A. A. Shiryaev

K3(U3O6)(Si2O7) and Rb3(U3O6)(Ge2O7): a pentavalent-uranium silicate and germanate.

A new uranium(V) silicate, K3(U3O6)(Si2O7), and the germanate analogue, Rb3(U3O6)(Ge2O7), have been synthesized under high-temperature, high-pressure hydrothermal conditions and characterized by single-crystal X-ray diffraction. Their structures contain uranate columns formed of triple octahedral chains of the alpha-UF5 type linked by disilicate (or digermanate) units to form a 3-D framework structure. The valence state of uranium is confirmed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and magnetic susceptibility.

Sorption and Speciation of Uranium on Silica Colloids

Sorption behavior of uranium onto silica colloids is studied to derive surface complexation data. Several spectroscopic and structural methods, i.e., Time-resolved Laser-induced Fluorescence (TRLIF), X-ray Absorption Fine Structure (XAFS), and Small-angle X-ray Scattering (SAXS) are employed for characterization of colloid suspensions and surface-sorbed species. TRLIF and EXAFS allowed identification of two principal uranium species on the silica surface – bidentate coordinated uranyl complexes of the following stoichiometries (≡SiO)2UO2 and (≡SiO)2UO2OH−. No polynuclear species or surface precipitates are formed as shown by EXAFS and SAXS examination of suspensions at various silica/uranium ratios. However, when uranium coverage of the silica surface exceeds 7%, the colloid stability as determined by electrophoretic mobility increases. Potentially, this could affect colloid-facilitated transport.

Photoluminescence of silicon-vacancy defects in nanodiamonds of different chondrites

Photoluminescence spectra show that silicon impurity is present in lattice of some nanodiamond grains (ND) of various chondrites as a silicon-vacancy (SiV) defect. The relative intensity of the SiV band in the diamond-rich separates depends on chemical composition of meteorites and on size of ND grains. The strongest signal is found for the size separates enriched in small grains; thus confirming our earlier conclusion that the SiV defects preferentially reside in the smallest (less than 2 nm) grains. The difference in relative intensities of the SiV luminescence in the diamond-rich separates of individual meteorites are due to variable conditions of thermal metamorphism of their parent bodies and/or uneven sampling of nanodiamonds populations. Annealing of separates in air eliminates surface sp2-carbon, consequently, the SiV luminescence is enhanced. Strong and well-defined luminescence and absorption of the SiV defect is a promising feature to locate cold (< 250 {\deg}C) nanodiamonds in space.

A small-angle X-ray scattering study of aqueous solutions of uranium(IV) complexes with lacunar heteropolytungstates

The behavior of aqueous and acid solutions of U(IV) complexes with lacunar heteropolyanions was studied by small-angle X-ray scattering. The polytungstate forms in solutions and in crystals are generally similar. The polyanionic complexes in solutions can exhibit ordering with a period of approximately 5.6–7.8 nm. Such species are probably precursors of the crystalline form.

Sorption of actinides onto nanodiamonds

Abstract Detonation nanodiamonds (ND) present a significant part of nanocarbons group, which could be produced on commercial scale by detonation of explosives in a closed chamber. Their unique properties of high surface area, low weight and radiation resistance make ND a prospective candidate for applications in sorption processes in radiochemistry. To study the influence of surface chemistry on sorption properties, apristine sample of ND was treated with acids and hydrogen. The surface chemistry of the samples was characterised by infrared spectroscopy, X-ray photoelectron spectroscopy and Boehm titration. The sorption properties of ND were tested fordifferent radionuclides. The sorption capacity of ND was shown to be higher than those of commonly used radionuclide sorbents like activated carbon and compariable to other members of nanocarbon group like graphene oxide and carbon nanotubes. The sorption properties were shown to be influenced by the presence of oxygen-containing groups on the surface of ND. This represents an opportunity to increase the sorption capacity of ND.

Catalytic reduction of U(VI) in H2SO4 solutions with hydrazine and formic acid in the presence of bimetallic platinum-ruthenium catalysts

The structure of bimetallic platinum-ruthenium catalysts was studied by X-ray phase analysis and X-ray absorption spectroscopy. The effects of strong electronic interaction of Pt and Ru atoms in formation of the binary catalyst were revealed. The kinetics of catalytic reduction of U(VI) with hydrazine and formic acid in sulfuric acid solutions on 5% (Pt-Ru)/SiO2 was studied. The mechanisms of these reactions with platinum and bimetallic platinum-ruthenium catalysts are similar. A catalytic synergistic effect is observed in heterogeneouscatalytic reactions of U(VI) reduction with hydrazine and formic acid in H2SO4 solutions.

Interaction of U, Np, and Pu with colloidal SiO2 particles

Interaction of U(VI), Np(V), and Pu(IV,V) ions with colloidal particles of amorphous SiO2 under the conditions simulating disposal sites of radioactive wastes and spent nuclear fuel was studied. Uranium and plutonium are quantitatively sorbed on the colloidal particles, which creates prerequisites for the colloidal transport of actinides.

DEPTH CONCENTRATIONS OF DEUTERIUM IONS IMPLANTED INTO SOME PURE METALS AND ALLOYS

Pure metals (Cu, Ti, Zr, V, Pd) and diluted Pd alloys (Pd-Ag, Pd-Pt, Pd-Ru, Pd-Rh) were implanted by 25-keV deuterium ions at fluences in the range (1.2–2.3) × 1022 m−2. The post-treatment depth distributions of deuterium ions were measured 10 days and three months after the implantation by using Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering (RBS). Comparison of the obtained results allowed us to make conclusions about relative stability of deuterium and hydrogen gases in pure metals and diluted Pd alloys. Very high diffusion rates of implanted deuterium ions in V and Pd pure metals and Pd alloys were observed. Small-angle X-ray scattering revealed formation of nanosized defects in implanted corundum and titanium.

Small-angle neutron scattering from extended defects in diamonds

The results of a small-angle neutron scattering investigation of natural and synthetic diamonds with variable concentrations and types of point defects and different degrees of deformation are reported. It is shown that deformation and/or annealing of diamonds lead to the formation of planar and three-dimensional defects, giving rise to small-angle scattering.

Structural features of quench products of melts in the chloride-carbonate-silicate systems revealed by vibrational and X-ray spectroscopy

Quench products of melts synthesized at 5 GPa and 1500°C in model system CaMgSi2O6–Na2CO3(±CaCO3)–KCl, were studied using vibrational (IR and Raman) and X-ray absorption spectroscopy (XANES). Correlations between structural peculiarities of the quenches with chemical composition are established. Increase of the CaMgSi2O6 content of the melts results in gradual substitution of the Са-bearing carbonate groups by Na-bearing, whereas Ca is progressively more bounded with silicate structural units. XANES spectra reveal that chlorine is predominantly present as (KxNa1–x)Cl complexes. XANES spectra also indicate distribution of potassium cations between chloride and silicate groups, although its partial bonding with carbonate groups in the melt is not excluded.