Janne Pakarinen

Evolution of the Uranium Chemical State in Mixed-Valence Oxides

A fundamental question concerning the chemical state of uranium in the binary oxides UO2, U4O9, U3O7, U3O8, and UO3 is addressed. By utilizing high energy resolution fluorescence detection X-ray absorption near edge spectroscopy (HERFD-XANES) at the uranium M4 edge, a novel technique in the tender X-ray region, we obtain the distribution of formal oxidation states in the mixed-valence oxides U4O9, U3O7, and U3O8. Moreover, we clearly identify a pivot from U(IV)-U(V) to U(V)-U(VI) charge compensation, corresponding with transition from a fluorite-type structure (U3O7) to a layered structure (U3O8). Such physicochemical properties are of interest to a broad audience of researchers and engineers active in domains ranging from fundamental physics to nuclear industry and environmental science.

Homogeneous hydrolysis of thorium by thermal decomposition of urea

Abstract Thorium was precipitated homogeneously from a thorium nitrate solution by the thermal decomposition products of urea. The kinetics of the hydrolysis were studied at 90 and 100°C by pH measurement during the initial 5 h and the precipitation efficiencies of thorium and radium were measured over a 24 h period. Precipitation of the radium daughters was closely followed with the aim of co-precipitation of radium with thorium. The CO2 formed during urea decomposition dissolved in the solution, forming CO32− during the experiment upon reaching a sufficiently high pH level (>7). This allowed radium to co-precipitate partially, thus reducing the activity of the filtrate. After filtration or centrifugation, the precipitate is composed of nanocrystalline thorium dioxide (crystallite size ~10 nm), with weakly bound H2O and CO2.

Trends in the valence band electronic structures of mixed uranium oxides

The valence band electronic structures of mixed uranium oxides (UO2, U4O9, U3O7, U3O8, and β-UO3) have been studied using the resonant inelastic X-ray scattering (RIXS) technique at the U M5 edge and computational methods. We show here that the RIXS technique and recorded U 5f-O 2p charge transfer excitations can be used to test the validity of theoretical approximations.

Inert Gas Measurement of Single Bubble in CeO2

Uranium dioxide (UO2), an oxide with a fluorite crystal structure, is the main fuel used in commercial light water reactors. Inert fission gases such as Xe and Kr significantly impact the performance of UO2 during reactor operation and in storage. These gases have a large yield of approximately 25% and have a low solubility in UO2, resulting in the formation of large density of fission gas bubbles [1]. Such bubbles cause the fuel to swell, which promotes clad outward creep that shortens the cladding lifetime. Characterization of the inert fission gas content in bubbles can help us understand fuel swelling and fuel pin pressurization. This is performed for Xe bubbles in cerium dioxide, CeO2, which is considered a surrogate for UO2 due to similar crystal structure and properties.

Investigations of a Type 316L Steam Dryer Plate Material Suffering from IGSCC After a Few Years in BWR Conditions.

Detailed microstructural characterization has been performed on a sample made of Type 316L stainless steel, removed from a boiling water reactor (BWR) steam dryer inner roof plate, which had suffered from intergranular stress corrosion cracking (IGSCC) after only a few years of operation. The aim of the characterisation was to increase the understanding of possible material related characteristics that could have affected the observed cracking.