Olga N. Batuk

Multiscale Speciation of U and Pu at Chernobyl, Hanford, Los Alamos, McGuire AFB, Mayak, and Rocky Flats.

The speciation of U and Pu in soil and concrete from Rocky Flats and in particles from soils from Chernobyl, Hanford, Los Alamos, and McGuire Air Force Base and bottom sediments from Mayak was determined by a combination of X-ray absorption fine structure (XAFS) spectroscopy and X-ray fluorescence (XRF) element maps. These experiments identify four types of speciation that sometimes may and other times do not exhibit an association with the source terms and histories of these samples: relatively well ordered PuO2+x and UO2+x that had equilibrated with O2 and H2O under both ambient conditions and in fires or explosions; instances of small, isolated particles of U as UO2+x, U3O8, and U(VI) species coexisting in close proximity after decades in the environment; alteration phases of uranyl with other elements including ones that would not have come from soils; and mononuclear Pu-O species and novel PuO2+x-type compounds incorporating additional elements that may have occurred because the Pu was exposed to extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly demonstrate instances of novel complexity in the Å and μm-scale chemical speciation and reactivity of U and Pu in their initial formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short geological but significant sociological times. They also show that incorporating the actual disposal and site conditions and resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and U in the environment.

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.

Synthesis and characterization of thorium, uranium and cerium oxide nanoparticles

Abstract We describe the synthesis of cerium, thorium and uranium oxide nanoparticles embedded in a mesoporous matrix as template in a kind of nanocasting technique. The solid matrix is used as a template to obtain and stabilize the actinide oxide nanoparticles. We apply high resolution transmission electron microscopy (HR-TEM) to show evidence of metal oxide incorporation into the matrix pores and analyze their structure. Measured interplanar distances and calculated lattice parameters for synthesized nanosized CeO2−x and ThO2 samples differ from their bulk crystalline counterparts. We obtain with our synthesis CeO2−x particles containing both Ce4+ and larger sized Ce3+. The lattice parameter for these ceria nanoparticles is found to be larger than the bulk value due to the presence of Ce3+ with its larger ionic radius. The presence of Ce3+ was established by means of high resolution X-ray emission spectroscopy (HRXES), applied to the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as one might generally expected for these nanoclusters. However, the lattice distance decrease for our particles is remarkable, up to 5%, indicating that contact with the surrounding silica matrix may exert a bond distance shortening effect such as through significant external pressure on the particle surface.

Determination of the ionic composition and oxidation state of uranium on the surface of oxides UO2+x from the XPS data

The elemental and ionic composition of uranium oxides UO2+x was determined using a procedure based on characteristics of the structure of the X-ray photoelectron spectra of outer and inner electrons of various uranium oxides. Although the bulk composition of the initial sample was close to the stoichiometry of UO2, the surface had the composition UO2.26 (6% U4+, 68% U5+, and 26% U6+, XPS). Samples kept at 70 and 150°C for a long time in simulated natural water (pH 7.0 ± 0.1) containing Ca2+, Na+, K+, and HCO3− ions were also examined. In the course of 6-month leaching, the fluorite-like structure of UO2+x is preserved, but secondary U6+ phases, including schoepite, are formed on the sample surface. The oxygen coefficient kO = 2 + x in UO2+x on the surface of the examined samples was determined by XPS in relation to the temperature and time of keeping in water.

Nature of nano-sized plutonium particles in soils at the Hanford Site

Abstract The occurrence of plutonium dioxide (PuO2) either from direct deposition or from the precipitation of plutonium-bearing solutions in contaminated soils and sediments is well described, particularly for the Hanford site in Washington State. However, past research has suggested that plutonium at the Hanford site may exist in chemical forms in addition to PuO2. Although the majority of the plutonium is present as oxide, we present evidence for the formation of nano-sized mixed plutonium- iron phosphate hydroxide structurally related to the rhabdophane group minerals in 216-Z9 crib sediments from Hanford using both transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS). The iron-plutonium phosphate formation may depend on the local microenvironment in the sediments, availability of phosphate, and hence the distribution of these minerals may control long-term migration of plutonium in the soil.

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.

Nanoscale heterogeneity as remnant hexagonal-type local structures in shocked Cu-Pb and Zr

Extended X-ray absorption fine structure spectroscopy was used to determine the local structure in: (1) Zr that had undergone quasistatic elongation; (2) Zr that had undergone plastic deformation by shock at pressures above and below the ω-phase transformation; and (3) shocked Cu that contained a few percent of insoluble Pb. Below the transition pressure, Zr samples showed only general disorder as increases in the widths of the Zr-Zr pair distributions. Above this pressure, Zr that was a mixture of the original hcp and the high pressure ω-phase when measured by diffraction showed two sets of peaks in its distribution corresponding to these two phases. Some of the ones from the ω-phase were at distances substantially different from those calculated from the diffraction pattern, although they are still consistent with small domains exhibiting stacking faults associated with hexagonal-type structural components exhibiting variability in the [0001] basal plane spacing. A similar result, new pairs at just over...

Interaction of neptunium and technetium with UO2+x

Interaction of uranium dioxide with highly mobile radionuclides 237Np and 99Tc was studied under oxidative conditions. Sorption of these radionuclides at different pH was measured, and the mechanism of redox reaction occurring in the course of their sorption were determined. In alkaline solution, Np(V) is reduced on the UO2+x surface and is sorbed in the form of tetravalent species. In neutral solutions, Np is sorbed in the form of Np(V). This is due to the fact that the stoichiometry of the UO2+x surface corresponds to U4O9. In acid solution, U(VI) is leached to form surface UO2. Although the free surface area of a UO2+x sample is low, the Np distribution coefficients Kd at pH > 6 are relatively high: log Kd > 2. Unlike Np, Tc(VII) is not reduced on the UO2+x surface. However, the sorption capacity of uranium dioxide for Tc(IV) is high.