Gaseous UF Research Articles

Classification of gaseous UF6 assay by femtosecond LIBS in the 424.4 nm spectral region using numerical HOGSVD-DTW features

This technical note presents experimental results using numerical features of fs-LIBS data to classify the assay value of a gaseous UF6 material.

Feasibility Study for an Active 238UF6 Gas Target for Photo-Fission Experiments

Abstract A series of fission experiments in the actinide region has been started at the superconducting Darmstadt linear accelerator S-DALINAC. For detailed investigations on, e.g., the energy dependence of fission modes, the population of fission isomers, or even the search for parity non-conservation effects (PNC) in the photon-induced fission process of 238 U, high luminosities are needed. Increasing target thickness reduces mass and angular resolutions. One possible solution is the utilization of an active gas target containing UF 6 . In order to test UF6 as an admixture to standard counting gases (e.g. argon) and to study its properties, an ionization chamber has been built at Technische Universitat Darmstadt. After testing the chamber with pure argon as a counting gas to evaluate signal quality and to determine the drift velocity, gaseous UF 6 was filled into the chamber in steps of one mass-percent uranium for each measurement, where both signal quality and drift velocity at different admixtures have been determined. Up to two percent of uranium in the counting gas one finds that the drift velocity increases with UF 6 content, while overall a good signal quality and energy resolution of the ionization chamber is preserved.

ChemInform Abstract: Redetermination of the Thermochemistry of Gaseous UF5, UF2, and UF.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A Relativistic DFT study of the structure and vibrational frequencies of the gaseous UF 4

We have compared the performance of the widely used generalized gradient approximation (GGA) functions for calculating the bond lengths, vibrational frequencies, total energies and population analysis of four geometric symmetries( T d , C 3v , C 2v and D 2d ) of gaseous UF 4 molecule. To describe the uranium and fluorine atoms we employed all-electron scalar relativistic double-numeric-polarized (DNP) basis sets. The calculated vibrational frequencies are in good agreement with the experimental and referenced data. Calculations show that energy of T d symmetry is the lowest among the four geometric symmetries, therefore, the stability of T d symmetry is the best among those four symmetries in respect to the calculated values.

Uranium Isotope Exchange between Gaseous UF6 and Solid UF5

Uranium Isotope Exchange between Gaseous UF₆ and Solid UF₅

The infrared spectrum and molecular structure of gaseous UF 4

The infrared spectrum of gaseous UF 4 was measured in the range of 75–700 cm −1. Absorptions found at 537 and 114 cm −1 were the asymmetric stretching frequency v 3 and the deformation frequency v 4 respectively. In addition, a re-analysis of previously published gas electron diffraction data has been made, testing four geometric models for the UF 4 molecule ( T d, C 3v, C 2v and D 2d symmetry). The spectroscopic and electron diffraction data are compatible with a tetrahedral symmetry for the UF 4 molecule. The bond distance r α (U-F) is 201.7(5) pm. By means of joint analysis of the electron diffraction and spectroscopic data, the symmetric stretching frequency was estimated to be v 2 = 123 cm −1. Coriolis constants were calculated using the Δv PR value of the v 3 band. The molecular parameters of tetrahedral UF 4 were used for the calculation of the entropy S°(1050 K, UF 4, g) = 492.8 ± 3.0 J mol −1 K −1.

The infrared spectrum and molecular structure of gaseous UF 4

The infrared spectrum of gaseous UF 4 was measured in the range of 75–700 cm −1. Absorptions found at 537 and 114 cm −1 were the asymmetric stretching frequency v 3 and the deformation frequency v 4 respectively. In addition, a re-analysis of previously published gas electron diffraction data has been made, testing four geometric models for the UF 4 molecule ( T d , C 3 v , C 2 v , and D 2 d symmetry). The spectroscopic and electron diffraction data are compatible with a tetrahedral symmetry for the UF 4 molecule. The bond distance r α (U-F) is 201.7(5) pm. By means of joint analysis of the electron diffraction and spectroscopic data, the symmetric stretching frequency was estimated to be v 2 = 123 cm −1. Coriolis constants were calculated using the Δv PR value of the v 3 band. The molecular parameters of tetrahedral UF 4 were used for the calculation of the entropy S°(1050 K, UF 4, g) = 492.8 ± 3.0 J mol −1 K −1.

Redetermination of the thermochemistry of gaseous UF5, UF2, and UF

Because of new studies questioning an earlier determination of the thermochemistry of UF5(g), we have reexamined the gaseous equilibrium Ag+UF5=AgF+UF4 by mass spectrometry over a broader temperature range, and have obtained more definitive results, leading to Δf H0298 (UF5,g)=−1929±10 kJ mol−1. Included was an independent thermochemical study of AgF which yielded Δf H0298 (AgF,g)=7.5±6 kJ mol−1 and D00 (AgF)=3.66±0.06 eV. Additionally, second-law measurements of the gaseous reaction U+UF2=2UF corroborated earlier results showing that D(U–F)>D(FU–F), while the reverse holds in the U–Cl and U–Br systems. A revised set of bond dissociation energies and enthalpies of formation for the gaseous UFn species is presented, consistent with all key thermochemical values. This new information is discussed in terms of other results in the literature.

Development of an enrichment measurement technique and its application to enrichment verification of gaseous UF 6

A technique has been developed to verify the enrichment of gaseous UF 6 inside cylinders. This technique combines an X-ray fluorescent measurement of the total amount of gaseous uranium with a measure of the 185.7 keV gamma ray from 235U using two collimators to obtain an enrichment that is independent of the pressure of the gaseous uranium and independent of the deposit that forms on a surface in contact with UF 6. This technique has the required sensitivity to determine whether the process gas is of uranium enrichment ≤ 20% or >20%.

The intercalation of uranium hexafluoride into graphite

Intercalation of UF 6 into graphite, both from the gaseous phase and from the Ledon 113 solution, was studied. The amount of intercalated UF 6 from the gaseous phase was found to be inversely proportional to the size of graphite particles. Intercalation increases with the increasing temperature and surface area of graphite. The contact of gaseous UF 6 with graphite led to the formation of β-UF 5 that is not intercalated, In the Ledon solution β-UF 5 is not formed. ‘Passivation’ of graphite by elementary fluorine also prevents the formation of β-UF 5 but the amount of intercalated UF 6 decreases. The intercalation of UF 6 into graphite from the gaseous phase is accompanied by the increase of the distance between the parallel carbon atom layers up to the values of about 884 pm. Ternary intercalates graphite -UF 6-Ledon 113 are formed during the intercalation of UF 6 from the Ledon 113 solutions and the distance between the parallel carbon atom layers 848–875 pm. Thermogravimetry in the presence of air revealed that the binary intercalates graphite -UF 6 decompose in a 3-step reaction while the ternary intercalates decompose in a 4-step reaction, In both cases uranium hexafluoride is not released but acts as a fluorination agent on the graphite carbon.

X-ray fluorescent determination of uranium in the gaseous phase

The pressure of uranium in gaseous UF 6 inside a cylinder can be measured using X-ray fluorescence. A highly collimated source, highly collimated detector, and a very rigid geometry are required. This measurement technique is independent of the deposit that forms on the inside wall of the cylinder due to chemical reactions of UF 6 gas with impurities. The technique is applicable for gaseous UF 6 at low pressures where other techniques fail. The deyectability limit, assuming a 25 mCi 57Co fluorescing source, is less than 1 Torr gas pressure in a 30 min count time.

Extremely low temperature behaviour of the thermodynamic properties of gaseous UF 6 under an exact quantum approach

The thermodynamic functions of molecules of type XF 6 are calculated under an exact quantummechanical approach, which also yields general expressions valid for other types of molecules. The formalism is used to analyze the behaviour of gaseous UF 6 at very low temperatures (around and below 1°K), where symmetry effects caused by the Pauli principle lead to results which are very markedly different from those obtained from a semi-classical approximation. It is shown that this approximation becomes sufficiently accurate only for temperatures about ten times the rotational temperature.

Gas temperature and density of UF_6 determined by two-wavelength UV absorption

The new technique of Wampler and Gentry for determining the temperature and density of a gas by measuring UV gas absorption at two wavelengths [F. B. Wampler and R. A. Gentry, J. Appl. Phys. 52, 1583 1981)] has been applied to gaseous UF(6). At 266 nm the absorption cross section sigma of UF(6) appears to be constant, sigma = (1.15 +/- 0.01) 10(-18) cm(2) from 0-100 degrees C. The absorption cross section at 245 nm over the same temperature range may be represented with the empirical polynomial sigma = [1.37 +/- 0.05 + (9.7 +/- 1.5) 10(-3). T - (4.2 +/- 1.1)10(-5). T(2)] 10(-18) cm(2), where T is in degrees Celsius. Differences in (dsigma/dT)lambda at 266 and 245 nm allow both UF(6) temperature and density to be determined by UV absorption measurements. The strengths and limitations of this technique are discussed.

Laser study of the photophysical and photochemical properties of gaseous UF6

Abstract In view of possible laser isotope separation schemes of uranium, the photophysical and photochemical properties of gaseous UF 6 have been the subject of experimental and theoretical investigations in the last few years. In particular the use of laser light to exite the fluorescence emission of the UF 6 molecules has provided a fruitful tool for the study of the dynamics and structure of the excited electronic states. The fluorescence emission from UF 6 excited in the visible-UV (A-X band: 340–410 nm) exhibits different time behaviour and quantum yield as a function of excitation wavelength. These experimental data can be explained in terms of a model involving two excited electronic states. In addition due to the very low quantum yield ( −4 ) a very efficient quenching mechanism must be present. This mechanism may arise from the following processes: (i) Deactivation of the excited molecule through the internal degrees of freedom of either the same molecule or through collisions with other molecules. (ii) Dissociation process. We may note, of course, that only process (ii), if followed by a chemical or physical removal of the photoproducts, would be useful for a laser isotope separation scheme. New experimental data are reported in order to elucidate the quenching process. Mixtures of UF 6 and H 2 in different ratios have been irradiated at 360 nm and 400 nm in a monel cell carefully passivated. The two excitation wavelengths were chosen in order to excite preferentially one or the other of the two electronic states, which contribute to the A-X absorption band. After irradiation, a definite pressure drop in the reaction cell due to photochemical reactions has been observed. The stoichiometry of the photochemical reactions that we expect to occur is: 2UF 6 + H 2 → 2UF 5 (solid) + HF (1) With a stoichiometric UF 6 :H 2 initial pressure ratio, a pressure drop very near to the expected value of 2/3 of the initial pressure has been found. This result enables us to rule out the possibility of UF 6 decomposition into UF 4 , which would lead to a final pressure higher than that expected. The experimental time behaviour of the overall pressure drop has been interpreted in terms of the following reaction mechanism: F + H 2 → HF + H (3) H + UF 6 → UF 5 + HF (4) H + H + M → H 2 + M (5) F + UF 5 → UF 6 (6) in which M represents all possible collisional partners. According to this reaction scheme, the following conclusions have been drawn: (i) UF 6 molecules excited at 360 nm dissociate with a yield that is almost 1; (ii) H atoms produced by reactions (3) undergo reaction (4) rather than (5). (iii) Under excitation at 400 nm a photodissociation quantum yield of 0.71 has been found. Thus the dissociation process seems to be the predominant quenching mechanism of UF 6 excited in the A-X band.

Near-UV photodissociation of gaseous UF 6 in the presence of H 2

Mixtures of UF 6 and H 2 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 UF 6 into UF 5+ F. A very high dissociation quantum yield has been found.

The enthalpy of formation of uranium hexafluoride

The energy of combustion of high-purity uranium in fluorine to form uranium hexafluoride was measured in a bomb calorimeter. The standard enthalpies of formation of crystalline and gaseous UF 6, ΔH f o(UF 6, c, 298.15 K) = −(2197.7 ± 1.8) kJ mol −1 and ΔH f o(UF 6, g, 298.15 K) = −(2148.1 ± 1.8) kJ mol −1, were derived. These values are approximately 11 kJ mol −1 more negative than previous measurements based on fluorine calorimetry.

Temperature dependence of the fluorescence lifetime of gaseous UF 6 excited at 374 nm

The visible fluorescence of UF 6 excited at 374 nm has been studied in the temperature range 273–313 K. The quenching rate has been found to change with temperature according to the Arrhenius equation. Values of the unimolecular decay times are also reported.

Near UV photophysics of gaseous UF 6

We report the spectrum, quantum efficiency and decay time of the fluorescence of UF 6 excited in the A band. The quantum efficiency drops down for excitation below 380 nm. The Stern-Volmer plot presents a two-slope behaviour.

Fluorescence from gaseous UF 6 excited by a near-UV dye laser

Preliminary data are reported on the visible fluorescence of gaseous UF 6 excited by a dye laser at 374 nm. A decay time of 500 ns at p = 0 and a quenching rate of 5.7 × 10 −12 cm 3 molec −1 s −1 have been measured at room temperature.

Formation of U(V) in a molten fluoride salt at 550–650°C by reaction of gaseous UF 6 with dissolved UF 4

The reaction of gaseous UF 6 with UF 4 dissolved in molten LiFBeF 2ThF 4 (72-16-12 mole%) was studied over the temperature range 550–650°C. Chemical and spectrophotometric evidence showed that U(V) was formed in the salt according to the reaction UF 6( g) + UF 4( d) = 2UF 5( d) , in which ( g) and ( d) denote gas and dissolved species, respectively. The studies were conducted using gold apparatus, since gold was the only material found to be inert to both gaseous UF 6 and dissolved UF 5. When UF 5 was present in low concentrations, it disproportionated slowly according to the reverse of the above reaction; the rate was second-order with respect to the UF 5 concentration. The results indicated that, when UF 5 was present in high concentrations, its vaporization from the salt and subsequent disproportionation in the vapor phase were significant. Under all conditions studied, however, the pressure of uranium-containing species in the vapor phase was less than 0·004 atm.