René Bès

University of Helsinki | HY · Department of Physics

The evolution of the uranium chemical state in uranium compounds, principally in the oxides, is of concern in the context of nuclear fuel degradation under storage and repository conditions, and in accident scenarios. The U–O system shows complicated phase relations between single-valence uranium dioxide (UO2) and different mixed-valence compounds...

The self-terminating chemistry of atomic layer deposition (ALD) ideally enables the growth of homogeneously distributed materials on an atomic scale. This study investigates the ALD of zinc oxide (ZnO) on mesoporous zirconium oxide (ZrO2) using zinc acetylacetonate [Zn(acac)2] and synthetic air in a fixed-bed powder ALD reactor. A broad variety of...

The binary uranium-oxygen system contains numerous single-valence and mixed-valence uranium oxide compounds, spanning over a wide phase field which is defined by the oxygen-to-uranium ratio (O/U) and temperature. Uranium dioxide (UO2), the principal component of commercial nuclear fuel, is nominally associated to the tetravalent uranium state. It i...

The self-terminating chemistry of atomic layer deposition (ALD) ideally enables the growth of homogeneously distributed materials on an atomic scale. This study investigates the ALD of zinc oxide (ZnO) on mesoporous zirconium oxide (ZrO2) using zinc acetylacetonate [Zn(acac)2] and synthetic air in a fixed-bed powder ALD reactor. A broad variety of...

We report positron lifetime and X‐ray absorption spectroscopy results in Si‐doped GaN crystals grown by the hydride vapor phase epitaxy. Pushing the Si doping to high concentrations leads to surprisingly strong compensation effects. Positron experiments show that the concentrations of Ga vacancies are not high enough to be efficient compensation ce...

The radioactive 129I is a top-priority radionuclide due to its the long half-life (1.57 × 107 years) and high mobility. Because of the planned and accidental releases to the environment, specific separation technologies are required to limit the potential radiation dose to human beings. Zirconium oxides are known for their adsorption capability and...

In this paper we present laboratory-scale X-ray absorption spectroscopy applied to the research of nanometer-scale thin films. We demonstrate the Cu K edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of CuI and CuO thin films grown with atomic layer deposition. Film thicknesses in the investigat...

The development of active catalysts for carbon dioxide (CO2) hydrogenation to methanol is intimately related to the creation of effective metal-oxide interfaces. In this work, we investigated how the order of addition of copper and zinc on zirconia influences the catalytic properties, the catalytic activity and selectivity toward methanol. Regardin...

Bentonite is one of the materials used to construct engineered barriers in high-level radioactive nuclear waste geological disposal with its many advantageous features such as low hydraulic conductivity, self-sealing ability, durability, adsorption and immobilization of metals and radionuclides and reduction of microbial activity. Many of these pro...

The uranium valence electronic structure in the prototypical undistorted perovskite KUO3 is reported on the basis of a comprehensive experimental study using multi-edge HERFD-XAS and relativistic quantum chemistry calculations based on density functional theory. Very good agreement is obtained between theory and experiments, including the confirmat...

In the recent years, the advent of an efficient and compact laboratory-scale spectrometer for x-ray absorption spectroscopy experiments has been extensively reported in the literature. Such modern instruments offer the advantage to routinely use x-ray absorption spectroscopy on systematic studies, which is usually unconceivable at synchrotron radia...

Valence band electronic structure of mixed uranium oxides (UO2, U4O9, U3O7, U3O8, UO3) has been studied by the resonant inelastic X-ray scattering (RIXS) technique at the U M5 edge and by computational methods. We show here that the RIXS technique and recorded U 5f-O 2p charge transfer excitations can be used to proof the validity of theoretical ap...

The evolution of the crystal lattice of samples made of UO2 doped with different concentrations of Nd in stoichiometric and hypo-stoichiometric conditions has been systematically studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The substitution of a trivalent cation for the U⁴⁺ initial position is responsible for creating...

Invited contribution after selection of the corresponding article as an ACS Editor's Choice on March 11, 2020. The voice-over presentation provides an overview of the main outcomes of the published research. DOI of the corresponding article: 10.1021/acs.inorgchem.9b03702. The presentation can also be viewed directly from the publisher's website: ht...

Securing sufficient and environmentally sustainable electricity production is one of the main challenges for the future. Among electricity sources, fission and fusion reactors do not suffer from intermittencies observed in solar or wind resources and allow an efficient and flexible basis for the highly demanding electricity network, especially around b...

A comprehensive analysis of X-ray absorption data obtained at the U L3-edge for a systematic series of single-valence (UO2 , KUO3 , UO3) and mixed-valence uranium compounds (U4O9 , U3O7 , U3O8) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectros-copy (XANES) and extended X-ray absorption fine struc...

Valence band electronic structure of mixed uranium oxides (UO2, U4O9, U3O7, U3O8, b-UO3) has been studied by the resonant inelastic X-ray scattering (RIXS) technique at the U M5 edge and...

The charge compensation mechanisms in UyNd1−yO2±x and its consequence on the overall O stoichiometry (or O/M ratio where M = Nd + U) have been studied through the uranium valence state mixture evolution as a function of Nd content up to y = 0.62 by means of high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) at t...

We report that by using crystal optics and laboratory-based X-ray source, one may perform easily X-ray Absorption Spectroscopy (XAS) state-of-the-art measurements at the U L3-edge. Our results are in excellent agreement with synchrotron-based studies for UO2, KUO3 and β-UO3. The laboratory scaled approach constitutes an inexpensive solution to the...

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...

The charge compensation mechanisms in U1-xGdxO2 and Th1-xGdxO2-x/2 have been systematically studied using X-ray Absorption Spectroscopy (XAS) upon gradually increasing the Gd content. Gd doped nuclear fuels are widely used for optimizing the fresh core neutronics, yet when Gd³⁺ is substituted into U⁴⁺ or Th⁴⁺ lattice position in UO2 or ThO2, respec...

In this paper, the development of a new positron lifetime spectrometer dedicated to high radioactive materials such as nuclear power plant’s mixed oxide fuels and structural steels is discussed. In the case of nuclear fuels, the behaviour of nanoscale defects, which are mainly created by the high radioactivity and high irradiation flux of neutrons...

The objective of this work was to study the molybdenum chemistry in UO2 based materials, known as SIMFUELS. These materials could be used as an alternative to irradiated nuclear fuels in the study of fission products behaviour during a nuclear severe accident. UO2 samples doped with 12 stable isotopes of fission products were submitted to annealing...

We report and discuss here the unambiguous uranium valence state determination on the complex compound [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] by using high-energy-resolution fluorescence detection-X-ray absorption near-edge structure spectroscopy (HERFD-XANES). The spectra at both U L3- and M4-edges confirm that all five nonequivalent U atoms are sol...

Many research programs have been carried out aiming to understand the fission products behaviour during a Nuclear Severe Accident. Most of these programs used highly radioactive irradiated nuclear fuel, which requires complex instrumentation. Moreover, the radioactive character of samples hinders an accurate chemical characterisation. In order to o...

We report here the direct experimental observation of the preferential xenon incorporation site in uranium dioxide and analyse how its incorporation evolves with the annealing temperature. We show that High Energy Resolution Fluorescence Detection X-ray Absorption Near Edge Structure in combination with first-principles calculations enable a precis...

Uranium-americium mixed oxides could be used as transmutation targets to lower Am inventory in spent nuclear fuels. Due to (241)Am activity, these materials are subjected to α-self-irradiation which provokes crystallographic disorder. Previous studies on U-Am mixed oxides gave first insight into α-radiation tolerance of these compounds, but have ne...

The MARS beamline at the SOLEIL synchrotron is dedicated to the characterization of radioactive material samples. One great advantage of the beamline is the possibility to characterize about 380 radionuclides by different X-ray techniques in the same place. This facility is unique in Europe. A wide energy range from around 3.5 keV to 36 keV K-edges...

Li2TiO3 ceramic is one of the promising solid breeding candidates for fuel generation in deuterium–tritium Fusion reactors. The Tritium (T) release characteristics consist of a complex combination of gas diffusion stages inside the solid. Considering that this ceramic will produce high concentration of gaseous transmutation products (3H and 4He) wh...

The Breeder Units (BUs) are the core and key components of the future deuterium-tritium Fusion reactors, performing the triple function of (1) tritium breeding starting from the nuclear reaction: 6Li + 1n  T + 4He + 4,8 MeV, (2) heat conversion and removal and (3) neutron and gamma-ray shielding. Considering the similarity in atomic mass with Trit...

First-principles calculations have been carried out to study the incorporation of rare gas atoms, with a focus on xenon, in titanium nitride. The density functional theory–generalized gradient approximation method has been used to calculate the formation energy of several point defects (vacancies, interstitials, divacancies, Frenkel pairs) and then...

Helium behavior in a sodium borosilicate glass (SON68) dedicated to the immobilization of high-level nuclear waste is examined. Two experimental approaches on nonradioactive glass specimens are implemented: pressurized helium infusion experiments and 3He ion implantation experiments. The temperature variation of helium solubility in SON68 glass was...

Oxide islands are observed on polycrystalline titanium nitride surface after annealing at 1000 °C for 10 h under secondary vacuum (PO2PO2 about 10−6 mbars). EBSD orientation mapping revealed that the growth of these islands is affected by the orientation of the underlying substrate grain. Based on the size and population density of the islands, the...

Dense TiN and TiC samples were prepared by hot pressing using micrometric powders. Xenon species (simulating rare gas fission products) were then implanted into the ceramics. The samples were annealed for 1 h at 1500 °C under several degraded vacuums with PO2 varying from 10−6 to 2×10−4 mbars. The oxidation resistance of the samples and their reten...

This work concerns the generation IV future nuclear reactors such as gas-cooled fast reactor (GFR) for which refractory materials as titanium nitride (TiN) are needed to surround fuel and act as a fission product diffusion barrier. This study is about Xe thermal behavior in sintered titanium nitride. Microstructure effects on Xe behavior have been...

First-principles calculations have been carried out to study the incorporation of rare gas atoms, with a focus on xenon, in titanium nitride. The density functional theory–generalized gradient approximation method has been used to calculate the formation energy of several point defects (vacancies, interstitials, divacancies, Frenkel pairs) and then...

Ce travail se place dans le cadre des réacteurs nucléaires de 4ème génération tels que les réacteurs à neutrons rapides et caloporteur gaz (RNR-G), pour lesquels des matériaux réfractaires comme le nitrure de titane (TiN) enroberont le combustible afin de permettre la rétention des produits de fission. Cette étude a porté sur le comportement thermi...

Titanium nitride has been proposed as a fission product barrier in fuel structures for gas cooled fast reactor (GFR) systems. The thermal migration of Cs was studied by implanting 800 keV 133Cs++ ions into sintered samples of TiN at an ion fluence of 5 × 1015 cm-2. Thermal treatments at temperatures ranging from 1500 to 1650 °C were performed under...