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.     

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.     

Depuration from SiF4 of industrial gaseous waste

The problem of wet treatment of gases containing fluorine compounds produced in industrial processes, with particular reference to the phosphatic fertilizers and ceramic industries, is here considered.With this purpose and also in view of the low limits imposed by law, the chemical-physical properties of the SiF₄ and HF absorption in H₂SiF₆ solutions are discussed. In connection with these properties and with fluorine content in the gases to be treated , several devices and plant schemes are examined, also considering the problem to dispose of liquids at high Fluorine concentration for its recoveryThe results obtained in some industrial plants are finally presented and discussed.     

Conversion de UF6 appauvri en U3O8

Uranium hexafluoride (UF₆), to or from isotopic enrichment plants is stored and transported, as a solid, in tanks containing 2 to 12 metric tons of material. Sampling must be carry out after complete melting obtained by heating of the tank. This sampling process is difficult and hazardous by risks of local solidification (sealing), of reaction with air moisture (Fluorhydric Acid, highly corrosive and toxic is formed), of chemical and radioactive contamination (in case of leaking), of loss of expensive material (especialy if enriched UF₆), and of over-filling of sampling pot (possible domage during warming up of itagain).The described new device was concepted and developed by COGEMA Laboratories and is used for two years in sampling facilities of enrichment plant of PIERRELATTE. It permits to warrant sample validity and eliminate all the hereabove risks.It allows seeing and adjusting volume of the samples and their flow, and permits measurement of temperature and pressure, specified for UF₆.This new device is usable for many others materials which present some risks and difficults, as Fluorine and its derivates, chlorine, liquefied inflammable gases etc.     

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.     

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.     

Role of elemental fluorine in nuclear field

The preparation of fluorine gas by Henri Moissan by electrolysis of molten fluorides can be considered as one of the most important discoveries during the last centuries. Indeed, in addition to its use in many industrial fields (microelectronic, surface cleaning, pharmacology, medicine, …), fluorine gas is strongly involved in nuclear field for the preparation of UF₆. The latter allows the natural uranium enrichment via the gaseous diffusion process. Due to the increase of the energy demand in industrialised and emergent countries, the production of UF₆ and consequently of F₂ should increase drastically during the next decades. The aim of this paper is to summarise the evolution of the process to produce fluorine from its discovery to the present process. Few aspects on the researches done for a better understanding of the fluorine evolution reaction are presented. The use of fluorine in the nuclear field is also discussed.     

Catalytic Degradation of Sulfur Hexafluoride by Rhodium Complexes

The development of a safe and efficient method for the degradation of SF₆ is of current environmental interest, because SF₆ is one of the most potent greenhouse gases. SF₆ is thermally and chemically extremely inert, and therefore, it has been used in various industrial applications. However, this inertness results in a major challenge for its depletion. We report on a process for a catalytic degradation of SF₆ in the homogeneous phase by using rhodium complexes as precatalysts. The SF₆ activation reactions feature mild reaction conditions, low catalyst loadings, and a high selectivity. The employment of phosphines and hydrosilanes for scavenging the sulfur and fluorine atoms of the SF₆ molecule allows the selective transformation of SF₆ into nongaseous and nontoxic compounds.     

Synthesis of vicinal bromo-fluoro organic compounds using elemental fluorine

The direct action of fluorine on bromine at ?78° produces BrF which, without any isolation or purification, adds readily across various double bonds providing there is some hydrogen donor in the reaction mixture. Only trans addition of the elements of BrF was observed. When the reaction was applied to enons an easy elimination of HF can take place thus producing α-bromo enons.In our previous work we described the reaction of some olefins with IF, a reagent which was prepared $\text{in}$$\text{situ}$ by the action of F₂ on I₂[1]. The literature deals with several other fluorohalogens, among them the three known bromo-fluoro compounds, BrF, BrF₃ and BrF₅, although only the last two are well characterized[2]. However, because of the instability and the high reactivity of these compounds, they have hardly been employed in organic chemistry[3].We wish to describe here for the first time, an efficient and convenient method of synthesizing vicinal bromo-fluoro compounds using the primary source of the fluorine atoms, namely elemental fluorine itself[4].When a mixture of F₂/N₂ is bubbled through a cold (?78°C), dilute solution of Br₂ (20 mmolar) in CFCl₃ (Freon) the bromine disappears and mainly BrF is produced. However, despite previous efforts, it seems that this compound cannot be isolated since it disproportionates easily to the very reactive BrF₃ and halogen on the aromatic ring can not be ascertained. The third product proved to be the corresponding bromo-ethoxy compound XV (15% yield). Prolonged chromatography or traces of HF from the reaction mixture results in the elimination of HF thus producing, quantitatively, 6-bromocumarin XVI[8]. 2-Bromo-3-fluoro cyclopentanone XVII (oil) if formed in 90% yield and isolated in almost pure form. As in the case of cumarin, XVII readily loses HF producing the 2-bromoclopentenone (XVIII)[9] in quantitative yield.     

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.     

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.     

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.     

Can the highest occupied molecular orbital of the uranyl ion be essentially 5f?

Recent relativistic calculations on the uranyl ion suggest that the low wave numbers of the first electron transfer bands are due to a bonding 5f-like orbital containing the two loosest bound electrons. Comparison with UF₆ makes it more likely that if indeed σ_u (and not π_u) is the highest occupied MO, it is rather due to “pushing from below” by U 6p (like N 2s in N₂).     

Fluorine‐Free Synthesis of High‐Purity Ti3C2Tx (T=OH,O) via Alkali Treatment

MXenes, 2D compounds generated from layered bulk materials, have attracted significant attention in energy‐related fields. However, most syntheses involve HF, which is highly corrosive and harmful to lithium‐ion battery and supercapacitor performance. Here an alkali‐assisted hydrothermal method is used to prepare a MXene Ti₃C₂T_x (T=OH, O). This route is inspired from a Bayer process used in bauxite refining. The process is free of fluorine and yields multilayer Ti₃C₂T_x with ca. 92 wt % in purity (using 27.5 m NaOH, 270 °C). Without the F terminations, the resulting Ti₃C₂T_x film electrode (ca. 52 μm in thickness, ca. 1.63 g cm⁻³ in density) is 314 F g⁻¹ via gravimetric capacitance at 2 mV s⁻¹ in 1 m H₂SO₄. This surpasses (by ca. 214 %) that of the multilayer Ti₃C₂T_x prepared via HF treatments. This fluorine‐free method also provides an alkali‐etching strategy for exploring new MXenes for which the interlayer amphoteric/acidic atoms from the pristine MAX phase must be removed.     

Fluorine‐Free Synthesis of High‐Purity Ti3C2Tx (T=OH,O) via Alkali Treatment

MXenes, 2D compounds generated from layered bulk materials, have attracted significant attention in energy‐related fields. However, most syntheses involve HF, which is highly corrosive and harmful to lithium‐ion battery and supercapacitor performance. Here an alkali‐assisted hydrothermal method is used to prepare a MXene Ti₃C₂T_x (T=OH, O). This route is inspired from a Bayer process used in bauxite refining. The process is free of fluorine and yields multilayer Ti₃C₂T_x with ca. 92 wt % in purity (using 27.5 m NaOH, 270 °C). Without the F terminations, the resulting Ti₃C₂T_x film electrode (ca. 52 μm in thickness, ca. 1.63 g cm^?3 in density) is 314 F g^?1 via gravimetric capacitance at 2 mV s^?1 in 1 m H₂SO₄. This surpasses (by ca. 214 %) that of the multilayer Ti₃C₂T_x prepared via HF treatments. This fluorine‐free method also provides an alkali‐etching strategy for exploring new MXenes for which the interlayer amphoteric/acidic atoms from the pristine MAX phase must be removed.     

Fluoride volatility method for reprocessing of LWR and FR fuels

Fluoride volatility method is based on direct fluorination of powdered spent fuel with fluorine gas in a flame fluorination reactor, where the volatile fluorides (represented mainly by UF₆, partially NpF₆) are separated from the non-volatile ones (e.g. PuF₄, AmF₃, CmF₃, fluorides of majority of fission products), the objective being to separate a maximum fraction of uranium component from plutonium, minor actinides and fission products. The current research and development work in the area of fluoride volatility method is focused on the experimental program carried out at the semi-technological line called FERDA, which is a follow-up of the previous FREGAT-2 technology. The experimental test program, launched in 2004 by the Nuclear Research Institute ?e? plc, has been focused mainly to the study of flame fluorination process, which is considered to be the crucial unit operation of the technology. The fluorination experiments were realized in the first instance with pure uranium oxide fuel and later on with simulated spent oxide fuel. Follow-on tests are planed with oxide fuels with inert matrixes. The experimental program is further supplemented by the system studies focused mainly to the process flow-sheet design and calculations and to the requisite modification of some apparatuses for the future verification of the process with irradiated fuel in hot conditions.     

The electronic structure of the BrF3 and BrF5 molecules

Ionization potentials of the BrF₃ and BrF₅ molecules have been calculated by the SCF DV Xα method. The calculations hale been carried out in numerical Hartree-Fock basis sets, or, to be more specific, in bases that are extensions of these bases since “virtual” bromine 4d-functions are added to the numerical HF bases. The results are used for the interpretation of the experimental PDS of these compounds.