Actinide Oxides Research Articles

Probing the local structure of nanoscale actinide oxides: a comparison between PuO2 and ThO2 nanoparticles rules out PuO2+x hypothesis.

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO2 and ThO2 nanoparticles displayed as dispersed colloids, nanopowders, or nanostructured oxide powders allow to establish a clear relationship between the size of the nanocrystals constituting these oxides and their corresponding An(iv) local structure investigated by EXAFS spectroscopy. Particularly, the first oxygen shell of the probed An(iv) evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu–O shell on the Fourier transformed EXAFS signal of the PuO2 samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO2+x species.

Unraveling the structural stability and the electronic structure of ThO2 clusters.

Unraveling the correlations between the geometry, the relative energy and the electronic structure of metal oxide nanostructures is crucial for a better control of their size, shape and properties. In this work, we investigated these correlations for stoichiometric thorium dioxide clusters ranging from ThO2 to Th8O16 using a chemically-driven geometry search algorithm in combination with state-of-the-art first principles calculations. This strategy allows us to homogeneously screen the potential energy surface of actinide oxide clusters for the first time. It is found that the presence of peroxo and superoxo groups tends to increase the total energy of the system by at least 3.5 eV and 7 eV, respectively. For the larger clusters, the presence of terminal oxygen atoms increases the energy by about 0.5 eV. Regarding the electronic structure, it is found that the HOMO-LUMO gap is larger in systems containing only bridging oxygen atoms (∼2-3.5 eV) than for systems containing oxo groups (∼1-3 eV), peroxo groups (∼0-2 eV), and superoxo groups (∼0-1 eV). Furthermore, while the LUMO is always dominated by thorium orbitals, the composition of the HOMO changes in the presence or the absence of oxo, peroxo and/or superoxo groups: in the presence of peroxo groups, it is dominated by thorium orbitals, in all other cases, it is dominated by oxygen orbitals, and is rather localized in the presence of terminal oxo or superoxo groups. These correlations are of great interest for synthesizing clusters with tailored properties, especially for applications in the field of nuclear energy and heterogeneous catalysis.

Characterization of modified murataite based ceramics as a perspective hosts for actinides, fission, and corrosion products of HLW

Murataite-based ceramics containing 10 wt% rare earth (REs = La, Ce, Nd, Ho) or actinide (An = Th, U) oxides were produced in a resistive furnace at a temperature of 1500 °C. The samples were composed of major murataite-type phases and minor perovskite, crichtonite, and zirconolite/pyrochlore. Light (Ce-group) REs are concentrated in the perovskite whereas Nd and Ho are mainly accommodated in the murataite. U and Th enter predominantly murataite. Ce and U regardless from their oxidation state in the batch (Ce2O3/CeO2, UO2/UO3) exist in the murataite as Ce(III) or U(IV) with some contribution due to U(V) for the uranium-doped sample.

Nanoscale oxygen defect gradients in UO2+x surfaces

Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in [Formula: see text] surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first-principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering additional insight into defect formation pathways and kinetics during [Formula: see text] surface oxidation.

Extreme multi-valence states in mixed actinide oxides

To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U1−yMyO2±x materials is essential. These compounds show complex chemical properties, originating from the fact that actinides and rare earths may occur with different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce changes in the oxygen stoichiometry, with dramatic effects on the properties of the fuel. First studies on U1−yAmyO2±x indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of these compounds, over a wide compositional domain, by combining XRD, XAS and Raman spectroscopy. We provide evidences of the co-existence of four different cations (U4+, U5+, Am3+, Am4+) in U1−yMyO2±x compounds, which nevertheless maintain the fluorite structure. Indeed, we show that the cationic sublattice is basically unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice.

Thermal and diffusional properties of (Th,Np)O2 and (U,Np)O2 mixed oxides

Molecular dynamics (MD) simulations were performed to determine thermal expansion, thermal conductivity and diffusional properties of Th1-xNpxO2 and U1-xNpxO2 mixed oxides (MOX). The linear thermal expansion coefficient (LTEC) of Th1-xNpxO2 MOX increases with NpO2 concentration, while that of U1-xNpxO2 MOX decreases. The degradation of thermal conductivity in U1-xNpxO2 is predicted to be far less significant compared to Th1-xNpxO2 because defect-phonon scattering is less pronounced in U1-xNpxO2. Addition of 6.25 atom% NpO2 in ThO2 degrades the thermal-conductivity of ThO2 by 24.0–12.5% in the 750–1000 K temperature range whereas up to 50 atom% NpO2 doping in UO2 degrades the thermal-conductivity only by 13–2.3%. Analytical expressions have been derived that describe the predicted lattice parameters and thermal conductivities over the full temperature and compositional ranges. Oxygen diffusivity is higher in UO2 and NpO2 compared to ThO2. With the addition of Th4+ or U4+ to NpO2, the diffusivity decreases due to the increase in the migration barriers caused by the larger ionic radius of Th4+ or U4+. The addition of Np4+ to ThO2 or UO2 decreases oxygen diffusion due to the preference for the oxygen vacancy to be adjacent to Np4+, even though the migration barriers decrease due to the smaller size of Np4+. Our MD calculated binding energies of the oxygen vacancy can be correlated with the isolated oxygen Frenkel pair defect energies (O-FPisolated) of individual actinide oxides calculated using same interatomic potential set. Moreover, MD calculated oxygen vacancy binding energy is consistent with that calculated using density functional theory.

Sample seal-and-drop device and methodology for high temperature oxide melt solution calorimetric measurements of PuO2

Thermodynamic properties of refractory materials, such as standard enthalpy of formation, heat content, and enthalpy of reaction, can be measured by high temperature calorimetry. In such experiments, a small sample pellet is dropped from room temperature into a calorimeter operating at high temperature (often 700 °C) with or without a molten salt solvent present in an inert crucible in the calorimeter chamber. However, for hazardous (radioactive, toxic, etc.) and/or air-sensitive (hygroscopic, sensitive to oxygen, pyrophoric, etc.) samples, it is necessary to utilize a sealed device to encapsulate and isolate the samples, crucibles, and solvent under a controlled atmosphere in order to prevent the materials from reactions and/or protect the personnel from hazardous exposure during the calorimetric experiments. We have developed a sample seal-and-drop device (calorimetric dropper) that can be readily installed onto the dropping tube of a calorimeter such as the Setaram AlexSYS Calvet-type high temperature calorimeter to fulfill two functions: (i) load hazardous or air-sensitive samples in an air-tight, sealed container and (ii) drop the samples into the calorimeter chamber using an "off-then-on" mechanism. As a case study, we used the calorimetric dropper for measurements of the enthalpy of drop solution of PuO2 in molten sodium molybdate (3Na2O·4MoO3) solvent at 700 °C. The obtained enthalpy of -52.21 ± 3.68 kJ/mol is consistent with the energetic systematics of other actinide oxides (UO2, ThO2, and NpO2). This capability has thus laid the foundation for thermodynamic studies of other Pu-bearing phases in the future.

Thermal transport study in actinide oxides with point defects

Abstract We use a molecular dynamics simulation to explore thermal transport in oxide nuclear fuels with point defects. The effect of vacancy and substitutional defects on the thermal conductivity of plutonium dioxide and uranium dioxide is investigated. It is found that the thermal conductivities of these fuels are reduced significantly by the presence of small amount of vacancy defects; 0.1% oxygen vacancy reduces the thermal conductivity of plutonium dioxide by more than 10%. The missing of larger atoms has a more detrimental impact on the thermal conductivity of actinide oxides. In uranium dioxide, for example, 0.1% uranium vacancies decrease the thermal conductivity by 24.6% while the same concentration of oxygen vacancies decreases the thermal conductivity by 19.4%. However, uranium substitution has a minimal effect on the thermal conductivity; 1.0% uranium substitution decreases the thermal conductivity of plutonium dioxide only by 1.5%.

First-principles investigation of the bulk properties of americium dioxide and sesquioxides

We report an investigation of the ground-state properties of americium dioxide and its sesquioxides using first-principles calculations. In order to take into account strong 5$f$ electronic correlations, we apply the generalized gradient approximation (GGA)+$U$ as well as the hybrid functionals to these compounds. We present a systematic study of several bulk properties such as structural, elastic, energetic, electronic, and magnetic properties as a function of the effective Coulomb $U$ and exchange $J$ GGA+$U$ parameters in the range of 0.0-8.0 and 0.00-0.75 eV, respectively. The values ($U, J$) = (6.00 eV, 0.75 eV) are those providing a correct description of bulk properties by comparison to the available experimental data. We discuss the effect of loss of symmetry caused by DFT+$U$ on several properties. We also discuss the effect of the spin-orbit coupling on these properties. Furthermore, we highlight that in order to reach the ground state of americium dioxide and sesquioxides using GGA+$U$ the monitoring of the occupation matrices of 5$f$ correlated orbitals is crucial. Our computational scheme provides reliable and accurate results concerning several bulk properties that have not been studied experimentally yet. For instance, our elastic constants calculated for AmO$_ 2$ follow the same trend as those of other actinide oxides (UO$_2$ , NpO$_2$ , and PuO$_2$) and can be used as reference. We have also computed the electronic and static dielectric constants, as well as the Am and O Born effective charge of americium dioxide. Finally, we find the A-type hexagonal structure to be the most stable structure at low temperature for americium sesquioxides. For this phase, we predict the internal structural parameters as well as the bulk modulus. We provide data concerning the formation enthalpy of the Am$_2$ O$_3$ bcc cubic structure.

Quantum-Mechanical Investigation of the Structures and Energetics of Uranium and Plutonium Incorporated into the Magnetite (Fe3O4) Lattice

Understanding the behavior of radionuclides is key to designing and implementing effective waste storage and remediation schemes. Incorporation into minerals is one process that may reduce the mobility of these contaminants. This study uses quantum-mechanical modeling to evaluate the incorporation of U and Pu into magnetite (Fe3O4), a common mineral and steel corrosion product. The incorporation from solid and aqueous sources (e.g., PuO2(s) and UO22+(aq)) with various oxidation states (Pu +3; U and Pu +4, +5, +6) is explored. Charge balancing is achieved via Fe lattice vacancies. The incorporation energies (ΔEinc) depend strongly on the stability of different actinide oxide phases and the hydration energies of aqueous species (e.g., ΔEinc of 1.38 and 2.88 eV for U5+ from δ-U2O5(s) and UO2+(aq), respectively). The calculated bonding environment of incorporated U (U–Oax = 2.16–2.24 A, CN = 2; U–Oeq = 2.30–2.34 A, CN = 4) aligns with the range of experimental results. Analysis of incorporation energies as we...

Comparison of thermomechanical properties of (Ux,Th1-x)O2, (Ux,Pu1-x)O2 and (Pux,Th1-x)O2 systems

The thermomechanical properties of mixed actinide oxide and its dependence on actinide composition are of significant interest for the safety in reactor design and operation. In this study, the elastic properties of (Ux,Th1-x)O2, (Ux,Pu1-x)O2 and (Pux,Th1-x)O2 solid solutions for 50-50 cation composition and thermal conductivity of (Pux,Th1-x)O2 system, for x = 0.3 and 0.5, have been investigated in the temperature range from 300 to 1500 K using Molecular dynamics (MD) simulations. Compared to pure oxides, reduced thermal conductivity is reported for (Pux,Th1-x)O2 mixture due to non-uniform cation sublattice. The degree of reduction in conductivity is the largest in (Pux,Th1-x)O2 than that of previously published (Ux,Th1-x)O2 and (Ux,Pu1-x)O2. This is attributed to the largest mismatch in lattice parameter between PuO2 and ThO2. In general, the elastic properties, for all pure and mixed oxide fuels, linearly decrease with the increase in temperature, and this is commonly observed also in other studies. The rate of change in elastic modulus (ε) with temperature follows the trend, for pure oxides: εPuO2>εUO2>εThO2 and for mixed oxides: εUPuO2>εUThO2>εPuThO2. In most cases, for (Ux,Th1-x)O2 and (Ux,Pu1-x)O2, the elastic modulus is similar to the linear interpolation between the corresponding pure oxides. However, for (Pux,Th1-x)O2, significant deviation from the linearity is observed and that is justified by differences in oxygen mobility in the mixed oxide fuels. In addition, for all oxides, except Pu0.5Th0.5O2, our results predict εB>εY>εG where B, Y and G being the bulk elastic properties. A calculation of elastic modulus at 0 K for all oxides is presented.

Actinide molecular ion formation in collision/reaction cell of triple quadrupole ICP-MS/MS and its application to quantitative actinide analysis

A study for the quantitative actinide analysis by using a triple quadrupole ICP-MS/MS, Agilent 8900, was carried out. We confirmed the formation percentages of ionic species of each actinide in several kinds of collision/reaction cell modes. It was found that the dominant ions in oxygen gas mode were oxide ions, and the ratio of monoxide ion/dioxide ion increases with atomic number in the case of same oxygen gas flow rate. The percentages of actinide ion and oxide ions depend on the oxygen gas flow rate, and the number of oxygen attached on actinides increases with the oxygen gas flow rate.

Development of a Mathematical Model for Denitration of Actinide Nitrates under the Action of UHF Radiation

Physicochemical processes occurring in a UHF furnace were analyzed. The information structure of the process as a management object was constructed. A system of differential equations describing the processes that occur in a UHF furnace in the course of evaporation of actinide nitrates was constructed. A mathematical model of preparation of actinide oxide powders by denitration in a UHF furnace was developed. It allows the calculation experiments to be performed and the operation of the installation and control systems to be optimized.

The interaction of molecular hydrogen with α-radiolytic oxidants on a (U,Pu)O2 surface

In order to assess the impact of α-radiolysis of water on the oxidative dissolution of spent fuel, an un-irradiated, annealed MOX fuel pellet with high content of Pu (∼24 wt%), and a specific α-activity of 4.96 GBq/gMOX, was leached in carbonate-containing solutions of low ionic strength. The high Pu content in the pellet stabilizes the (U,Pu)O2(s) matrix towards oxidative dissolution, whereas the α-decays emitted from the surface are expected to produce ∼3.6 × 10−7 mol H2O2/day, contributing to the oxidative dissolution of the pellet.Two sets of leaching tests were conducted under different redox conditions: Ar gas atmosphere and deuterium gas atmosphere. A relatively slow increase of the U and Pu concentrations was observed in the Ar case, with U concentrations increasing from 1·10-6 M after 1 h to ∼7 × 10-5 M after 58 days.Leaching under an atmosphere starting at 1 MPa deuterium gas was undertaken in order to evaluate any effect of dissolved hydrogen on the radiolytic dissolution of the pellet, as well as to investigate any potential recombination of the α-radiolytic products with dissolved deuterium. For the latter purpose, isotopic analysis of the D/H content was carried out on solution samples taken during the leaching. Despite the continuous production of radiolytic oxidants, the concentrations of U and Pu remained quite constant at the level of ∼3 × 10-8 M during the first 30 days, i.e. as long as the deuterium pressure remained higher than 0.8 MPa. These data rule out any oxidative dissolution of the pellet during the first month.The un-irradiated MOX fuel does not contain metallic ε-particles, hence it is mainly the interaction of radiolytic oxidants and dissolved deuterium with the surface of the mixed actinide oxide that causes the neutralization of the oxidants. This conclusion is supported by the steadily increasing levels of HDO measured in the leachate samples.

Laser‐induced oxidation of UO2: A Raman study

AbstractA reliable method to probe and characterise the oxidation of actinide oxides by means of Raman spectroscopy is introduced. The present so‐called Raman laser heating method enables studying the behaviour of various compounds at high temperatures and under a given atmosphere with the unique alteration of a small amount of sample, which is certainly advantageous in terms of safety when handling hazardous or radioactive materials. The approach is based on a dual use of the laser beam, which is at the same time employed as excitation source for the Raman analysis and as heating source, by exploiting the possibility to vary the beam power density reaching the sample surface. A high laser power density can lead to a significant increase of the analyte surface temperature by up to several hundred of degrees. A sufficiently low power density allows us to subsequently acquire the corresponding Raman spectrum at the same point without distorting the measurement. In this work, UO2 powder has been subjected to Raman laser heating in air as a proof of this method's applicability, attaining a sequential acquisition of the characteristic Raman spectra of the different oxides involved in the oxidation from UO2 to U3O8. The temperature at which such sequence of phase transformations started to occur was estimated to be around (560 ± 40) K. The temperature at the sample surface was estimated from the Stokes/anti‐Stokes intensities ratio, using a similar set‐up to that used in the Raman laser heating experiments. These results are particularly appealing in remote analyses, like those required in the study of nuclear fuel and nuclear waste.

Ligand induced shape transformation of thorium dioxide nanocrystals.

Nanocrystals (NCs) with size and shape dependent properties are a thriving research field. Remarkable progress has been made in the controlled synthesis of NCs of stable elements in the past two decades; however, the knowledge of the NCs of actinide compounds has been considerably limited due the difficulties in handling them both experimentally and theoretically. Actinide compounds, especially actinide oxides, play a critical role in many stages of the nuclear fuel cycle. Recently, a non-aqueous surfactant assisted approach has been developed for the synthesis of actinide oxide NCs with different morphologies, but an understanding of its control factors is still missing to date. Herein we present a comprehensive study on the low index surfaces of thorium dioxide (ThO2) and their interactions with relevant surfactant ligands using density functional calculations. A systematic picture on the thermodynamic stability of ThO2 NCs of different sizes and shapes is obtained employing empirical models based on the calculated surface energies. It is found that bare ThO2 NCs prefer the octahedral shape terminated by (111) surfaces. Oleic acid displays selective adsorption on the (110) surface, leading to the shape transformation from octahedrons to nanorods. Other ligands such as acetylacetone, oleylamine, and trioctylphosphine oxide do not modify the equilibrium shape of ThO2 NCs. This work provides atomic level insights into the anisotropic growth of ThO2 NCs that was recently observed in experiments, and thus may contribute to the controlled synthesis of actinide oxide NCs with well-defined size and shape for future applications.

A low-temperature synthesis method for AnO2 nanocrystals (An = Th, U, Np, and Pu) and associate solid solutions

Actinide oxalate decomposition under hot compressed water is proposed as a milder production route for nanometric sized (mixed) actinide oxides.

Synthesis and characterisation of PuPO4 - a potential analytical standard for EPMA actinide quantification

Transmutation nuclear fuels contain weight percentage quantities of actinide elements, including Pu, Am and Np. Because of the complex spectra presented by actinide elements using electron probe microanalysis (EPMA), it is necessary to have relatively pure actinide element standards to facilitate overlap correction and accurate quantitation. Synthesis of actinide oxide standards is complicated by their multiple oxidation states, which can result in inhomogeneous standards or standards that are not stable at atmospheric conditions. Synthesis of PuP4 results in a specimen that exhibits stable oxidation-reduction chemistry and is sufficiently homogenous to serve as an EPMA standard. This approach shows promise as a method for producing viable actinide standards for microanalysis.

Electronic structure of mixed caesium actinide oxides Cs2AnO4 (An = U, Np, Pu, Am)

Relativistic multireference CASSCF/CASPT2 calculations have been performed on the mixed caesium actinide oxide molecules Cs2AnO4 with An = U, Np, Pu and Am. Probing the lowest-energy spin multiplicities the spin-orbit-free (SF) ground and low-energy excited states have been evaluated and characterised. After optimizing the molecular geometries of the SF ground states further calculations have been performed taking into account spin-orbit (SO) coupling. The SO ground and vertical low-lying excited states have been characterised.

Mechanical behaviour of porous lanthanide oxide microspheres: Experimental investigation and numerical simulations

Actinide oxide microspheres are considered as promising substituents to powder precursors for the production of ceramic pellets of nuclear fuel or targets. Porous microspheres of sub-millimetric size are synthesised using the Weak Acid Resin process. Controlling their microstructure and their mechanical properties is essential to predict the microstructure of green compacts and sintered pellets. Here, cerium and gadolinium are used to mimic actinides as metal cation. Single microspheres are crushed experimentally using a micropress in a Scanning Electron Microscope (SEM) to investigate their mechanical properties and visualise their fracture behaviour. The results are compared to numerical simulations based on the Discrete Element Method (DEM). In DEM, a microsphere is modelled as an assembly of bonded spheres representing aggregates. Bonds may fracture in tension or shear. A limited number of material parameters (aggregate elastic modulus, bond strength) are sufficient for the accurate simulation of the fracture behaviour of a microsphere.