods have been developed to synthesize β-UF₅. UF₆ is reduced by H₂ in liquid anhydrous HF at room temperature. This reaction which is hindered kinetically at room temperature can be catalyzed by metallic Au which is applied as a foil in the stirred reaction mixture.In addition, it was found that anhydrous HCl catalyzes the reaction, too. 200 mbar of HCl were added, together with 4 bar H₂, to UF₆ in liquid HF, and the reaction mixture was magnetically stirred for 66 hours. β-UF₅ could be obtained in 91 % yield as a very pure product. This latter method is recommended for large scale production of β-UF₅.Reactions of UF₆ with other reducing agents like HCl, SO₂, and CO in liquid HF were studied. These reactions give poor yields or impure products.UF₆ yields with CO and Au in the presence of HF a carbonyl compound of Au with the very high ν_CO at 2205 cm^?1. Analytical and spectroscopic data suggest the formula Au(CO)₂U₂F₁₁.     

Laser induced dielectric breakdown studies of the reaction UF6 + H2

Laser induced dielectric breakdown (LIDB) has been documented in UF₆ at pressures ranging from 8–100 torr. A high power, line selectable TEA CO₂ laser has been used as the source to induce the dielectric breakdown (DB). Reactions of the fragmented UF₆ with H₂ have been studied at various pressure ratios. In all cases a rapid and large pressure drop and heavy deposits of suspended particulates were observed and attributed to the LIDB driven reaction 2UF₆ + H₂ → 2UF₆ + H₂ → 2UF₅ + 2HF.     

Analyse des composes fluores corrosifs par chromatographie gazeuse : Controle du fluor libre

A purely chromatographie and sufficiently general method is described for the analysis of molecular fluorine in permanent inert gases or in other corrosive fluorine compounds (UF₆, HF). The method is also applied to the analysis of permanent inert gases (even in trace quantities) in the same fluorine compounds. In this method, one or several chemical “precolumns” are placed in series with a gas-liquid chromatography column. The “precolumn” system retains or destroys the corrosive compounds (UF₆, HF, F₂) and the chromatography column separates the components formed.     

Beiträge zur Chemie der Uranfluoride und -oxidfluoride. II. Darstellung und Schwingungsspektren von α- und β-Uranpentafluorid

On the Chemistry of Uranium Fluorides and Oxide Fluorides. II. Preparation and Vibrational Spectra of α- and β-Uranium Pentafluoride By reaction of a saturated solution of UF₆ in anhydrous HF with HBr β-UF₅ was prepared in a simple manner. β-UF₅ was changed into α-UF₅ by heating in the presence of UF₆. The IR and RAMAN spectra of the dimorphs are reported and discussed.     

UF6 plasma chemistry reconstitution experiments using high-temperature fluorine

Prior results indicate techniques have been developed for fluid mechanical confinement of high-temperature uranium hexafluoride (UF₆) plasma for long test times while simultaneously minimizing uranium compound deposition on the walls. Follow-on investigations were conducted to demonstrate a UF₆/argon injection, separation, and reconstitution system for use with rf-heated uranium plasma confinement experiments applicable to UF₆ plasma core reactors. A static fluorine batch-type regeneration test reactor and a flowing preheated fluorine/UF₆ regeneration system were developed for converting all the nonvolatile uranium compound exhaust products back to pure UF₆ using a single reactant. Pure fluorine preheat temperatures up to 1000 K resulted in on-line regeneration efficiencies up to about 90%; static batch-type experiments resulted in 100% regeneration efficiencies but required significantly longer residence times. A custom-built, ruggedized time-of-flight (T.O.F.) mass spectrometer, sampling, and data acquisition system permitted on-line quantitative measurements of the UF₆ concentrations down to 30 ppm at various sections of the exhaust system; this system proved operational after long-time exposure to corrosive UF₆ and other uranium halides.     

Surface reactivity of uranium hexafluoride (UF6)

The present article reviews a selection of results obtained in the AREVA/CNRS/UCA joint research laboratory. It focuses on interfaces formed by uranium hexafluoride (UF₆) with chemical filter (purification), carbon (UF₆ storage), and metallic substrate (corrosion). As a matter of fact, along the nuclear fuel cycle, metallic surfaces of the fluorination reactors, cooling systems (for the liquefaction of UF₆), and storage containers are in contact with UF₆, either in the gas or in the liquid phase. For the removal of volatile impurities before the enrichment, surface of chemical filters with a high specific surface area must be enhanced for both selectivity and efficiency. To store depleted UF₆ (²³⁸U), graphite intercalation compounds are proposed and preliminary results are presented.     

Composes d'addition entre les pentafluorures d'uranium et d'antimoine,preparation des composes UF5. n SbF5 (n = 1 ou2)

The adduct UF₅.2SbF₅ has been obtained from the reaction of UF₅ with SbF₅ and the reaction of UF₆ with SbF₅ in the presence of freon 114. From this preparation and also from studies on UF₆, SbF₅ solutions, the fluorinating properties of UF₆ were found to be enhanced by the presence of SBF₅. An x-ray diffraction study has shown that crystals of UF₅.2SbF₅ are monoclinic, space group P2₁/c, with unit cell dimensions a = 8.036(3) Å ; b = 14.112(13) Å ; c = 10.028(17) Å ; β = 96.91(7)°.The adduct UF₅.SbF₅ is obtained by thermal decomposition of UF₅.2SbF₅.     

[UF6]2−: A Molecular Hexafluorido Actinide(IV) Complex with Compensating Spin and Orbital Magnetic Moments

The first structurally characterized hexafluorido complex of a tetravalent actinide ion, the [UF₆]^2? anion, is reported in the (NEt₄)₂[UF₆]?2 H₂O salt ( 1 ). The weak magnetic response of 1 results from both U^IV spin and orbital contributions, as established by combining X‐ray magnetic circular dichroism (XMCD) spectroscopy and bulk magnetization measurements. The spin and orbital moments are virtually identical in magnitude, but opposite in sign, resulting in an almost perfect cancellation, which is corroborated by ab initio calculations. This work constitutes the first experimental demonstration of a seemingly non‐magnetic molecular actinide complex carrying sizable spin and orbital magnetic moments.     

Compatibility tests of low vapour pressure perfluoropolyether oils with UF6

An extensive study has been carried out in order to evaluate the possible use of the perfluoropolyether fluids, obtained from HFP photooxidation and specially purified and selected, as lubricants or vacuum working fluids, in presence of UF₆.Some physicochemical properties are reported and the results of the tests experienced on perfluoropolyether cuts put into contact with UF₆, in different conditions for long aging time at high temperature in presence of some typical engineering materials.The results, shown in this work, allowed a verification of the long term compatibility of the perfluoropolyether special fractions against UF₆ at temperatures up to 130°C.These results still offer a contribution in order to solve some technological problems on UF₆ processing.     

The reaction of uranium hexafluoride with silver fluoride in anhydrous hydrogen fluoride and the chemical properties of the product Ag2UF8

UF₆ reacts with AgF dissolved in anhydrous hydrogen fluoride to precipitate Ag₂UF₈. Ag₂UF₈ has some unexpected properties: On reaction with water it produces O₂ and reduced uranium. No adequate explanation could be found of why UF₆ and AgF combined in this manner should produce a powerful oxidant. Raman spectra and chemical properties of the solid products are given.     

SF6-sensitized dissociation of UF6 with a pulsed CO2 laser

The dissociation of UF₆ sensitized by SF₆ excited with a pulsed CO₂ laser in the presence of H₂ and CO as scavengers has been investigated. In the SF₆-UF₆-H₂ system the dissociation yields have been determined as a function of the laser frequency, the fluence, and H₂ partial pressure. A maximum dissociation yield has been found at a laser frequency of 935 cm^?1. No obvious dissociation of UF₆ was observed in the UF₆-SF₆ system without F-atom scavengers.     

Sensitized multiphoton dissociation of UF6 IN SF6-UF6 mixtures by a tea CO2 laser

UF₆ undergoes decomposition in the presence of SF₆ when mixtures of both are irradiated with a TEA CO₂ laser. The mechanism for UF₆ decomposition may involve vibrational energy transfer from excited SF₆ and laser absorption from the same laser pulse by excited UF₆ in its vibrational quasi-continuum     

Long‐range π‐type hydrogen bond in dimers CH2?HF,CH2O?H2O,and CH2O?NH3

Using four basis bets, (6‐311G(d,p), 6‐31+G(d,p), 6‐31++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for the dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. In contrast with the above three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD (T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Correlated one‐electron wave functions

Using four basis sets, 6‐311G(d,p), 6‐31+G(d,p), 6‐311++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the acidic H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. By contrast with above the three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen‐bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is a noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD(T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Near-UV photodissociation of gaseous UF6 in the presence of H2

Mixtures of UF₆ and H₂ in different ratios have been irradiated at 360 and 400 nm by means of a filtered mercury lamp. A significant pressure drop has been observed at both excitation wavelengths due to the dissociation of UF₆ into UF₅+ F. A very high dissociation quantum yield has been found.     

NOUF6 Revisited: A Comprehensive Study of a Hexafluoridouranate(V) Salt

We have synthesized NOUF₆ by direct reaction of NO with UF₆ in anhydrous HF (aHF). Based on the unit cell volume and powder diffraction data, the compound was previously reported to be isotypic to O₂PtF₆, however, detailed structural data, such as the atom positions and all information that can be derived from those, were unavailable. We have therefore investigated the compound by using single‐crystal and powder X‐ray diffraction, IR, Raman, NMR, EPR, and photoluminescence spectroscopy, magnetic measurements, as well as chemical analysis, density determination, and quantum chemical calculations.     

Phase analysis of the solidified KF–(LiF–NaF–UF4)–ZrF4 molten electrolytes for the electrowinning of uranium

The X-ray diffraction (XRD) phase analysis of different solidified uranium-based fluoride systems ((LiF–NaF)_eut–UF₄; (KF–LiF–NaF)_eut–UF₄; (LiF–NaF)_eut–UF₄–ZrF₄ and (KF–LiF–NaF)_eut–UF₄–ZrF₄) were examined in order to provide the basis for pyro-electrochemical extraction of uranium in molten fluorides. Several uranium-based species (Na₂UF₆, Na₃UF₇, K₂UF₆, K₃UF₇, UO₂, K₃UO₂F₅) were identified in the solidified melts. The role of oxygen in argon atmosphere was found to be critical in the formation of uranium species during the melting and solidification. In order to reduce the accumulated level of free oxygen traces in our experiments, zirconium (in the form of ZrF₄) was used inside the melt as an oxygen buffer. It was found that ZrF₄ can really stabilize the uranium species by complexation and protects them against the oxygenation. The results of this work highlight the importance of oxygen removal for obtaining pure deposit in the electrorefinning of uranium.     

Interaction potentials for UF6 with itself and with rare-gas atoms

Potential curves have been calculated for the UF₆-UF₆ and UF₆-rare gas atom interactions using statistical models for the UF₆ charge density and the intermolecular interaction. Agreement with available experimental data is good.     

Uranium (V) fluorides

Rhenium and uranium hexafluorides oxidise iodine in iodine pentafluoride at ambient temperature to give the I₂⁺ cation. With UF₆ additional reaction occurs to give β-uranium pentafluoride as one product (J.A. Berry, A. Prescott, D.W.A. Sharp, and J.M. Winfield, $\text{J. Fluorine Chem}$., 1977, $\text{10}$, 247). Further work on the latter reaction together with an electronic spectroscopic study of the oxidation of I₂ by phosphorus pentafluoride in IF₅, suggests that the fate of the I₂⁺ cation depends on the nature and quantity of the oxidising agent. Oxidation of I₂ by PF₅ can be conveniently followed by monitoring its visible spectrum. The reaction occurs over several hours and eventually an apparent equilibrium between I₂ and I₂⁺ results. Formation of I₂⁺UF₆^?is rapid and, with the mole ratio UF₆:I₂ > 10:1, UF₅ is precipitated rapidly from solution, I₂⁺ being oxidised further, apparently to IF₅. With a smaller UF₆:I₂ mole ratio UF₅ is contaminated by I₂, the latter is presumed to result from the disproportion-ation of an I^I or I^III fluoride.β-UF₅ is very soluble in acetonitrile and reacts with thallium(I) fluoride in this solvent to give Tl^IUF₆. It reacts with trimethyl(methoxo)silane to give (CH₃)₃SiF, U(OCH₃)₅, and an insoluble solid, believed to be a mixture of U^V methoxide, fluorides. Both reactions are conveniently followed by near i.r. spectroscopy.     

The fluorination of uranium and vanadium oxides with some metal fluorides

UO₃ reacts with CrF₃ or CuF₂ forming UO₂F₂ and Cr₂O₃ or CuO respectively. Further fluorination occurs above 800°C to form UF₆ though the presence of excess CrF₃ gives mainly UF₄. The fluorination of U₃O₈ with CrF₃ gave UO₂F₂, UF₄ and Cr₂O₃ but with CuF₂ gave UO₂F₂, CuO and Cu₂O. VO₂ reacts with excess CrF₃ forming VF₃, VF₅ and Cr₂O₃. If there is a deficiency of CrF₃ the products are VOF₃, V₃O₅ and Cr₂O₃. CuF₂ and VO₂ form VOF₃, CuO and Cu₂O.