Carbon atom diffusion in monoclinic zirconia from bulk to the (1¯11) surface is studied by Density Functional Theory and atomic Kinetic Monte Carlo (KMC) calculations. The aim is to characterize the behaviour of 14C in one type of nuclear waste in its deep disposal site during a geological timescale at 50 °C. In bulk, diffusion coefficient expression Dbulk=1.6×10−3(cm2s)×e(−1.19 eVkBT) shows a very slow diffusion of 10−22 cm2/s at 50 °C. The carbon atom at the subsurface frontier in the bulk side prefers to diffuse towards the surface rather than staying in the bulk or moving laterally. Diffusion from subsurface to surface is accessible at 50 °C with a minimum migration energy calculated at 0.20 eV. On the surface, diffusion is more probable than in the bulk with a diffusion coefficient expression Dsurf=1.1×10−3(cm2s)×e(−0.76 eVkBT) equals to 10−15 cm2/s at 50 °C. Atomic KMC simulation shows one dimensional diffusion along an identified path on the surface.

In the framework of the geological repository of the used fuel claddings of pressurized water reactor, carbon behavior in bulk zirconium is studied by periodic Density Functional Theory calculations. The C interstitial sites were investigated and it was found that there are two possible carbon interstitial sites: a distorted basal tetragonal site and an octahedral site. There are four types of possible atomic jumps between them. After calculating the migration energies, the attempt frequencies and the jump probabilities for each possible migration path, kinetic Monte Carlo (KMC) simulations were performed to simulate carbon diffusion at the macroscopic scale. The results show that carbon diffusion in pure Zr bulk is extremely limited at the storage temperature (50 degrees C). Since there are defects in Zr bulk, in a second step, the effect of atomic vacancy was studied and it was proved that vacancies cannot increase carbon diffusion. (C) 2016 Elsevier B.V. All rights reserved.

The sorption of uranium onto ZrP2O7 in the presence of oxalic acid has been investigated as a function of temperature (20, 40 and 60 °C). Using several complementary analytical methods to characterize the solid surface, it has been shown that the oxalic acid interact with the zirconium diphosphate affecting its surface reactivity. A significant influence of temperature on the sorption reaction has been revealed in the batch experiments. Temperature dependence sorption data and microcalorimetric measurements have been used to determine enthalpy change associated to the sorption reaction. The results have shown that oxalic acid has an important effect on uranium sorption, which is more evident at 60 °C.

Herein, we describe the structural investigation of one possible uranyl binding site inside a nonstructured protein. This approach couples spectroscopy, thermodynamics, and theoretical calculations (DFT) and studies the interaction of uranyl ions with a phosphopeptide, thus mimicking a possible osteopontin (OPN) hydroxyapatite growth-inhibition site. Although thermodynamical aspects were investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) and isothermal titration calorimetry (ITC), structural characterization was performed by extended X-ray absorption fine structure (EXAFS) at the U LIII -edge combined with attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. From the vibrational and fluorescence spectra, several structural models of a UO2 (2+) /peptide complex were developed and subsequently refined by using theoretical calculations to fit the experimental EXAFS obtained. The structural effect of the pH value was also considered under acidic to moderately acidic conditions (pH 1.5-5.5). Most importantly, the uranyl/peptide coordination environment was similar to that of the native protein.

Synchrotron-based microprobe x-ray absorption spectroscopy (XAS) has been used to study the local atomic structure of chromium in chromia-doped uranium dioxide (UO2) grains. The specimens investigated were a commercial grade chromia-doped UO2 fresh fuel pellet, and materials from a spent fuel pellet of the same batch, irradiated with an average burnup of similar to 40 MW d kg(-1). Uranium L-3-edge and chromium K-edge XAS have been measured, and the structural environments of central uranium and chromium atoms have been elucidated. The Fourier transform of uranium L-3-edge extended x-ray absorption fine structure shows two well-defined peaks of U-O and U-U bonds at average distances of 2.36 and 3.83 angstrom. Their coordination numbers are determined as 8 and 11, respectively. The chromium Fourier transform extended x-ray absorption fine structure of the pristine UO2 matrix shows similar structural features with the corresponding spectrum of the irradiated spent fuel, indicative of analogous chromium environments in the two samples studied. From the chromium XAS experimental data, detectable next neighbor atoms are oxygen and uranium of the cation-substituted UO2 lattice, and two distinct subshells of chromium and oxygen neighbors, possibly because of undissolved chromia particles present in the doped fuels. Curve-fitting analyses using theoretical amplitude and phase-shift functions of the closest Cr-O shell and calculations with ab initio computer code FEFF and atomic clusters generated from the chromium-dissolved UO2 structure have been carried out. There is a prominent reduction in the length of the adjacent Cr-O bond of about 0.3 angstrom in chromia-doped UO2 compared with the ideal U-O bond length in standard UO2 that would be expected because of the change in effective Coulomb interactions resulting from replacing U4+ with Cr3+ and their ionic size differences. The contraction of shortest Cr-U bond is similar to 0.1 angstrom relative to the U-U bond length in bulk UO2. The difference in the local chromium environment between fresh and irradiated UO2 is discussed based on the comparison of quantitative structural information obtained from the two chromia-doped fuel samples analyzed.

Microstructural changes in a set of commercial grade UO2 fuel samples have been investigated using synchrotron based micro-focused X-ray fluorescence (μ-XRF) and X-ray diffraction (μ-XRD) techniques. The results are associated with conventional UO2 materials and relatively larger grain chromia-doped UO2 fuels, irradiated in a commercial light water reactor plant (average burn-up: 40MWdkg−1). The lattice parameters of UO2 in fresh and irradiated specimens have been measured and compared with theoretical predictions. In the pristine state, the doped fuel has a somewhat smaller lattice parameter than the standard UO2 as a result of chromia doping. Increase in micro-strain and lattice parameter in irradiated materials is highlighted. All irradiated samples behave in a similar manner with UO2 lattice expansion occurring upon irradiation, where any Cr induced effect seems insignificant and accumulated lattice defects prevail. Elastic strain energy densities in the irradiated fuels are also evaluated based on the UO2 crystal lattice strain and non-uniform strain. The μ-XRD patterns further allow the evaluation of the crystalline domain size and sub-grain formation at different locations of the irradiated UO2 pellets.

The effects of temperature and solvation on uranyl ion adsorption at the water/rutile TiO2(110) interface are investigated by Density Functional Theory (DFT) in both static and Born–Oppenheimer molecular dynamics approaches. According to experimental observations, uranyl ion can form two surface complexes in a pH range from 1.5 to 4.5. Based on these observations, the structures of the complexes at 293K are first calculated in agreement with vacuum static calculations. Then, an increase in temperature (293 to 425K) induces the reinforcement of uranyl ion adsorption due to the release of water molecules from the solvation shell of uranyl ion. Finally, temperature can modify the nature of the surface species.

The energy transfer at room temperature between Tb3+ and Eu3+ ions sorbed onto SrTiO3 powders is investigated, using Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS). Several published works deal with the energy transfer between two lanthanide ions in co-doped matrices but it is the first time that transfer processes between two lanthanide ions sorbed on a solid surface is reported. The results show that the energy transfer between sorbed Tb3+ and Eu3+ ions on strontium titanate is a non-radiative process and follows a dipole-dipole type interaction. Moreover, the higher the acceptor ions Eu3+ concentration, the more efficient the energy transfer.It is shown that no energy migration between the Tb3+ donor ions occurs. A formalism based on the model of Inokuti-Hirayama is used and allows one to fit the non-exponential Tb3+ fluorescence decay. It is thus possible to evaluate the critical radius (R-0) of the influence sphere of the sorbed Tb3+ ions. According to the previous works, two sorption sites are considered for the sorbed rare-earth. The calculated radii are similar to those obtained for other couples of donor-acceptor lanthanide ions reported in the literature. (C) 2012 Published by Elsevier B.V.

In the field of nuclear waste management, prediction of radionuclide migration through the geosphere has to take into account the effects of organic matter. This work deals with the effects of organic acids (citric and oxalic acid) on speciation of uranium(VI) sorbed onto zirconium diphosphate (ZrP2O7). Surface properties of zirconium diphosphate and its uranium(VI) sorption capability in the presence and absence of organic acids were previously studied. The preliminary study suggested that organic acids take part in the sorption equilibria. In order to understand the interactions between organic ligands (citrate or oxalate), uranium(VI) and zirconium diphosphate, a luminescence spectroscopy study was carried out. Luminescence measurements indicated that only one uranium(VI) surface complex is formed when citric or oxalic acid is present. Moreover, the total carbon content in the studied samples indicated that organic ligands remain on the surface when uranium(VI) sorption is carried out. Uranium sorption edges were then fitted with the FITEQL4.0 code [15] and the Constant Capacitance Model (CCM). Spectroscopy information was used to constraint the modeling. The best fit for the U(VI)/citrate/ZrP2O7 and U(VI)/oxalate/ZrP2O7 systems considered the formation of a ternary surface complex.

During XPS analysis, the soft X-ray-induced reduction of metals such as Cr(VI) and Ce(IV) in oxides has been reported in the literature and some mechanisms have been proposed to explain this phenomenon. The reduction of U(VI) by the beam during X-ray Photoelectron Spectroscopy has been already reported in the literature but only for U(VI) sorbed or precipitated onto solids with reducing properties (as micas or pyrites) for whose Fe(II) can also induce the reduction of U(VI), or onto TiO(2) whose the photocatalytic properties are well known. The objective of this paper is to investigate the effects of X-ray beam on U(VI) bulk compounds (UO(3), UO(2)(OH)(2), (UO(2))(2)SiO(4), UO(2)(CH(3)COO)(2) and UO(2)C(2)O(4)). Successive U4f, U5f, C1s XPS spectra were recorded and compared as a function of the irradiation time. The XPS photoreduction of U(VI) into U(IV) is only observed for uranyl compounds containing organic matter (uranyl acetate and uranyl oxalate). Considering the evolution of the C1s signal during the X-ray irradiation, a significant decrease of the C = O component simultaneously to the U(VI) reduction is observed, which suggests a desorption of CO or other volatile organic products from the solid surface. All these results on U(VI) bulk compounds indicate the important role of organic carbon species in the photoreduction process and to explain these observations, a photoreduction mechanism has been suggested. Copyright (C) 2010 John Wiley & Sons, Ltd.

Iodine is one of the most important fission products due to its high fission yield, significant radiobiological hazard and potential volatility. Its environmental and biological risks have been extensively studied in case of a severe reactor accident. Nevertheless, little information is available about iodine behaviour under normal Pressurize Water Reactor (PWR) operating conditions.The work reported explores the behaviour of different iodine species (I(-), I(3), I(2) HOI and IO(-)) during full power periods, transient periods (power reductions and depressurizations) and shutdowns. Thermodynamic calculations were conducted, and their results are compared with previous predictions and with the experimental data provided by nuclear power plants (NPP).Based on thermodynamic calculations and NPP feedback, it was concluded that iodine speciation depends primarily on the redox potential and water radiolysis.The experimental values confirm that the iodine ionic form I(-) is the major species during normal operation (I(2) < 2%) and shutdowns (I(2) < 9%).During shutdowns:High [I(2)] (20-40%) can be observed in the presence of fuel failures following an iodine spike during power or pressure variations. The fuel oxidation by radiolysis products can lead to I(2) formation inside the gap and its subsequent release through cladding defects.Once in the primary coolant, I(2) is transformed into I(-) or IO(3)(-)/IO(4)(-), depending on the water oxidation conditions.The lithium concentration and the primary coolant temperature seem to have a secondary influence on iodine speciation, while the existence of a redox potential threshold appears to be the main factor controlling the formation of volatile and non-volatile iodine forms.This paper summarizes the major results of the iodine thermodynamic studies and PWR feedback, permitting some possible recommendations for inclusion in the NPP guidelines in order to master iodine's behaviour. Future work is proposed. Redox potential measurements at high temperatures, coupled with thermodynamic estimations and radiolysis analysis, should be considered as useful tools to specify the optimal conditions for limiting iodine volatisation and I(2) absorption. (C) 2011 Elsevier Ltd. All rights reserved.

Density Functional Theory (DFT), based on both static and Born–Oppenheimer Molecular Dynamics approaches, has been used to investigate the effect of hydrogen bonds and temperature on the water monolayer adsorption on the rutile TiO2 (110) face. It was demonstrated that the difference between some previous theoretical results and experimental data is due to too slim slab thickness model and/or too small surface area. According to the present static calculations, water monolayer adsorbs molecularly on the five-fold titanium atoms of an optimised five-layer slab thickness, due to the stabilising lateral hydrogen bonds between molecules. From the molecular dynamics simulations, two adsorption mechanisms were described as a function of temperature. Finally, it was pointed out that the dynamics of water adsorption is strongly influenced by the structural model used. When temperature increases, the monolayer dissociates gradually. However, because of the periodic boundary conditions, the 1×1 surface unit needs to be extended to at least 2×5 to get an accurate representation of the monolayer dissociation ratio. In these conditions, this ratio is around 20%, 25% and 33% at 270, 350 and 425K, respectively.

Hydration of zirconium diphosphate (ZrP2O7) conduced to formation of active sites in solid/liquid interface. In ZrP2O7/NaClO4 0.5 M suspensions, active sites and their acidity constants are quite determined but the presence of some impurities is now studied. This work was conducted to determine the surface properties changes produced by oxalic and citric acid during the hydration process. Moreover the presence of organic acids with ZrP2O7 modified reveals an increase in uranium sorption constants. The zirconium diphosphate has been characterized using X-ray powder diffraction (XRD), Scanning electron microscopy (SEM) and Particle induced X-ray emission and Neutron (PIXE). Furthermore, the specific surface area, measured by the BET method, was 3.5 m2/g. The pH corresponding to the isoelectric point, determined by Zeta Potential measurements and mass titration was 3.6. The sites density calculated using titration curves was around of 5.37 s/nm2 for NaClO4 0.5 M, 13.71 s/nm2 for NaClO4 0.5 M/citric acid 0.1 M and 7.33 s/nm2 NaClO4 0.5 M/oxalic acid 0.1 M. The surface acidity constants and species distribution in surface were calculated by means of simulation of the titration curves with the FITEQL code (constant capacitance model), for ZrO and PO amphoteric sites of ZrP2O7. The uranyl sorption edge was determined for NaClO4 0.5 M. It spreads between pH 3 and 4.5 for complete sorption according to the previously published results. In the ZrP2O7–citrate modified surface, the uranyl sorption edge begin at pH 2 and was almost complete at pH 3.2 while ZrP2O7–oxalate modified surface edge started at 50% of sorption at pH of 1.5 and was complete at pH 3.

The hexacyanometallate family is well known in transition metal chemistry because the remarkable electronic delocalization along the metal-cyano-metal bond can be tuned in order to design systems that undergo a reversible and controlled change of their physical properties.We have been working for few years on the description of the molecular and electronic structure of materials formed with [Fe(CN) 6 ] n-building blocks and actinide ions (An = Th, U, Np, Pu, Am) and have compared these new materials to those obtained with lanthanide cations at oxidation state +III.In order to evaluate the influence of the actinide coordination polyhedron on the threedimensional molecular structure, both atomic number and formal oxidation state have been varied : oxidation states +III, +IV.EXAFS at both iron K edge and actinide L III edge is the dedicated structural probe to obtain structural information on these systems.Data at both edges have been combined to obtain a three-dimensional model.In addition, qualitative electronic information has been gathered with two spectroscopic tools : UV-Near IR spectrophotometry and low energy XANES data that can probe each atom of the structural unit : Fe, C, N and An.Coupling these spectroscopic tools to theoretical calculations will lead in the future to a better description of bonding in these molecular solids.Of primary interest is the actinide cation ability to form ioniccovalent bonding as 5f orbitals are being filled by modification of oxidation state and/or atomic number.

The purpose of this work is the study of the interaction mechanisms between U(VI) ions and SrTiO3 surfaces as a function of pH and temperature (25, 50, 75 and 90 degrees C) by coupling thermodynamic and spectroscopic approaches. First, the reactivity towards U(VI) for both surface sites of the strontium titanate ( Ti-O and Sr-O) has been investigated as a function of the temperature. The N-2-BET specific area was measured: 2.4 +/- 0.2 m(2) g(-1). The surface site density has been determined from potentiometric titrations (6 sites/nm(2) for each site Ti-O and Sr-O). The potentiometric titration data have been simulated, for each temperature, using the FITEQL 4.0 software and the constant capacitance model, taking into account both protonation of the Sr-CH surface sites and deprotonation of the Ti-OH ones (one pK model). The intrinsic strontium protonation constant increases with an increasing temperature, while the titanate deprotonation one decreases. Moreover, both enthalpy and entropy changes corresponding to the surface acid-base reactions have been evaluated using the van't Hoff relation. The uranium(VI) ions are sorbed onto SrTiO3 Surfaces in the 0.5-5.0 pH range with an initial cation concentration equal to 10(-4) M. The U(VI) surface complexes were identified by using time-resolved laser-induced fluorescence spectroscopy (TRLFS). For all the Studied samples, the fluorescence spectra and the corresponding lifetime values do not change with the pH and the temperature. Two U(VI) complexes sorbed onto SrTiO3 were detected and the corresponding lifetimes are 60 +/- 5 and 12 +/- 2 mu s whatever the temperature (25, 50, 75 and 90 degrees C). The sorption edges were simulated with the FITEQL 4.0 code. The sorption equilibrium constants of the U(VI)/SrTiO3 system between 25 and 90 degrees C were obtained with the constant capacitance model (CCM), considering two reactive surface sites. According to the spectroscopic characterization, two types of surface complexes, namely [( SrOH)( TiOH)UO2](2+) and [( TiOH)( TiO)UO2](2+), were considered. Finally, enthalpy (Delta H-r degrees) and entropy (Delta S-r degrees) changes were calculated from the temperature-dependent sorption constants, by the application of the van't Hoff formalism. The formation of the [( SrOH)(--TiOH)UO2](2+) surface complex was found to present an endothermic character associated to an increase in the disorder of the system. On the contrary, the formation of the [( TiOH)( TiO)UO2](2+) surface complex led to an exothermic process with only a slight increase in the disorder of the system. (C) 2009 Elsevier Inc. All rights reserved.

Protactinium occupies a key position in the actinide series between thorium and uranium. In aqueous acidic solution, it is stable at oxidation state (V), occurring either as an oxocation or as a naked ion, depending on the media. Very few structural information on the hydration sphere of Pa(V) in acidic medium is available, in particular in hydrofluoric acid. Combined EXAFS and theoretical calculations have been used in this work to characterize the protactinium coordination sphere at various HF concentrations. The correlation of the XAFS data with quantum chemical calculations provides complementary structural and electronic models from ab initio techniques. At HF concentrations from 0.5 to 0.05 M, both theoretical calculations and EXAFS data suggest that the protactinium coordination sphere is mainly composed of fluoride ions. At the lowest HF concentration, the occurrence of a monooxo bond is observed with EXAFS, in agreement with the literature. A comparison of these data with related neptunium(V) and plutonium(V) diooxocations in perchloric acid is also presented.

This work presents an investigation of the interaction mechanisms between uranyl ions and a solid phosphate, the zirconium oxophosphate: Zr2O(PO4)2. Both thermodynamic and structural points of view are developed. Indeed, prior to any simulation of the retention data, it is necessary to precisely characterize the system under study in order to gain information at a molecular scale. First, the intrinsic surface properties of this synthetic compound have been investigated for different temperatures ranging from 25 to 90 degrees C. Mass and potentiometric titrations show that the surface site density remains constant between 25 and 90 degrees C, while the experimental point of zero charge slightly decreases from 4.8 to 4.5 with an increasing temperature. The potentiometric titration data are simulated, for each temperature, using the constant capacitance model and taking into account two surface sites ([TRIPLE BOND]ZrO and [TRIPLE BOND]PO) with a total surface site density equal to 7.0 sites nm(-2). For both reactive sites, the intrinsic protonation constants do not change with the temperature, while the deprotonation ones increase. These results led to the determination of the associated enthalpy and entropy changes according to the van't Hoff relation. Second, the speciation of U(VI) at the solid/solution interface has been studied using two complementary spectroscopic techniques probing the sorbed uranyl ions: time-resolved laser-induced fluorescence spectroscopy (TRLFS) and X-ray absorption spectroscopy (EXAFS). The substrate presents two different reactive surface sites against uranium retention, which are constituted by the oxygen atoms of the surface PO4 groups and the oxygen atoms linked to the zirconium atoms. Two inner-sphere complexes are thus present on the substrate, their relative proportion depending on the pH value of the suspension. The effects of the temperature (25-90 degrees C) on the surrounding uranium were checked using the TRLFS technique. The uranyl sorption constants onto the Zr2O(PO4)2 substrate were determined taking into account the structural investigation. The surface complexation modeling was performed using the constant capacitance model included in the FITEQLv4.0 code. The four adsorption edges obtained at 25, 50, 75, and 90 degrees C were simulated. The modeling of these experimental data was realized considering two surface complexes (([TRIPLE BOND]ZrOH)2UO(2+)2, ([TRIPLE BOND]PO)2UO2) according to the structural investigation. The constant value associated with the ZrO site does not change with the temperature, while the one corresponding to the PO site increases. Finally, the enthalpy and entropy changes associated with the uranyl sorption constants have been determined using the van't Hoff relation.

Periodic DFT calculations using plane waves basis sets with the GGA formalism were performed in order to study the behavior of the UO22+ uranyl ion at the water–Ni(111) interface. First, the adsorption of water molecules interacting with an optimized surface model has been studied. At low coverage, isolated water molecules adsorb preferentially on top of the surface nickel atoms, the plane defined by the H2O molecule being almost parallel to the surface. When the water coverage increases from 1/9ML to 2/3ML (ML = monolayer coverage), hydrogen bonds are created leading to the formation of water hexamers, as suggested experimentally on Ni(111) and other metallic surfaces [A. Michaelides, A. Alavi, D.A. King, Physical Review B 69 (2004) 113404. M. Nakamura, M. Ito, Chemical Physics Letters 325 (2000) 293–298]. Higher water coverage induces the formation of an additional water layer physisorbed over the cyclic hexamers. In a second step, the behavior of UO22+ with respect to the hydrated Ni(111) face has been investigated. Evidence that two adsorption modes can take place was put forward: a first one with an outer sphere adsorption mechanism, where two water molecules of the uranyl ion first hydration shell are shared with four water hexamers, and a second one through a strong Ni–O−yle bond formation. Even though the second surface complex is energetically the most stable, the necessary activation energy to reach it makes it improbable.

Uranium monocarbide (UC) or ternary alloys are considered to be possible candidates for future nuclear fuels. Although the crystallographic and electronic structure of UC has been addressed in past investigations, discrepancies in the literature data have fostered a new investigation of the UC phase. We report here a reinvestigation of the UC phase by complementary X-ray spectroscopy and quantum chemical calculations. A combination of X-ray powder diffraction and extended X-ray absorption fine structure analysis at the uranium L-III edge led to the crystallographic determination of the UC phase of the NaCl type. For electronic structure investigation, a combination of uranium X-ray absorption near-edge spectroscopy at the L-III edge and at the N-IV,N-V edges with quantum chemical calculations allowed us to define the evolution of the metal charge in comparison with metallic uranium on the one hand and uranium dioxide on the other hand.