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.     

Uranium and fluorine cycles in the nuclear industry

In the nuclear fuel cycle, fluorine is currently not recycled. In this paper, we have examined the possible routes to implement such a cycle. Because UF₆ deconversion requires an excess of water, aqueous HF is produced. Two alternatives are then possible: either separate HF from H₂O or recycle the HF-H₂O in the deconversion process. Alternative UF₆ deconversion could also be implemented to resorb the high UF₆ inventory.     

Rotating frame spin-lattice relaxation through anisotropic chemical shift in polycrystalline UF6

A minimum in the temperature dependence of the ¹⁹F rotating frame spin-lattice relaxation time T_1? in polycrystalline UF₆ has been observed which enables one to determine the correlation time for slow reorientations of the UF₆ octahedra directly. The dependence of the depth of the T_1? versus temperature minimum on the external magnetic field H₀ at a fixed value of the rotating field H₁ provides an independent proof that fluctuations of the ¹⁹F screening constant tensor significantly contribute to the fluorine spin-lattice relaxation rate in this system.     

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.     

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.     

Electrochemistry of fluorine production

The electronic properties of carbon-fluorine films (denoted C-F) formed on carbon electrodes in KF-2HF during fluorine evolution reaction were investigated in aqueous solution containing a redox couple and in mercury. It was shown that the passivating C-F films behave as electronic conductors. STM measurements have shown composition heterogeneities at the surface of fluorinated HOPG (conducting ans insulating areas). The influence of the amount of insulating graphite fluorides on the surface of the electrodes was demonstrated. Thus, the high anodic overvoltage observed during fluorine evolution on C/C-F anodes in KF-2HF is mainly attributed to the poor wettability of the electrodes by the melt, which results in a small electroactive area. A new model was proposed for representing the electrode/electrolyte interface; it includes the presence of a “fluidized” layer between the surface C-F film and the fluorine gas film. The “fluidized” layer is composed of liquid KF-2HF melt and dissolved fluorine gas. The influence of the mass transfer phenomenon occurring in that layer was pointed out mainly by impedance measurements. Finally, the contributions of the C-F film, η_C-F, and of the “fluidized” layer, η_fluid, to the total anodic overvoltage, η_T, were studied using a numerical calculation method. Both contributions must be taken into account for a global understanding of the fluorine evolution process.     

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.     

New reprocessing system for spent nuclear reactor fuel using fluoride volatility method

Our proposed spent nuclear fuel reprocessing technology named FLUOREX, which is a hybrid system using fluoride volatility and solvent extraction, meets the requirements of the future thermal/fast breeder reactors (coexistence) cycle. We have been done semi-engineering and engineering scale experiments on the fluorination of uranium, purification of UF₆, pyrohydrolysis of fluorination residues, and dissolution of pyrohydrolysis samples in order to examine technical and engineering feasibilities for implementing FLUOREX. We found that uranium in spent fuels can be selectively volatilized by fluorination in the flame type reactor, and the amount of uranium volatilized is adjusted from 90% to 98% by changing the amount of F₂ supplied to the reactor. The volatilized uranium is purified using UO₂F₂ adsorber for plutonium and purification methods such as condensation and chemical traps for fission products provide a decontamination factor of over 10⁷. Most of the fluorination residues that consist of non-volatile fluorides of uranium, plutonium, and fission products are converted to oxides by pyrohydrolysis at 600-800 °C. Although some fluorides of fission products such as alkaline earth metals and lanthanides are not converted completely and fluorine is discharged into the solution, oxides of U and Pu obtained by pyrohydrolysis are dissolved into nitric acid solution because of the low solubility of lanthanide fluorides. These results support our opinion that FLUOREX has great possibilities for being a part of the future spent nuclear fuel cycle system.     

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.     

Novel method of synthesis of C60F48 with improved yield and selectivity

A novel two-stage method of preparation of C₆₀F₄₈ with 96% purity and 80% yield is reported. A C₆₀ embedded into a MnF₂ matrix is reacted with molecular fluorine under dynamic conditions, i.e. in flow of fluorine gas and with sublimation of volatile products, which results in formation of C₆₀F₃₄-C₆₀F₃₈ mixtures with >90% yield. Subsequent fluorination of the mixture thus obtained in the closed reactor at elevated pressure directly leads to the final product. C₆₀F₄₈ thus synthesized has been characterized by means of EI-MS, MALDI-MS, IR-spectroscopy and X-ray photoelectron spectroscopy (XPS). The problems of fullerene burning and degradation in the fluorine atmosphere are discussed.     

Analysis on holdup during processing UF6

Holdup in processing UF₆ is analyzed in the present article. Under normal operation conditions of temperature and pressure, UF₆ stays in gas phase. While if water moisture ingresses, chemical reaction between UF₆ and moisture results in solid or liquid productions that can deposit on structure surfaces. The present report focuses on the chemistry of holdup and how to measure it, and safeguards and criticality safety concerns. An available statistic theoretical model is also discussed.     

Determination of fluorine in UF4 by high-temperature hydrolysis

The method of high-temperature hydrolysis separating fluorine from UF₄ is described. The determination of the content of fluorine by different methods is performed and compared.     

Investigation of chemical changes in uranium oxyfluoride particles using secondary ion mass spectrometry

Understanding how environmental conditions may affect sample composition is critical to the interpretation of laboratory analyses from environmental sampling. We prepared a set of UO₂F₂ particle samples from the hydrolysis of UF₆ and stored these samples in environmental chambers at different temperature, humidity and lighting conditions. The NanoSIMS ion microprobe was used to measure the UF⁺/U⁺ secondary ion ratio of individual particles. Monitoring variations in this ratio may provide insights on changes in particle composition over time and in response to environmental exposure. This report presents the baseline measurements carried out on freshly-prepared particle samples to determine the initial amount of fluorine.     

Solid-state NMR and EPR study of fluorinated carbon nanofibers

Carbon nanofibers were fluorinated in two manners, in pure fluorine gas (direct fluorination) and with a fluorinating agent (TbF₄ during the so-called controlled fluorination). The resulting fluorinated nanofibers have been investigated by solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). This underlines that the fluorination mechanisms differ since a (CF)_n structural type is obtained, whatever the temperature, with the controlled reaction, whereas, during the direct process, a (C₂F)_n type is formed over a wide temperature range. Through a careful characterization of the products, i.e. density of dangling bonds (as internal paramagnetic centers), structural type (acting on molecular motion) and specific surface area (related to the amount of physisorbed O₂), the effect of atmospheric oxygen molecules on the spin-lattice nuclear relaxation has been underlined.     

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     

Photodissociation of UF6 and multiphoton ionization of UF5

Multiphoton ionization of UF₅ as a probe of laser-induced dissociation of UF₆ is investigated using an ionization chamber. Results are given on the fluence dependence of the multiphoton ionization process and on the pressure dependence of the UF₆ photodissociation.     

Surface fluorination of the cathode active materials for lithium secondary battery

LiMn₂O₄ was treated with F₂ at room temperature (RT), 373 and 473 K under 1.3, 6.6 and 13.2 kPa-F₂. XPS data indicate that two kinds of fluorine species may exist on the sample surface and the ratio of these fluorines is affected by choosing the reaction condition. The peak indicating Mnⁿ⁺ bonded to fluorine appeared in the XPS spectra of Mn2p3/2 electron. From the results of the charge/discharge measurements, the efficiency of charge/discharge process for the sample fluorinated under 1.3, 6.6 and 13.2 kPa-F₂ below 373 K was larger than that of untreated one. The discharge capacity of the fluorinated sample was also larger than that of untreated one. The discharge capacity, the loss of discharge capacity during 50 charge/discharge cycles, F/O ratio measured from XPS data and the intensity of the peak indicating Mnⁿ⁺ bonded to fluorine in the XPS spectra were closely related to each other. The optimal fluorination condition was under 1.3 kPa-F₂ at RT for 1 h.     

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.     

Analysis of solid uranium samples using a small mass spectrometer

A mass spectrometer for isotopic analysis of solid uranium samples has been constructed and evaluated. This system employs the fluorinating agent chlorine trifluoride (ClF₃) to convert solid uranium samples into their volatile uranium hexafluorides (UF₆). The majority of unwanted gaseous byproducts and remaining ClF₃ are removed from the sample vessel by condensing the UF₆ and then pumping away the unwanted gases. The UF₆ gas is then introduced into a quadrupole mass spectrometer and ionized by electron impact ionization. The doubly charged bare metal uranium ion (U²⁺) is used to determine the U²³⁵/U²³⁸ isotopic ratio. Precision and accuracy for several isotopic standards were found to be better than 12%, without further calibration of the system. The analysis can be completed in 25 min from sample loading, to UF₆ reaction, to mass spectral analysis. The method is amenable to uranium solid matrices, and other actinides.     

Ca(AsF3)(AsF6)2: the first alkaline earth metal cation coordinated to AsF3

Ca(AsF₃)(AsF₆)₂ was prepared by the reaction of CaF₂ with excess AsF₅ in AsF₃ solvent. The compound crystallizes in an orthorhombic crystal system, space group Pnma, with a =1034.9(4) pm, b = 1001.7(4) pm and c = 1088.4(4) pm, V = 1.1283(8) nm³ and Z = 4. Calcium is coordinated to eight fluorine atoms, with six fluorine atoms located at the corners of a regular trigonal prism originating from six AsF₆ units. Two rectangular faces of the trigonal prism are capped by fluorine atoms from two fluorine bridged AsF₃ molecules. For the first time, AsF₃ is shown to serve as a bridging ligand to two metal cations, with bridging distances of F(AsF₃)-Ca = 241.1 and 243.2 pm. It was found, again for the first time, that the bridging As-F distances are shorter (172.4 and 173.1 pm) than the terminal As-F distance (184.5 pm). The Raman spectrum shows vibrational modes that are readily assigned to AsF₃ and AsF₆⁻.