Relativistic bond lengthening of UO 2 2+ and UO2

Summary Relativistic calculations on UO₂ [1] have shown that relativity leads to substantial bondlengthening in this compound, in contrast to the bond contraction found almost exclusively for other compounds. The bond lengthening isnot caused by the relativistic expansion of the 5f valence AO of U, which is the primary bond forming orbital on U in UO₂. The origin of the bond lengthening can be traced back to the semi-core resp. subvalence character of the U 6p AO. The valence character of 6p shows up in an increasing depopulation of the 6p upon bond shortening, and hence loss of mass-velocity stabilization. The core character of 6p shows up in large off-diagonal mass-velocity matrix elements 5p|h _MV|6p which are shown to have an overall bond lengthening effect. The larger expansion in UO₂ than in UO ₂ ²⁺ is due to destabilization of U levels in UO₂, caused by repulsion of the two additional 5f electrons.The present analysis corroborates the picture of relativistic bond length effects of Ref. [2].     

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.     

Adsorption behavior of uranium(VI) and other ionic species on cross-linked chitosan resins modified with chelating moieties

Cross-linked chitosan resins with catechol (catechol-type chitosan, type 1 and type 2), iminodiacetic acid (IDA-type chitosan), iminodimetylphosphonic acid (IDP-type chitosan), phenylarsonic acid (phenylarsonic acid-type chitosan), or serine (serine-type chitosan) were prepared for the collection and concentration of uranium(VI). The adsorption behavior of U(VI) and other ionic species, such as metal ions and oxo-acid ions, on the cross-linked chitosan (base material) and chitosan resins modified with chelating moieties was examined using a column procedure. Especially, the catechol-type chitosan (type 2) adsorbed U(VI) at pH 2-7, and selectively collected U(VI) at acidic pH regions by forming a stable chelate with hydroxyl groups of catechol moiety introduced to the chitosan. Also, the adsorption properties of cationic and anionic species present in aquatic media were elucidated. The adsorption ability for U(VI) was in the order: catechol-type chitosan (type 2) > serine-type chitosan > phenylarsonic acid-type chitosan > the others. The catechol-type chitosan (type 2) was useful for the collection and concentration of uranium(VI).     

Evaluation of sorption of uranium onto metakaolin using X-ray photoelectron and Raman spectroscopies

Metakaolin prepared from a natural clay mineral ore of aluminium kaolinite is a promising low cost and high activity aluminosilicate material that has been investigated for studying the sorption behavior of uranium. Here, metakaolin was characterized using X-ray photoelectron spectroscopy (XPS) and the effects of pH, contact time and initial metal ion concentration on its sorption behavior were studied. The sorption process was found to initially be rapid (∼60% at time 0 min) but became slower with time; equilibrium was established within 24 h (∼80% sorption). The data were applied to study the kinetics of the sorption process. The Langmuir and Dubinin-Radushkevich (D-R) sorption isotherms were used to describe partitioning behavior for the system at room temperature. The binding of metal ions was found to be pH dependent, with optimal sorption occurring at pH 5. The retained metal ions were eluted with 5 mL of 0.1 M HNO₃. Raman spectroscopy and XPS were used to evaluate the sorption mechanism of U(VI).     

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.     

Application of bromine trifluoride for pre-concentration and determination of rare-earth elements in fuel uranium dioxide

Radioactive indicator's (¹⁵²Eu) method was used to study the behavior of rare-earth element (REE) micro-impurities at fluorination of fuel UO₂ with bromine trifluoride. The proposed process is very simple and carried out under a blanket layer of Freon-113 in the glassy carbon (vitreous) crucibles.It was shown that uranium matrix completely removes from reaction sphere in the form of UF₆ and REE contaminations quantitatively remain in crucible owing to practical non-volatility of their fluorides. High purity of fuel UO₂ causes small amount of the non-volatile rest (0.3-2 wt.% from initial weight) that considerably facilitates a problem of confident diagnostics of the strictly limited REE content in this material. Removal of uranium and concentration of REE from analyzed test samples via fluorination is several hundred times more rapid scheme of pre-sampling to atomic emission spectral determination of REE micro-quantities in UO₂ than traditionally used extraction and ion-exchange ways of the preliminary concentration of contaminations and removal of uranium.     

Dehydrocoupling reactions of amines with silanes catalyzed by [(Et2N)3U][BPh4]

Dehydrocoupling reactions of primary amines RNH₂ with PhSiH₃ were catalyzed by [(Et₂N)₃U][BPh₄] to give the corresponding aminosilanes PhSiH_3−n(NHR)_n (n=1–3), the relative yields of the products were found to be dependent on the experimental conditions and on the nature of R. For a primary silane (PhSiH₃), the reactivity of RNH₂ follows the order primary>secondary>tertiary. Similar dehydrocoupling reactions using secondary amines with secondary silanes were found to be less reactive. Homodehydrocoupling of the silane was found not to be a competing reaction at room temperature. The hydride [(RNH)₂UH][BPh₄], which is plausibly formed in the reaction of [(RNH)₃U][BPh₄] with PhSiH₃ is a likely intermediate in the catalytic cycle.     

Darstellung und Struktur von U2Ta6O19, einer neuen Verbindung mit „Jahnberg‐Struktur”︁, sowie Anmerkungen zu den ersten Oxidchloriden in den Systemen Th/Nb/O/Cl und Th/Zr(Hf)/Nb/O/Cl

Synthesis and Crystal Structure of U₂Ta₆O₁₉, a New Compound with “Jahnberg‐Structure” and a Note to the First Oxide Chlorides in the Systems Th/Nb/O/Cl and Th/Zr(Hf)/Nb/O/Cl Black crystals of U₂Ta₆O₁₉ with hexagonal shape were obtained (at T₁) by chemical transport using HCl (p (HCl, 298 K) = 1 atm; silica tube) as transport agent in a temperature gradient (T₂ → T₁; 1000 °C → 950 °C) and using a mixture of UO₂, Ta₂O₅, and HfO₂ (or ZrO₂) (1 : 2 : 2) as starting materials (at T₂). For the structure determination the best result was achieved in space group P6₃/mcm (No. 193, a = 6.26(2) Å, c = 19.86(6) Å). U₂Ta₆O₁₉ is isotypical to Th₂Ta₆O₁₉. In the crystal structure each uranium atom is surrounded by oxygen atoms like a bi‐capped trigonal antiprism and tantalum atoms like a pentagonal bipyramid (CN = 7). Like the “Jahnberg Structures” both coordination polyhedra arrange themselves in separate layers (U–O‐polyhedra, in o‐, Ta–O‐polyhedra in p‐layers) so that in the direction of the c‐axis the sequence of layers is p‐p‐o. Using chemical transport it was possible to prepare the compounds Th₁₂Nb₁₆O₆₃Cl₂ and Th₈M₄Nb₁₆O₆₃Cl₂ (M = Zr, Hf), which are the first quaternary and quinquinary examples in these systems. They crystallize isotypically.     

关于铀同位素标准物质的几个问题

讨论了铀同位素标准物质(UIRM)研制中的一些技术问题,如IRM中同位素丰度的定值,IRM不确定度的影响因素等,并介绍了国内外UIRM研制情况。     

Lab on valve-multisyringe flow injection system (LOV-MSFIA) for fully automated uranium determination in environmental samples

The hyphenation of lab-on-valve (LOV) and multisyringe flow analysis (MSFIA), coupled to a long path length liquid waveguide capillary cell (LWCC), allows the spectrophotometric determination of uranium in different types of environmental sample matrices, without any manual pre-treatment, and achieving high selectivity and sensitivity levels. On-line separation and preconcentration of uranium is carried out by means of UTEVA resin. The potential of the LOV-MSFIA makes possible the fully automation of the system by the in-line regeneration of the column. After elution, uranium(VI) is spectrophotometrically detected after reaction with arsenazo-III. The determination of levels of uranium present in environmental samples is required in order to establish an environmental control. Thus, we propose a rapid, cheap and fully automated method to determine uranium(VI) in environmental samples. The limit of detection reached is 1.9 ηg of uranium and depending on the preconcentrated volume; it results in ppt levels (10.3 ηg L⁻¹). Different water sample matrices (seawater, well water, freshwater, tap water and mineral water) and a phosphogypsum sample (with natural uranium content) were satisfactorily analyzed.