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.     

UF6 and UF4 in Liquid Ammonia: [UF7(NH3)]3− and [UF4(NH3)4]

From the reaction of uranium hexafluoride UF₆ with dry liquid ammonia, the [UF₇(NH₃)]^3? anion and the [UF₄(NH₃)₄] molecule were isolated and identified for the first time. They are found in signal‐green crystals of trisammonium monoammine heptafluorouranate(IV) ammonia (1:1; [NH₄]₃[UF₇(NH₃)] ? NH₃) and emerald‐green crystals of tetraammine tetrafluorouranium(IV) ammonia (1:1; [UF₄(NH₃)₄] ? NH₃). [NH₄]₃[UF₇(NH₃)] ? NH₃ features discrete [UF₇(NH₃)]^3? anions with a coordination geometry similar to a bicapped trigonal prism, hitherto unknown for U^IV compounds. The emerald‐green [UF₄(NH₃)₄] ? NH₃ contains discrete tetraammine tetrafluorouranium(IV) [UF₄(NH₃)₄] molecules. [UF₄(NH₃)₄] ? NH₃ is not stable at room temperature and forms pastel‐green [UF₄(NH₃)₄] as a powder that is surprisingly stable up to 147 °C. The compounds are the first structurally characterized ammonia complexes of uranium fluorides.     

The determination of plutonium by mass spectrometry using a [242]-plutonium tracer

An isotopic dilution method is described for the determination ot plutonium in samples of irradiated uranium using a ²⁴²Pu tracer. An aliquot of tracer is added to the sample and the mixture treated to ensure isotopic exchange; plutonium is then separated by an ion exchange procedure and an isotopic analysis made using an M.S.5. mass spectrometer. The precision (3 σ) for an aliquot containing 0.1 μg plutonium is 0.6%. A possible application of the method would be its use for control analyses of the feed solution in a chemical plant processing natural uranium fuel elements as, for example, the Windscale primary separation plant.     

Production of Gas-Phase Uranium Fluoroanions Via Solubilization of Uranium Oxides in the [1-Ethyl-3-Methylimidazolium]<Superscript>+</Superscript>[F(HF)<Subscript>2.3</Subscript>]<Superscript>−</Superscript> Ionic Liquid

A new methodology for gas-phase uranium ion formation is described in which UO₂ is dissolved in neat N-ethyl,N′-methylimidazolium fluorohydrogenate ionic liquid [EMIm⁺][F(HF)_2.3^?], yielding a blue-green solution. The solution was diluted with acetonitrile and then analyzed by electrospray ionization mass spectrometry. UF₆^? (a U(V) species) was observed at m/z?=?352, and other than cluster ions derived from the ionic liquid, nothing else was observed. When the sample was analyzed using infusion desorption chemical ionization, UF₆^? was the base peak, and it was accompanied by a less intense UF₅^? that most likely was formed by elimination of a fluorine radical from UF₆^?. Formation of UF₆^? required dissolution of UO₂ followed by or concurrent with oxidation of uranium from the +?4 to the +?5 state and finally formation of the fluorouranate. Dissolution of UO₃ produced a bright yellow solution indicative of a U(VI) species; however, electrospray ionization did not produce abundant U-containing ions. The abundant UF₆^? provides a vehicle for accurate measurement of uranium isotopic abundances free from interference from minor isotopes of other elements and a convenient ion synthesis route that is needed gas-phase structure and reactivity studies like infrared multiphoton dissociation and ion-molecule dissociation and condensation reactions. The reactive fluorohydrogenate ionic liquid may also enable conversion of uranium in oxidic matrices into uranium fluorides that slowly oxidize to uranyl fluoride under ambient conditions, liberating the metal for facile measurement of isotope ratios without extensive chemical separations.

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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.     

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.     

Detection of uranium in industrial and mines samples by microwave plasma torch mass spectrometry

Microwave plasma torch (MPT), traditionally used as the light source for atomic emission spectrophotometry, has been employed as the ambient ionization source for sensitive detection of uranium in various ground water samples with widely available ion trap mass spectrometer. In the full‐scan mass spectra obtained in the negative ion detection mode, uranium signal was featured by the uranyl nitrate complexes (e.g. [UO₂(NO₃)₃]^?), which yielded characteristic fragments in the tandem mass spectrometry experiments, allowing confident detection of trace uranium in water samples without sample pretreatment. Under the optimal experimental conditions, the calibration curves were linearly responded within the concentration levels ranged in 10–1000 µg·l^?1, with the limit of detection (LOD) of 31.03 ng·l^?1. The relative standard deviations (RSD) values were 2.1–5.8% for the given samples at 100 µg·l^?1. The newly established method has been applied to direct detection of uranium in practical mine water samples, providing reasonable recoveries 90.94–112.36% for all the samples tested. The analysis of a single sample was completed within 30 s, showing a promising potential of the method for sensitive detection of trace uranium with improved throughput. Copyright © 2016 John Wiley & Sons, Ltd.     

Acetonitrile and triphenylphosphine oxide complexes of the uranium pentafluoride - antimony pentafluoride and uranium tetrafluoride oxide - antimony pentafluoride adducts

The new ternary adducts, UF₄O·SbF₅·2CH₃CN, UF₄O·2SbF₅·6L, UF₅·SbF₅· 2L (L = CH₃CN or (C₆H₅)₃PO) and UF₅·2SbF₅·5CH₃CN, have been prepared and studied by infrared, ¹⁹F n.m.r. and e.s.r. spectroscopy, mass spectrometry, X-ray powder diffraction and chemical analysis. The infrared spectra strongly suggest an ionic formulation with the uranium cationic species preferentially coordinated by the organic ligand.     

Massenspektrometrische Bestimmung von ppm-Mengen Thorium in Uranerzen durch Isotopenverdünnungsanalyse

Thorium is chemically separated from uranium ores in a non-quantitative way. In one sample the isotopic ratio ²³⁰Th/²³²Th is determined in a mass spectrometer. In another sample ionium is added as indicator for the isotopic dilution; the thorium of this particular sample is also analysed in a mass spectrometer. The content of ²³⁰Th and ²³²Th is calculated from the measurements of the isotopic ratios and the amount of the ionium indicator. The results are compared with the results obtained by other methods.     

Accurate measurement of uranium isotope ratios in soil samples using thermal ionization mass spectrometry equipped with a warp energy filter

A chemical and mass-spectrometric procedure for uranium isotopic analysis using a thermal ionisation mass spectrometer equipped with a Wide Aperture Retardation Potential energy filter has been developed and applied to uranium isotopic measurements for various soil samples. Soil samples were digested using a microwave digestor. Uranium was isolated from soil samples by the chemical separation procedure based on the use of anion-exchange resin and UTEVA extraction chromatography column. The isotope ratios were measured for two certified reference materials by using a VG Sector 54-30 thermal ionisation mass spectrometer in dynamic mode with Faraday cup and Daly ion counting system. Replicates of standard reference materials showed excellent analytical agreement with established values supporting the reliability and accuracy of the method. Precision of the ²³⁵U/²³⁸U ratio was achieved by a correction factor of 0.22% amu as a function of ion-beam intensity with sample loads of around 250?ng of U. The resulting reproducibility for standards and soil samples was better than 0.2% at two standard deviations (SD). Uranium isotopic compositions have been determined in several reference soil samples such as Buffalo river sediment, NIST 2704, river sediment SRM 4350b and ocean sediment NIST-4357 and a Chernobyl soil sample. There was a significant deviation from the natural uranium in comparison with Chernobyl soil samples.     

Simultaneous Plutonium and Uranium Isotopic Analysis from a Single Resin Bead - A Simplified Chemical Technique for Assaying Spent Reactor Fuels

Abstract

The method uses basic anion resin to adsorb plutonium and uranium from 7–8 M HNO₃ solutions containing dissolved spent reactor fuels. After equilibrating the resin with the solution, a single bead is used to determine the isotopic composition of plutonium and uranium on sample sizes as small as 10^?9 to 10^?10 g of each element per bead. Isotopic measurements are essentially free of isobaric interferences and fission product contamination in the mass spectrometer is eliminated. A very small aliquot of dissolver solution containing 10^?6 g of U and 10^?8 g of Pu is sufficient sample for chemically preparing several resin beads. A single prepared bead is loaded onto a rhenium filament and analyzed in a two-stage mass spectrometer using pulse counting for ion detection to obtain the high sensitivity required. Total quantity of the elements, in addition to isotopic abundances, can be determined by isotope dilution. Other areas where the method may be useful are: in plutonium production, isotope separations, and for trace detection of contamination on reactor parts.     

Isotopic “fingerprints” for natural uranium ore samples

A set of six samples, collected worldwide from various uranium ore mining facilities, was analysed for uranium isotopic composition by high accuracy isotope mass spectrometry. The goal of this article was twofold: to measure isotopic variations between samples of different geographical origin and to produce calibrated isotope ratios with the smallest achievable uncertainty (as defined according to the ISO Guide to the Expression of Uncertainty in Measurement). In the first step, the molar ratio of the isotopes ²³⁵U and ²³⁸U, n(²³⁵U)/n(²³⁸U), was measured using a UF₆-gas-inlet isotope mass spectrometer (VARIAN MAT 511). This instrument was calibrated against gravimetrically prepared synthetic isotope mixtures thus allowing SI-traceable measurements to be made. The ratios of the “minor isotopes” to ²³⁸U [n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U)] were determined in a second step using a thermal ionisation mass spectrometer with high abundance sensitivity (Finnigan MAT262-RPQ-PLUS). The mass-fractionation correction was done internally using the result of the n(²³⁵U)/n(²³⁸U) measurement. As a result, the complete measured uranium isotopic composition is traceable to the SI system. For all ratios n(²³⁴U)/n(²³⁸U), n(²³⁵U)/n(²³⁸U), and n(²³⁶U)/n(²³⁸U) significant differences for samples of different origin were found. Regarding the n(²³⁶U)/n(²³⁸U) results, only two samples, one of them from the Oklo reactor in Gabon, showed significant presence of ²³⁶U. For all other samples an upper limit for n(²³⁶U)/n(²³⁸U) of about 6 × 10⁻¹⁰, mainly dependent on the instrumentation, was found. As a result of this study we propose values for the isotope abundances of natural uranium for the “Best Measurement from a Single Terrestrial Source” and the “Range of Natural Variations” in the IUPAC-table of the “Isotopic Composition of the Elements.”     

Identification of the chemical forms of uranium compounds in micrometer-size particles by means of micro-Raman spectrometry and scanning electron microscope

We used a micro-Raman spectrometer with two different laser excitation sources (514 and 785 nm) and variable laser powers to identify some uranium chemical species contained in airborne particulate matter. In the first part of this paper, we demonstrate that characteristic Raman bands mentioned in the literature for several uranium compounds relevant in the nuclear industry (UO₂, UO₄·(4H₂O), U₃O₈, UO₂F₂ and UF₄) can be identified in particles in the few μm to 30 μm size range. In the second part of the paper, we describe a method to carry out Raman analysis on airborne uranium particles sampled along with a majority of other kinds of particles simply by dabbing adhesive carbon disks on dusty surfaces. This methodology involves an SEM equipped with an energy dispersive X-ray analyser and software for automated detection of particles specifically to locate uranium particles on the substrate before the Raman analysis. Then the sample holder is transferred to the micro-Raman spectrometer and particles are relocated using landmarks and simple geometric calculations. Raman analyses are carried out with the laser that gives the best signal to noise ratio. With such a method particles as small as 5 μm can be efficiently analysed, although most of the smaller particles cannot be analysed due to limited precision of the relocation process. This methodology was successfully applied to 20 particles collected in a nuclear facility.     

Study of the vibrational photochemical reation of UF_6+HCl and its isotopic selectivity

The photochemical vibrational reaction of UF_6+HCl was investigated by exciting theternary overtone of 3ν_3 of UF_6 in an intracavity CO laser system.It was found that the rateof the laser-driven reaction was much greater than that of the thermal reaction and thisphotochemical process had an isotopic selectivity.The uranium isotope enrichment factorsmeasured in experiments wore around 1,007 in a single step.     

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.     

Methodology for uranium compounds characterization applied to biomedical monitoring

Chronic exposure to uranium compounds led to the development of a methodology in order to characterize those compounds. This methodology, based on the recommendation of the I.C.R.P,¹ involves three main steps: the measurement of the uranium concentration and the particle size distribution at workstations; the characterization of the industrial compound, i.e. its physico-chemical properties; and the study of in-vitro solubility using chemical and cellular tests. Different methods for uranium analysis are presented. Results and comments on UF₄, UO₃, U₃O₈, UO₂ and U+UO₂ are given.     

A gravimetric and an X-ray fluorescence method for the determination of rubidium in Rb2U(SO4)3

Chemical characterization of rubidium uranium(IV) trisulfate, Rb₂U(SO₄)₃, a new chemical assay standard for uranium requires accurate analysis of rubidium. A gravimetric and an X-ray fluorescence method (XRF) for the determination of rubidium in this compound are described. In the gravimetric method, rubidium is determined as Rb₂Na[Co(NO₂)₆].H₂O without separating uranium with a precision of the order of ±0.5%. In the XRF method, the concentration ratio of rubidium to uranium, C_Rb/C_U, is determined in the solid samples by the binary ratio method using calibration between intensity ratios (I_Rb/I_U) and concentration ratios (C_Rb/C_U). The concentration of rubidium is derived using the uranium value which is known with a precision better than ±0.05%. The XRF method has a precision better than ±0.8% for rubidium determination.     

Application of gamma-ray spectrometry for the assay of uranium in crude UF4—An input material used for producing nuclear grade U-metal at the natural uranium conversion plants

A -spectrometric method has been developed for the assay of uranium in crude UF₄, which is used as a secondary source of input material for producing nuclear grade U-metal at natural uranium conversion plants. The method makes use of a NaI (Tl) detector coupled with a multichannel analyzer. The 1 MeV -ray of²³⁸U is used for calibration. A method for the fabrication of uniform -assay calibration standards is also suggested, based on the results of this investigation. The calibration standards were prepared by soaking the matrix in uranium solution and then drying the whole material. The amount of²³⁸U in the crude UF₄ sample was directly estimated by comparing the areas under the 1 MeV -ray peak of known calibration standards with the corresponding areas of the samples to be measured. 100 g each of the standard and the sample were counted. 5 crude UF₄ samples were analyzed by this method. The uranium contents in these samples were found to be in the range of 12.2–28.7 g. To compare the -ray spectrometry results with a completely independent method, chemical analysis by potentiometry of all the samples was also done. The -spectrometric results were found to agree within ±18% with the chemical analysis results.     

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.     

Characterisation of Radioactive Particles by SIMS

 Secondary ion mass spectrometry (SIMS) was optimised for characterisation of uranium- and plutonium-containing particles in soils, swipes and forensic samples. This was done by analysing in-house produced spherical UO₂-particles. Screening techniques as α-autoradiography together with SIMS analysis were employed to detect UO₂-particles in a soil sample from Chernobyl. The use of SIMS was exploited for the identification of uranium- and plutonium-containing particles and for the determination of their isotopic composition. The particles collected on swipe samples were transferred to a special adhesive support for the analysis by SIMS. Particles containing highly enriched uranium with diameters up to 10 μm were also detected in a forensic sample. For the measurements of the isotopic ratios a mass resolution of 1000 was used. At this resolution flat-top peaks were obtained which greatly improve the accuracy of the measurement. The isotopic composition of the particles was measured with a typical accuracy and precision of 0.5%. Statistically meaningful results can be obtained, for instance, from a specimen containing as few as 10¹⁰ atoms/μm³ of uranium in particles of UO₂ weighing a few picograms.