Nuclear fuels and materials present special problems to atomistic-scale modeling. At a metal-metal-oxide interface, the metal centers are charged on the oxide side, but neutral on the metallic side. The intimate contact necessitates that atomistic models for these materials be both compatible and consistent with one another at some level. A new "fragment’’ Hamiltonian (FH) model, at the atomistic level, is presented that reduces qualitatively to existing, successful models for metals, such as the embedded atom method, and ceramics, such as the charge equilibration models. Moreover, the FH model possesses both electron hopping and fundamental gaps that appear as separate terms in a generalized embedding function. The electron hopping contributions come from both one-electron and two-electron sources. These contributions appear as a result of the FH point of view, rather than being postulated. The model obeys certain wellknown theoretical limits that come from the nonlinearity of electron hopping processes as the volume of a crystal is changed. The generalized notion of embedding entails two variables instead of one. The ability to account for multiple charge states in the cations leads to the capability within the model to distinguish the qualitative differences among metallic, ionic, and covalent bonding environments. The details of all of these energies, among with fragmentfragment interactions, combine to determine the state of the atom in the material.

By contracting with the Parr Instrument Company and Bruker Optical Systems, we have developed a system for continuous monitoring of hydrothermal and room temperature reactions by Raman spectroscopy. Using the uranyl peroxide cage cluster {[UO2(O2)(OH)]16[UO2(OM)2]4}24- (denoted {U20R}) and a coordination polymer made from uranyl ions and 4,4’-biphenyldicarboxylate as model systems, we demonstrate the spectroscopically observable changes associated with reaction progress and crystallization.

Nonresonant inelastic x-ray scattering (NIXS) measurements probing the 5d → 5f electronic transitions at the uranium O4,5 edges in a localized electron system such as UO2 show a good agreement with many-electron atomic spectral calculations. The higher multipole spectra are split into two peaks that can be assigned as the 5d5/2 and 5d3/2 structures, despite the large electrostatic interactions. A new sum rule for the branching ratio of spin-orbit split core levels in electric multipole spectra, which generalizes the sum rule for dipole transitions in x-ray absorption spectroscopy, allows for a systematic analysis of the NIXS spectra. The branching ratio is linearly proportional to the expectation value of the angular part of the spin-orbit interaction in the initial state, where the rank of the multipole determines the coefficient of the linear dependence. This spin-orbit sum rule can be an important diagnostic tool for high-energy spectroscopies.

Synchrotron-based X-ray techniques are used increasingly to characterize actinide element speciation in heterogeneous media related to nuclear waste disposal safety. Especially techniques offering added temporal, spatial and energy resolved information are advancing our understanding of f-element physics and chemistry in general and of actinide element waste disposal in particular. Examples of investigations of uranium containing systems using both highly (energy) resolved X-ray emission spectroscopy (HRXES) techniques and spatially resolved techniques with focused X-ray beams are presented in this paper: polarization dependent partial fluorescence yield X-ray absorption near edge structure (PD-PFY-XANES) spectroscopic studies of a single Cs2UO2Cl4 crystal, which experimentally reveal a splitting of the σ, π, and δ components of the 6d valence states [1], and characterization of UO2/Mo thin films prepared on different substrates using a combination of techniques (2D and 3D micro- and nano-X-ray fluorescence, XANES and both holographic and ptychographic tomography).

In this study, sintered pellets were prepared from Zircaloy-2 oxide and UO2 as a parameter of content ratio (Zr contents were 0, 24.3, 49.0, 73.4, and 97.9 at% in metal). The sintered pellets were heated in 5%H2/Ar gas. UO2 pellets underwent simple thermal expansion caused by thermal vibration while Zircaloy-2 oxide pellets underwent thermal expansion and volume change with phase transformation. Finally, the 24.3, 49.0, and 73.5 at%Zr-UO2 pellet specimens showed both phenomena. However, phase transformation temperatures were lower than that of Zircaloy-2 oxide, and volume changes were much smaller. X-ray diffraction patterns obtained after thermal expansion measurements showed that the 24.3 at%Zr-UO2 specimen contained tetragonal and cubic (Zr, U)O2 while the 73.5 at%Zr-UO2 specimen contained mainly monoclinic ZrO2.

Results for a radiolysis model sensitivity study of radiolytically produced H2O2 are presented as they relate to Spent (or Used) Light Water Reactor uranium oxide (UO2) nuclear fuel (UNF) oxidation in a low oxygen environment. The model builds on previous reaction kinetic studies to represent the radiolytic processes occurring at the nuclear fuel surface. Hydrogen peroxide (H2O2) is the dominant oxidant for spent nuclear fuel in an O2-depleted water environment. The most sensitive parameters have been identified with respect to predictions under typical conditions. As compared with the full model with about 100 reactions, it was found that only 30 to 40 of the reactions are required to determine [H2O2] to one part in 10–5 and to preserve most of the predictions for major species. This allows a systematic approach for model simplification and offers guidance in designing experiments for validation.

Natural samples of soil, sediment and natural water were collected in the “Parc National du Mercantour” in France. Soil and sediment samples were studied to better understand the behaviors of radionuclides (RNs) in different natural compartments. Considering 137Cs and 241Am activities in depth (measured by α- and γ-spectrometries), two types of sediment profiles can be distinguished depending on the origin (Chernobyl accident or atmospheric nuclear weapon tests). Due to difficulties in modeling the dispersion of those RNs in natural samples, even in a protected area, semi-synthetic studies were conducted. Eu(III) was used as an analogue of Am(III). Eu behavior in water was studied by EXAFS and compared to speciation diagrams drawn in similar chemical conditions. Eu is mainly complexed by carbonate and phosphate ions. The mean Eu-O distance (2.46 Å) obtained by EXAFS is in agreement with predominant solid species determined by speciation diagrams and previous published studies.

Atomistic modeling is used to study the role of different alloying additions to metallic U-Zr nuclear fuels in terms of their ability to reduce lanthanide migration to the outer surface of the fuel and thus reduce their interaction with cladding. The Bozzolo-Ferrante-Smith (BFS) method for alloys is used to examine the behavior of each addition, the resulting phase structure, and the evolution of the fuel surface. Different behaviors are observed for each of the additives (In, Tl, Ga, Sb, Pd), all a result of the competition between the formation of bulk precipitates and the tendency of each additive to segregate to the surface. For each case, characteristic temperatures are determined indicating the range of temperatures in which each additive performs a different role. Sb and Pd additives are determined to be the most effective additions, properly balancing their ability to bind lanthanides in the fuel with their own segregating tendencies.

Many metallic actinide systems host partially filled 5f electrons in the low-energy spectrum. Consequently, they exhibit diverse quantum mechanical phenomena such as magnetism, superconductivity, a mysterious hidden-order phase, or heavy-fermion behavior. Here we present results of a unified theoretical method based on the self-consistent GW formalism for the electronic many-body self-energy. We calculate the dynamic electronic correlation spectra starting from materials specific first-principles electronic band-structure. In particular, we present results for four isostructural intermetallic actinides PuCoIn5, PuCoGa5, PuRhGa5, and UCoGa5. A common underlying property of these materials is a strong spin–orbit coupling split band structure that enables substantial spin fluctuations. In a feedback effect on the electronic structure they create electronic ‘hot spots’, where the single-particle spectral weight is maximum, resulting in a universal peak-dip-hump feature. These results are in good agreement with experiments, suggesting that actinides are adequately described by the intermediate Coulomb interaction regime, where both itinerant (peak) and localized (hump) features coexist.

Surface adsorption represents a competition between collision and scattering processes that depend on surface energy, surface structure and temperature. The surface reactivity of the actinides can add additional complexity due to radiological dissociation of the gas and electronic structure. Here we elucidate the chemical bonding of gas molecules adsorbed on Pu metal and oxide surfaces. Atmospheric gas reactions were studied at 190 and 300 K using x-ray photoelectron spectroscopy. Evolution of the Pu 4f and O 1s core-level states were studied as a function of gas dose rates to generate a set of Langmuir isotherms. Results show that the initial gas dose forms Pu2O3 on the Pu metal surface followed by the formation of PuO2 resulting in a layered oxide structure. This work represents the first steps in determining the activation energy for adsorption of various atmospheric gases on Pu.

The reaction kinetics of depleted uranium under constant hydrogen pressure (1 bar) have been measured as a function of reaction temperatures between 65 and 385 °C for as-polished and vacuum annealed samples. Enhanced hydrogen reactivity was observed on samples that underwent vacuum annealing prior to hydrogen exposure. The enhanced reactivity was found to be the result of enhanced nucleation rates on annealed samples since the specific rate per reacting unit area remained unaffected. X-ray photoelectron spectroscopy demonstrates that the nucleation kinetics were promoted on annealed samples as a result of the dehydration and partial reduction of the UO2+x outer oxide layer and the formation of an oxycarbide (UOxCy) sub-layer.

The five solid-solid phase transformations of pure Pu are typically represented in idealized thermal expansion plots as having sharp onsets and finishes with linear expansion behavior between the transitions. These behaviors are in reality less common, and the various transitions may have bursting behavior, curved onsets and finishes, and non-linear thermal expansion. In this presentation we will review the transformation behavior of diverse set of pure Pu types. These types include zone-refined pure Pu, electro-refined pure Pu, pure Pu doped with 1000 appm Ga, and alpha-phase Pu within an as-cast 1.9 atomic. % Ga alloy.

Most data available on the interaction of actinides with biological systems are based on physiological or biokinetic measurements, with scarce information on the structure of the actinide coordination site. This proceeding article describes an approach for structural elucidation of actinide biological complexes. Indeed most of c.a. actinide circulation pathways are unknown, as they accumulate mostly in bones, kidney and liver. In case of accidental release of radionuclide in the environment, internal contamination under either acute or chronic conditions has the potential to induce both radiological and chemical toxicity through significant interaction with the metabolome or proteome followed by possible functional modifications. For instance, the metalloproteins present primary, secondary and tertiary structures, and also different post-translational modifications, all playing a crucial role in interacting with their partners, which can be altered by actinide bonding. When tightly bound, metal ions are critical to the function, structure, and stability of the proteins, by disabling specific interactions through significant local or global conformational modifications. In order to overcome the intricacy of actinide chemistry combined with that of metalloproteins, a simplified study toward better understanding the interaction of actinides and biological systems using simple biomolecules such as amino acids has therefore been considered. Focus is made on the cation coordination site itself, given that conformational effects are not taken into account in this approach. In a first step, we have selected simple phosphorylated building blocks that may be considered as chemical representatives of some ubiquitous target metalloproteins or some important phosphorylated peptides or proteins.

The electronic structure of delta plutonium (δ-Pu) and plutonium compounds is investigated using photoelectron spectroscopy (PES). Results for δ-Pu show a small component of the valence electronic structure which might reasonably be associated with a 5f6 configuration. PES results for PuTe are used as an indication for the 5f6 configuration due to the presence of atomic multiplet structure. Temperature dependent PES data on δ-Pu indicate a narrow peak centered 20 meV below the Fermi energy and 100 meV wide. The first PES data for PuCoIn5 indicate a 5f electronic structure more localized than the 5fs in the closely related PuCoGa5. There is support from the PES data for a description of Pu materials with an electronic configuration of 5f5 with some admixture of 5f6 as well as a localized/delocalized 5f5 description.

The electronic response of actinide systems under high-pressure conditions is undoubtedly of broad scientific interest but simultaneously very difficult to qualify. We will focus here on the use of inelastic x-ray scattering and its significance for actinides research through recent examples. IXS indeed combines several advantages that turn it into a powerful probe of the electronic and valence properties of f-electron systems. Besides element and orbital selectivity, resonant IXS can overcome core-hole lifetime broadening thus providing sharper spectral features and finer details about the electronic structure. Second, non-dipolar transitions are allowed in non-resonant IXS at high momentum transfer; thus “giant dipolar” Fano-like resonances that overwhelms the electron response at the O4,5 edges can be avoided. Recent results of IXS under pressure in Am and U under pressure are presented along with perspectives for actinides research at SOLEIL synchrotron.

A strong resonance in the inverse photoelectron spectroscopy (IPES) of cerium oxide was reported recently. Here, it is shown that dominance of the indirect channel of the resonant inverse photoelectron spectroscopy (RIPES) is so complete that the photon energy dependence can be explained in terms of emission associated with a single photon energy.

Plutonium oxide heat sources are used to power space missions. The heat produced by alpha decay of the 238 isotope of Pu is converted to electricity in a thermopile, providing electricity during a substantial fraction of the 88 year half-life of the isotope. Decay of the Pu produces helium and uranium, and a fraction of the evolved helium is captured in the oxide matrix. All of the helium produced in decay can in principle be contained in the oxide lattice, where it occupies the tetrahedral sites. Some helium diffuses out at a rate that is somewhat dependent on the form and morphology of the fuel. Rates have previously been measured for oxide aged about 1 year. Current measurements on sealed heat sources as old as 34 years indicate that the rate of diffusion has changed only slightly over time. Possible mechanisms for helium release include bubble diffusion, point defect migration, agglomeration and movement of He at grain boundaries, and volume diffusion through the lattice sites. We observe primarily diffusion from site to site within the lattice, with an activation energy of 18.7 kcal/mole, independent of point defect movement, despite the rising concentration of helium in the lattice over time and the accumulation of radiation damage within the lattice. Because of the slow diffusion of helium from the fuel to the headspace, heat sources are anticipated to be stable over a long lifetime.

Oxygen potentials of PuO2-x were measured at temperatures of 1473 - 1873 K by thermo-gravimetry. The oxygen potentials were determined by in situ analysis as functions of oxygen-to-metal ratio and temperature. The measurement data were analyzed on the basis of defect chemistry and an approximate equation was derived to represent the relationship among temperature, oxygen partial pressure, and deviation x in PuO2-x.