Complexes triflates de l’uranium

Uranium triflate complexes. We review here the different preparations of uranium triflates that we have developed in the course of these last years in our laboratory. Protonation of 〚U〛–R and 〚U〛–NR₂ (R = alkyl) bonds with pyridinium triflate constitutes a general and efficient route towards triflate complexes. This method is very suitable for the preparation of organometallic compounds such as U(Cp)₃(OTf), U(Cp)₂(OTf)₂(py), U(Cp*)₂(OTf)₂, and U(Cot)(OTf)₂(py), which have been crystallographically characterised. The homoleptic species U(OTf)_n (n = 3, 4) are easily prepared by heating a mixture of uranium turnings or UH₃ in triflic acid. By adjusting the temperature to 120 or 180 °C, either U(OTf)₃ or U(OTf)₄ is isolated. Treatment of UO₃ with pure or aqueous solution of triflic acid leads to the non-solvated uranyl triflate UO₂(OTf)₂, which is more conveniently obtained by heating a suspension of UO₃ in triflic anhydride. This reactant is an excellent dehydrating agent and enables the preparation of UO₂(OTf)₂ and Ce(OTf)₄ from the hydrated starting materials.     

Preparation of uranium oxide powder for nuclear fuel pellet fabrication with uranium peroxide recovered from uranium oxide scraps by using a carbonate–hydrogen peroxide solution

This work studied a way to reclaim uranium from contaminated UO₂ oxide scraps as a sinterable UO₂ powder for UO₂ fuel pellet fabrication, which included a dissolution of the uranium oxide scraps in a carbonate solution with hydrogen peroxide and a UO₄ precipitation step. Dissolution characteristics of reduced and oxidized uranium oxides were evaluated in a carbonate solution with hydrogen peroxide, and the UO₄ precipitation were confirmed by acidification of uranyl peroxo–carbonate complex solution. An agglomerated UO₄ powder obtained by the dissolution and precipitation of uranium in the carbonate solution could not be pulverized into fine UO₂ powder by the OREOX process, because of submicron-sized individual UO₄ particles forming the agglomerated UO₄ precipitate. The UO₂ powder prepared from the UO₄ precipitate could meet the UO₂ powder specifications for UO₂ fuel pellet fabrication by a series of steps such as dehydration of UO₄ precipitate, reduction, and milling. The sinterability of the reclaimed UO₂ powder for fuel pellet fabrication was improved by adding virgin UO₂ powder in the reclaimed UO₂ powder. A process to reclaim the contaminated uranium scraps as UO₂ fuel powder using a carbonate solution was finally suggested.     

Progress in neutron activation analysis for uranium

A new type of extractant, sym-dibenzo-16-crown-5-oxyhydroxamic acid (HL) is introduced. The extractions of UO22+, Na+, K+, Sr2+, Ba2+ and Br- were studied with HL in chloroform. The results obtained show that UO22+ can be quantitatively extracted at pH values above 5, whereas the extractions of K+, Na+, Sr2+, Ba2+ and Br- are negligible in the pH range of 2 - 7. The dependence of the distribution ratio of U(VI) on both the concentration of the HL and pH are linear, and they have the same slope of 2. This suggests that U(VI) appears to form a 1:2 complex with ligand. Uranium(VI) can be selectively separated and concentrated from interfering elements such as Na, K, Sr and Br by solvent extraction with HL under specific conditions. The recovery of uranium is nearly 100% and the radionudear purity of uranium is greater than 99.99%. Therefore, neutron activation analysis has greatly improved the sensitivity and accuracy for the detection of trace uranium from seawater.     

Releases of radium from an abandoned uranium mine site: Žirovski Vrh uranium mine, Slovenia

Since the beginning of explorative uranium mining at the Žirovski Vrh uranium ore deposit area in 1968, a radioactivity monitoring programme has been carried out. The extent of the programme has varied according to the pre-operational, operational, and, finally, post-operational conditions. In this paper, our ten year results on the dissolved radium concentrations in surface waters, which have been contaminated and potentially affected by the uranium mining and milling activities, are reported. With the exception of waters drained from the hydrometallurgic waste site with radium content ranging from 2 to 9 kBqm⁻³, radium content is far below the drinking water limit of 1000 Bqm⁻³; in the Brebovŝčica stream, which collects all the waters affected by the mine, the present radium concentration does not exceed 10 Bqm⁻³.     

Why does uranium oxide phosphate contract on heating?

(U₂O)(PO₄)₂ is related to ultra-low expansion β-(Zr₂O)(PO₄)₂ ceramics, but shows a continuous thermal contraction. High-temperature neutron diffraction has allowed to follow accurately the thermal variations of its cell edges and to give a structural explanation to the phenomenon: like in β-(Zr₂O)(PO₄)₂, the dilatometric anomaly arises simultaneously from a contractive push-pull effect due to Coulombic repulsions and from a libration of the PO₄ and UO₇ polyhedra, but in the present case, the second mechanism predominates. The size of the tetravalent cation appears as a key parameter in monitoring the thermal expansion of ceramics of this family.     

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.”     

Assay of uranium in crude diuranate cakes and MgF2 slag produced at the natural uranium conversion plants by γ-ray spectrometry

A transmission-corrected -ray counting method has been employed for the assay of uranium in crude Na₂U₂O₇ cakes produced at the Uranium Conversion Facilities. A 3×3 NaI(TI) detector was used in conjunction with a 400-channel analyzer. The observed count rate of the 1 MeV -ray emitted by the²³⁸U in the sample was corrected for sample self-attenuation, measured with a⁶⁵Zn (-energy 1115 keV) transmission source. A calibration factor determined by measuring a standard of known amount of radioactive material in the same form and geometry as the unknown sample was used to convert the transmission corrected count rate to the amount of uranium in the weighted sample. Another -spectrometric method is described for the assay of the U-content in the MgF₂ slag produced during the magnesiothermic reduction of UF₄ to U-metal ingots at the natural U-conversion plant.     

Solvent extraction of uranium from aqueous solutions by α-benzoinoxime

Solvent extraction of uranium with α-benzoinoxime from aqueous solutions has been systematically investigated. The extraction equilibration was very fast and achieved at 60 s for uranium. The extraction of uranium was pH-dependent using α-benzoinoxime as extractant. The effect concentration of uranium and α-benzoinoxime was studied. The uranium loaded in the organic phase can be stripped efficiently with 93 % yield using 0.1 M HCl as the stripping agent in a single stripping step. A good selectivity for uranium was observed through α-benzoinoxime as extractant from aqueous solution with other interfering cation ions. Present study suggested that α-benzoinoxime can be used as a potential extractant for separation of uranium from aqueous solution using centrifugal extractor in industrial application.     

Synthesis and characterization of uranium bromide phosphate UBrPO4·2H2O and uranium hydroxide phosphate U(OH)PO4

Uranium chloride phosphate tetrahydrate UClPO₄·4H₂O was obtained by mixing uranium (IV) hydrochloric solution and concentrated phosphoric acid [1]. From crystal structure studies its formula was determined as dihydrate [2]. Using the same method, i.e. starting from uranium (IV) hydrobromic solution and H₃PO₄, two crystal forms of a new compound, uranium bromide phosphate UBrPO₄·2H₂O were synthesized. Their XRD patterns, UV-visible and infrared spectra are presented in this paper. The hydrolysis process of the chloride and bromide phosphates leads to the amorphous uranium hydroxide phosphate U(OH)PO₄·6H₂O.     

Accurate purification age determination of individual uranium–plutonium mixed particles

Age of individual uranium–plutonium (U/Pu) mixed particles with various U/Pu atomic ratios (1–70) were determined by inductively coupled plasma mass spectrometry. Micron-sized particles were prepared from U and Pu certified reference materials. The Pu reference was stored for 4–6 years since the last purification (July 14, 2008). The Pu purification age was obtained from the ²⁴¹Am/²⁴¹Pu ratio which was calculated from the product of three measured ratios of Pu and Am isotopes in the eluted fractions. These ratios were measured by a high-resolution inductively coupled plasma mass spectrometer equipped with a desolvation system. Femto-gram to pico-gram quantities of Am, U, and Pu in a sample solution were sequentially separated on a small anion-exchange column. The ²⁴¹Am/²⁴¹Pu ratio was accurately determined by spiking pure ²⁴³Am into the sample solution. The average determined age for the particles for the five independent U/Pu ratios was in good agreement with the expected age with high accuracy (difference age 0.27 years) and high precision (standard deviation 0.44 years). The described analytical technique can serve as an effective tool for nuclear safeguards and environmental radiochemistry.

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Figure Young (4?6 y) Pu purification age of individual U/Pu mixed micron-sized reference particles for the five independent U/Pu ratios (1?70) were determined with 0.27±0.44 y difference from the expected age. Sub pico-gram quantities of Am, U and Pu were sequentially separated a small column, and their isotope ratios were accurately measured using an ICP-MS by applying the ²⁴³Am spiking technique to the analysis and correcting the impurity and the contaminations.

     

Computational insights into uranium complexes supported by redox-active α-diimine ligands

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, ((Mes)DAB(Me))(2)U(THF) (1) and Cp(2)U((Mes)DAB(Me)) (2) ((Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum.     

Electrochemical separation of uranium and cerium in molten LiCl–KCl

The electrochemical separation of uranium from cerium in LiCl–KCl eutectic and the electrochemical behavior of Ce(III) were studied. According to the cyclic voltammogram of Ce(III) and the former result of U(III), electrodeposition potential was determined at ?1.65 V (vs Ag/AgCl). The uranium metal was successfully deposited and separated from cerium. The morphology of deposit and cross section of electrode were investigated by SEM, firstly uranium deposit alloys with stainless steel and forms a thin transition layer, and secondly the uranium metal layer grows from the transition layer. The separation factors of uranium/cerium on different recovery ratios were determined through a series of steps. It was found that the content of cerium in the deposit and separation factors declined with increasing the initial concentration of U³⁺ in molten salts; the separation factors remained stable at around 20 in different uranium recovery ratios.     

Radioactive contamination of the environment as a result of uranium production: a case study at the abandoned Lincang uranium mine, Yunnan Province, China

The distribution of radioactive pollutants, such as ~(222)Rn, U, Th and ~(226)Ra in the air, sur-face waters, soils and crops around the Lincang uranium mine, Yunnan Province, China, is studiedThe mechanical, geochemical and biogeochemical processes responsible for the transport andfate of the radioactive elements are discussed based on the monitoring data. The pollutants con-centrations of effluents from the mine tunnels were dependent on pH and SO_4~(2-) which were con-trolled by biochemical oxidation of sulfide in the ore/host rocks. Radon anomalies in air reached 4km from the tailings pile depending on radon release from the site, topography and climate. ~(238)Uand ~(226)Ra abnormities in stream sediments and soil were 40-90 cm deep and 790-800 m awaydownstream. Anomalies of radioactive contaminants of surface watercourses extended 7.5-13km from the discharge of effluents of the site mainly depending on mechanical and chemical proc-esses. There were about 2.86 ha rice fields and 1.59 km stream sediments contaminated. Erosionof tailings and mining debris with little or no containment or control accelerated the contaminationprocesses.     

Microwave-assisted dissolution of ceramic uranium dioxide in TBP–HNO3 complex

Microwave-assisted dissolution of ceramic uranium dioxide in tri-n-butyl phosphate (TBP)–HNO₃ complex was investigated. The research on dissolution of ceramic uranium dioxide in TBP–HNO₃ inclusion complex under microwave heating showed the efficiency of the use of this method. Nitric acid present in the inclusion complex participates both dissolution of UO₂, and oxidation of U(IV)–U(VI), the resulting UO₂(NO₃)₂ extracted with tri-n-butyl phosphate. Dissolution rate depends on both temperature of microwave dissolution process, and concentration of nitric acid present in the inclusion complex. The most intensive dissolution process is when the concentration of nitric acid ≥2 mol/L and the temperature of 120 °C. From the experimental data obtained by two kinetic models activation energies were calculated. At the average activation energy of UO₂ dissolution in TBP–HNO₃ complex equal 70 kJ/mol, and reaction order is close to one, i.e. the reaction takes place in an area close to kinetic.     

Release and speciation of uranium in low-ionic-strength groundwaters at an abandoned uranium mine in Val Vedello (Orobic Alp—Italy) by adsorptive cathodic stripping voltammetry

Adsorptive cathodic stripping voltammetry (AdCSV) with HDME and a chloranilic acid ligand was used in the trace analysis of uranyl ions at pH?=?2 in low-ionic-strength groundwaters around mining areas. Upon optimization, the limit of detection around 0.10?µg?L^?1 was found with linearity up to 10?µg?L^?1. In the abandoned mining area of Val Vedello (Orobic Alps, Italy), measured uranium concentrations in water ranged from 0.3?µg?L^?1 above the uranium mineralization levels to 145?µg?L^?1 in groundwaters percolating from mine galleries. Such uranium concentrations are related to natural weathering effects of CO₂ and/or hydrogen carbonate ion on uranium mineralizations under oxic conditions. A marked seasonal dependence was then found, in agreement with literature data on a pre-operational survey dating back to 1980–1981. No significant chemical impact of the abandoned mining activity on groundwater quality could be found. Accordingly, no significant increase in contaminants derived from the heat-burn of explosives, such as chloride and nitrate, in groundwaters from mine galleries was found.     

Electrochemistry of uranium in LiF–BeF2 melt

A study of the electrochemistry of uranium in LiF–BeF₂ system important for molten salt reactor concept was conducted at W and Ni electrodes. Cyclic voltammetry and chronopotentiometry methods were used. Two-step reduction mechanism for U⁴⁺ ions involving one electron exchange in soluble/soluble U⁴⁺/U³⁺ system and three electron exchange in the second step were found on W electrode. Both processes were identified as reversible and diffusion-controlled. Based on voltammetric and chronopotentiometric measurements, the diffusion coefficient of U⁴⁺ ions at 813 K was calculated: D(U⁴⁺) = 1.26 × 10⁻⁶ cm² s⁻¹ and D(U⁴⁺) = 1.28 × 10⁻⁶ cm² s⁻¹, respectively. Formation of U–Ni alloys was observed on Ni electrode.