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The nuclear security community has long been interested in the identification and quantification of nuclear material signatures to understand a material’s provenance, use, and ultimate application. New forensics signatures and methods intended for non-traditional or advanced nuclear fuel applications require fuel irradiation experiments to demonstrate viability and validity. Integral fuel irradiations have historically required significant costs and long timelines to design, irradiate, and characterize. This paper describes how a recently developed nuclear fuel irradiation testbed can be used to provide a low cost, rapid turnaround, modular test environment for irradiation and evaluation of nuclear fuel specimens for nuclear security applications. The irradiation testbed houses six small ‘MiniFuel’ samples within hermetically sealed capsules inside targets that can be removed in between each ∼25-day operating cycle of the High Flux Isotope Reactor (HFIR). As many as nine targets can be irradiated using a single irradiation position (reflector region) in HFIR, allowing for varying irradiation temperatures and burnups. A suite of hot cell capabilities have been established to perform post-irradiation examination for measuring performance (e.g., fuel swelling, fission gas release) and facilitating experiment disassembly for subsequent property measurements, microstructural analysis, or chemical assay. This new testbed allows fuel irradiations to be conducted on an accelerated timeframe to enable rapid proof of concept testing and to provide reference material for nuclear fuel security applications. Recent applications using this testbed include the testing of isotopic taggants in UO2 fuel (intentional forensics), testing of U-10Mo fuel for down-conversion of highly enriched uranium–fueled reactors, and the production of irradiated UO2 fuel material for signature analysis of its isotopic composition (plutonium, fission gases, etc.).

Uranium tetrafluoride hydrate (UFH) is formed by immersing anhydrous UF4 under water for 12 h. UFH is therefore clearly a chemical species of environmental concern, as anhydrous UF4 is an intermedi...

Uranium trioxide displays a complex chemical phase space, with at least six structurally distinct polymorphs accessible via different synthetic routes. Remarkably, despite its technological importance, full structural and electronic characterization of these polymorphs remains an open area of study. δ-UO3 in particular has attracted significant theoretical attention due to its high point group and space group symmetries, having U (VI) in octahedral coordination with polyhedra interconnected through corner-sharing to build a 3-D cubic lattice with space group symmetry Pm-3m and Z = 1. Critical experimental information, such as its optical vibrational spectra, are not known. Here, we study the Raman and infrared (IR) spectra of δ-UO3 together with the support of density functional theory (DFT) calculations for spectral interpretation. A symmetry analysis of the DFT-predicted phonon eigenmodes indicates that δ-UO3 should have two IR active modes and no Raman active modes. Experimental results, however, indicate significant Raman scattering from δ-UO3. We therefore propose four potential explanations for this apparent contradiction: a possible tetragonal distortion to the cubic cell, the existence of a surface impurity layer, vacancy scattering, and structural activation of Raman signal. We use powder X-ray diffraction and confocal Raman spectroscopy with depth profiling to investigate these possibilities and suggest future experiments to explore this phenomenon in more detail. Understanding the lattice dynamics of δ-UO3 is important for identification of technogenic U phases via Raman and infrared spectroscopy and our results indicate that the simple understanding of δ-UO3 as a high-symmetry cubic structure should be reconsidered.

Carbon fiber (CF) is a promising lightweight alternative to steel and is of significant interest for energy applications. As CF continues to find new uses and is exposed to new external conditions, a noninvasive method of monitoring its structural integrity is critical. Raman spectroscopy is a commonly used method for this monitoring; however, it is highly inferential, and the interpretation of the data is not always straightforward. In this work, we perform density functional theory (DFT) calculations to investigate changes in the vibrational properties of CF structural units (i.e., graphene and graphite) caused by monovacancy and Dienes defects as a foundation for modeling more complex defects that move our model toward that of realistic CF. Using large computational supercells, we can understand how these defects change the electronic structure and vibrational properties of graphene and graphite for interdefect distances near those of the lower experimental limit. The monovacancy opens an electronic bandgap at the K point. Although no such electronic gap is opened by the Dienes defect, both defects introduce flat defect bands near the Fermi energy. The Dienes defect creates long-range deviations of the phonons, leading to substantial broadening of the highest frequency optical modes in the band structure compared to that of the pristine material. In contrast, the phonon changes caused by the monovacancy are short range, and only minor changes in the band structure or phonon density of states were observed. These findings can assist in the interpretation of experimental results by providing atomic-scale insight into key electronic and vibrational features.

Single-phase β-UO3 is synthesized by flash heating UO2(NO3)·6H2O in air to 450 °C and annealing for 60 h under the same conditions. For the first time, we report the Raman spectra of pure β-UO3. To...

Multiphase mixtures of the uranium fluoride compounds UFx with x = 3, 4, 4.5, 5, whose local U–F bonding geometry is conserved, may result from UF6 reduction. One method for identifying multiphase mixtures is optical vibrational spectroscopy, but experimental Raman and IR spectra for UFx compounds are difficult to obtain. To supplement those experimental measurements, we use density functional perturbation theory (DFPT) with the on-site Coulomb correction (Hubbard + U) to calculate the phonon normal modes, their IR intensity, and their Raman activity for UF3, UF4, U2F9, and UF5. In addition, we use neutron spectroscopy to measure the phonon density of states of the most stable UF4. Our measurements on the Wide-Angular Range Chopper Spectrometer at the Spallation Neutron Source indicate that UF4 has a broad phonon spectrum centered at about 30 meV, but modeling of this spectrum indicates significant multiphonon scattering is present. DFPT calculations for U2F9, which shares structural features with UF4 and UF5, suggest a dynamic instability and we speculate that previous crystallographic measurements of this compound were possible only in multiphase mixtures. Analysis of the detailed atomic motions of phonons in UF5 indicate that a series of high energy modes (between 69 and 75 meV) are described by motion of single-coordinated F atoms (bonded to only one U atom). These single-coordinated F atoms are unique to the UF5 structure and those modes could be used to distinguish UF5 in a multiphase mixture using optical spectroscopy.

Amorphous uranium oxides are known to arise via industrial processes relevant to the nuclear fuel cycle yet evade rigorous structural characterization. A promising approach is to develop statistica...

Mixed-phase and low-symmetry systems are widely observed among uranium oxide materials. Amorphous-U2O7 forms during the calcination of studtite, a uranyl peroxide mineral. Using a genetic algorithm search for stable crystalline phases, we have identified a potentially stable phase of U2O7 that shares structural features with experimentally observed amorphous U2O7 samples. The crystalline structure is expected to undergo a solid–solid phase change around 12 GPa.

Hydrated uranyl fluoride, [(UO2F2)(H2O)]7·4H2O, is not stable at elevated water vapor pressure, undergoing a complete loss of fluorine to form a uranyl hydroxide hydrate. Powder X-ray diffraction data of the resultant uranyl hydroxide species is presented for the first time, along with Raman and infrared (IR) spectra. The new uranyl hydroxide species is structurally similar to the layered uranyl hydroxide hydrate minerals schoepite and metaschoepite, but has a significantly expanded interlayer spacing (c = 15.12 vs. 14.73 A), suggesting that additional H2O molecules may be present between the uranyl layers. Comparison of the Raman and IR spectra of this new uranyl hydroxide hydrate and synthetic metaschoepite ([(UO2)4O(OH)6]·5H2O) suggests that the equatorial environment of the uranyl ion may differ and that H2O molecules in the new species participate in stronger hydrogen bonds. In addition, the interlayer spacing of both this new uranyl hydroxide species and synthetic metaschoepite is shown to be sensitive to the environmental humidity, contracting and re-expanding with desiccation and rehydration. Structural distinction between the new uranyl hydroxide species and synthetic metaschoepite is confirmed by a comparison of the thermal behavior; unlike metaschoepite, the new hydrate does not form α-UO2(OH)2 upon dehydration.

A noncollinear relativistic PBEsol + U study of low-index actinide dioxides (AnO2, An = U, Np, or Pu) surfaces has been conducted. The importance of magnetic vector reorientation relative to the plane of the surface is highlighted; this has often been ignored in collinear nonrelativistic models. The use of noncollinear relativistic methods is key to the design of reliable computational models. The ionic relaxation of each surface is shown to be confined to the first three monolayers, and we have explored the configurations of the terminal oxygen ions on the reconstructed (001) surface. The reconstructed (001) surfaces are ordered as (001)αβ < (001)α < (001)β in terms of energetics. Electrostatic potential isosurface and scanning tunneling microscopy images have also been calculated. By considering the energetics of the low-index AnO2 surfaces, an octahedral Wulff crystal morphology has been calculated.

Triuranium octoxide $({\mathrm{U}}_{3}{\mathrm{O}}_{8})$ undergoes an orthorhombic to hexagonal structural phase transition near ${T}_{s}=305{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and a separate nonstructural phase transition at ${T}_{c}=210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The later transition has previously been associated with temperature-induced fluctuations in the uranium oxidation state. A discontinuity in the slope of electrical conductivity versus temperature measurement at $210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ has supported this idea. The orthorhombic phase has three crystallographic sites in two distinct oxidation configurations [2 U(V) and 1 U(VI)], whereas the hexagonal phase has one distinct uranium site. High-resolution x-ray diffraction measurements eliminate the possibility of superlattice Bragg reflections to less than 0.2 ${\mathrm{e}}^{\ensuremath{-}}$ scattering power and ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ is not metallic; consequently, the presence of oxidation fluctuations is required for charge balancing. Interestingly, the order-to-disorder transition occurs at a much lower temperature than the structural transition. Using temperature-dependent x-ray diffraction and Raman spectroscopy, we show anisotropic lattice expansion in the in-plane $b$ and $c$ lattice constants. A specific discontinuity in the temperature derivatives of the $b$ and $c$ lattice constants are the first reported structural signatures of the order-to-disorder transition, suggestive of a change in local U--O coordination. Phonon frequencies of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ measured by Raman spectroscopy show significant temperature-dependent dynamics. Redshifting of several modes between 40 and $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ cannot be explained by unit cell expansion alone because the unit cell volume decreases in this region. Instead, we show that phonon frequencies are highly correlated with the anisotropic lattice expansion/contraction along specific crystallographic directions.

Single-phase β-UO

e-UO3 is an exotic polymorph in the uranium trioxide system with an undetermined crystal structure and limited optical vibrational spectroscopic data. To improve understanding of this compound, we synthesize and investigate the crystal structure and optical vibrational spectra of e-UO3. Infrared spectra collected for e-UO3 are in good agreement with previously published results, and our studies extend the available data into the low-energy (600–100 cm–1) regime. For the first time, Raman spectra are presented for e-UO3 using both 785 and 532 nm excitation wavelengths. Previous reports suggest an impurity phase may be present in e-UO3 produced by calcination of U3O8; however, spectral center-of-mass calculations, principal component analyses, and Raman spectroscopic mapping employed to investigate this possibility indicate that the product of U3O8 calcined in O3(g) in this work is likely phase-pure. A possible novel structure solution for e-UO3 is determined via Rietveld refinement of powder X-ray diffraction data and is triclinic, P-1, with a = 4.01 A, b = 3.85 A, c = 4.18 A, and α = 98.26 °, β = 90.41 °, γ = 120.46 ° (Rwp = 8.30 %). The asymmetric unit of e-UO3 consists of U(VI) in hexagonal bipyramidal coordination with displaced equatorial oxygen. Further analysis reveals that e-UO3 is best described by a 2 × 1 × 2 supercell structure in P-1 with a = 8.03 A, b = 3.86 A, c = 8.37 A with α = 98.26 °, β = 90.41 °, and γ = 120.46 °, although a higher-symmetry structure is possible. Optical vibrational spectroscopic and structural measurements of e-UO3 presented here furthers our understanding of this complex uranium oxide and clarifies the origin of reported structural similarity to U3O8.

Solid-state carbon materials, such as graphite and graphene, are at the forefront of materials research because of their unique electronic, vibrational, and mechanical properties, leading to a broad range of potential and realized applications. One key application is their role as basic structural units of carbon fiber (CF), a lightweight alternative to steel. In CF, a delicate relationship exists between ultimate material strength and the atomic-scale density of defects contained at the inter- and intra-subunit levels. Computational studies provide insight into the stability of various types of defects that can form in these systems and connect with experimental observables such as bandgap and spectroscopic measurements. Therefore, the literature contains many computational studies that focus on changes induced by defects, including vacancies and Dienes transformations (Stone-Wales and Thrower defects). However, wide-ranging methods and cell sizes have been used, and property-specific information is often lacking. This review summarizes key literature findings, paying particular attention to changes in the electronic, vibrational, and mechanical properties induced by defects in graphitic materials relevant to CF.

Hydrated uranyl fluoride, [(UO2F2)(H2O)]7·4H2O, is not stable at elevated water vapor pressure, undergoing a complete loss of fluorine to form a uranyl hydroxide hydrate. Powder X-ray diffraction data of the resultant uranyl hydroxide species is presented for the first time, along with Raman and infrared (IR) spectra. The new uranyl hydroxide species is structurally similar to the layered uranyl hydroxide hydrate minerals schoepite and metaschoepite, but has a significantly expanded interlayer spacing (c = 15.12 vs. 14.73 Å), suggesting that additional H2O molecules may be present between the uranyl layers. Comparison of the Raman and IR spectra of this new uranyl hydroxide hydrate and synthetic metaschoepite ([(UO2)4O(OH)6]·5H2O) suggests that the equatorial environment of the uranyl ion may differ and that H2O molecules in the new species participate in stronger hydrogen bonds. In addition, the interlayer spacing of both this new uranyl hydroxide species and synthetic metaschoepite is shown to be sensitive to the environmental humidity, contracting and re-expanding with desiccation and rehydration. Structural distinction between the new uranyl hydroxide species and synthetic metaschoepite is confirmed by a comparison of the thermal behavior; unlike metaschoepite, the new hydrate does not form α-UO2(OH)2 upon dehydration.

We confirm that synthetic uranyl hydroxide hydrate metaschoepite [(UO)24O(OH)6]·5H2O is unstable against dehydration under dry conditions, and we present a structural and vibrational spectroscopic study of synthetic metaschoepite and its ambient temperature dehydration product. Complementary structural (X-ray diffraction and neutron diffraction) and vibrational spectroscopic techniques (Raman spectroscopy, infrared spectroscopy, and inelastic neutron scattering) are used to probe different components of these species. Analysis of the dehydration product suggests that it contains both pentagonally coordinated and hexagonally coordinated uranyl ions, necessitating that some uranyl ions undergo a coordination change during the dehydration of pentagonally coordinated metaschoepite. Vibrational spectra of metaschoepite and its dehydration product are interpreted with power spectra generated from ab initio molecular dynamics trajectories, allowing assignment of all major features. We identify the uranyl symmetric stretching modes of the four distinct uranyl ions in synthetic metaschoepite and clarify the assignment of lower energy Raman modes in both structures. The coanalysis of experimental and computational data reveals a strong coupling between the uranyl stretching modes and hydroxide bending modes in the anhydrous structure, leading to the presence of several high-energy combination bands in the inelastic neutron scattering data.

The corrosion and oxidation of actinide metals, leading to the formation of metal-oxide surface layers with the catalytic evolution of hydrogen, impacts the management of nuclear materials. Here, the interaction of hydrogen with actinide dioxide (AnO2, An = U, Np, or Pu) (011) surfaces by Hubbard corrected density functional theory (PBEsol+U) has been studied, including spin–orbit interactions and non-collinear 3k anti-ferromagnetic behavior. The actinide dioxides crystalize in the fluorite-type structure, and although the (111) surface dominates the crystal morphology, the (011) surface energetics may lead to more significant interaction with hydrogen. The dissociative adsorption of hydrogen on the UO2 (0.44 eV), NpO2 (−0.47 eV), and PuO2 (−1.71 eV) (011) surfaces has been calculated. It is found that hydrogen dissociates on the PuO2 (011) surface; however, UO2 (011) and NpO2 (011) surfaces are relatively inert. Recombination of hydrogen ions is likely to occur on the UO2 (011) and NpO2 (011) surfaces, whereas hydroxide formation is shown to occur on the PuO2 (011) surface, which distorts the surface structure.

The interaction of atomic and molecular hydrogen with the actinide dioxide (AnO2, An = U, Np, Pu) (111) surfaces has been investigated by DFT+U, where noncollinear 3k antiferromagnetic (AFM) behaviour and spin-orbit interactions (SOI) are considered. The adsorption of atomic hydrogen forms a hydroxide group, and is coupled to the reduction of an actinide ion. The energy of atomic hydrogen adsorption on the UO2 (0.82 eV), NpO2 (-0.10 eV), and PuO2 (-1.25 eV) surfaces has been calculated. The dissociation of molecular hydrogen is not observed, and shown to be due to kinetic rather than thermodynamic factors. As a barrier in the formation of a second hydroxyl group, an unusual charge distribution has been shown. This is possibly a limitation of a (1·1) unit cell method. The recombination of hydrogen ions on the AnO2 (111) surfaces is favoured over hydroxide formation.

The magnetic structure of the actinide dioxides (AnO2) remains a subject of intense research and is key to the development of high-accuracy computational models. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic groundstate, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO2, whereas a ferromagnetic (111) ground-state with R3 ̅m (No. 166) has been established for NpO2. This has a profound impact on future computational investigations. Band structure calculations have been performed to analyse these results.

The interaction of atomic and molecular hydrogen with the actinide dioxide (AnO2, An = U, Np, Pu) (111) surfaces has been investigated by DFT+U, where noncollinear 3k antiferromagnetic (AFM) behaviour and spin-orbit interactions (SOI) are considered. The adsorption of atomic hydrogen forms a hydroxide group, and is coupled to the reduction of an actinide ion. The energy of atomic hydrogen adsorption on the UO2 (0.82 eV), NpO2 (-0.10 eV), and PuO2 (-1.25 eV) surfaces has been calculated. The dissociation of molecular hydrogen is not observed, and shown to be due to kinetic rather than thermodynamic factors. As a barrier in the formation of a second hydroxyl group, an unusual charge distribution has been shown. This is possibly a limitation of a (1·1) unit cell method. The recombination of hydrogen ions on the AnO2 (111) surfaces is favoured over hydroxide formation.

Inelastic neutron scattering (INS) is uniquely sensitive to hydrogen due to its comparatively large thermal neutron scattering cross-section (82 b). Consequently, the inclusion of water in real samples presents significant challenges to INS data analysis due directly to the scattering strength of hydrogen. Here, we investigate uranyl fluoride (UO2F2) with inelastic neutron scattering. UO2F2 is the hydrolysis product of uranium hexafluoride (UF6), and is a hygroscopic, uranyl-ion containing particulate. Raman spectral signatures are commonly used for inferential understanding of the chemical environment for the uranyl ion in UO2F2, but no direct measurement of the influence of absorbed water molecules on the overall lattice dynamics has been performed until now. To deconvolute the influence of waters on the observed INS spectra, we use density functional theory with full spectral modeling to separate lattice motion from water coupling. In particular, we present a careful and novel analysis of the Q-dependent Debye–Waller factor, allowing us to separate spectral contributions by mass, which reveals preferential water coupling to the uranyl stretching vibrations. Coupled with the detailed partial phonon densities of states calculated via DFT, we infer the probable adsorption locations of interlayer waters. We explain that a common spectral feature in Raman spectra of uranyl fluoride originates from the interaction of water molecules with the uranyl ion based on this analysis. The Debye–Waller analysis is applicable to all INS spectra and could be used to identify light element contributions in other systems.

The magnetic structure of the actinide dioxides (AnO2) remains a field of intense research. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic ground-state, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO2, whereas a ferromagnetic (111) ground-state with R3[combining macron]m (No. 166) has been established for NpO2. Band structure calculations have been performed to analyse these results.

Under humid conditions, uranyl fluoride ([(UO2F2)(H2O)]7·4H2O) undergoes a loss of fluorine to form a uranyl hydroxide species, which can be further hydrated to form a uranyl peroxide species. X-ray diffraction data of the uranyl peroxide product is presented for the first time. In addition, the temperature and humidity conditions under which these reactions occur have been clarified by a 220-day experiment using microRaman spectroscopy to track chemical changes in individual particles of uranyl fluoride. At 25 and 35∘C, uranyl fluoride is found to be stable at 32% relative humidity but not stable at and above 59% relative humidity. We show that water vapor pressure is the driving factor in formation of both the hydroxide and peroxide products. The kinetics of the transformation from uranyl fluoride into uranyl hydroxide is consistent with a denucleation reaction following the absorption of water molecules.

A noncollinear relativistic PBEsol+U study of the low-index actinide dioxides (AnO2, An = U, Np, Pu) surfaces has been conducted. The surface properties of the AnO2 have been investigated and the importance of the reorientation of magnetic vectors relative to the plane of the surface is highlighted. In collinear nonrelativistic surface models, the orientation of the magnetic moments is often ignored; however, the use of noncollinear relativistic methods is key to the design of reliable computational models. The ionic relaxation of each surface is shown to be confined to the first three monolayers and we have explored the configurations of the terminal oxygen ions on the reconstructed (001) surface. The reconstructed (001) surfaces are ordered as (001)αβ < (001)α < (001)β in terms of energetics. Electrostatic potential isosurface and scanning tunneling microscopy images have also been calculated. By considering the energetics of the low-index AnO2 surfaces, an octahedral Wulff crystal morphology has been calculated.

Uranium oxides are readily amenable to investigation using Raman spectroscopy, and this technique is frequently used as a chemical analysis tool. We show, in triuranium octoxide (U3O8), the presence of previously unreported Raman peaks located below 100 cm−1. By maximum intensity, the strongest peak in U3O8 appears at 54 cm−1 and is resolution limited, making this mode an ideal candidate for chemically identifying U3O8 using Raman spectroscopy. Detailed peak analysis indicates that the main spectral feature between 300 and 500 cm−1 is more accurately described by a septet than a triplet. Two samples of differing oxygen content show only minor differences in bulk crystal structure, but subtle changes in lattice dynamics are suggestive of defect scattering in analogy to UO2+x.

The magnetic structure of the actinide dioxides (AnO2) remains a subject of intense research and is key to the development of high-accuracy computational models. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic groundstate, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO2, whereas a ferromagnetic (111) ground-state with R3 ̅m (No. 166) has been established for NpO2. This has a profound impact on future computational investigations. Band structure calculations have been performed to analyse these results.

A thorough understanding of the chemistry of PuO2 is critical to the design of next-generation nuclear fuels and the long-term storage of nuclear materials. Despite over 75 years of study, the ground-state magnetic structure of PuO2 remains a matter of much debate. Experimental studies loosely indicate a diamagnetic (DM) ground-state, whereas theoretical methods have proposed either a collinear ferromagnetic (FM) or anti-ferromagnetic (AFM) ground-state, both of which would be expected to cause a distortion from the reported Fm3[combining macron]m symmetry. In this work, we have used accurate calculations based on the density functional theory (DFT) to systematically investigate the magnetic structure of PuO2 to resolve this controversy. We have explicitly considered electron-correlation, spin-orbit interaction and noncollinear magnetic contributions to identify a hereto unknown longitudinal 3k AFM ground-state that retains Fm3[combining macron]m crystal symmetry. Given the broad interest in plutonium materials and the inherent experimental difficulties of handling this compound, the results presented in this paper have considerable implications for future computational studies relating to PuO2 and related actinide structures. As the crystal structure is coupled by spin-orbit interactions to the magnetic state, it is imperative to consider relativity when creating computational models.

Nuclear power generation is an important way to satisfy rising global energy needs without increasing dependence on coal and petroleum. However, conventional nuclear fuels, such as uranium and plutonium dioxides, raise several safety concerns. Thorium dioxide is a potentially safer alternative for nuclear reactors and, as such, is the subject of renewed research interest. Owing to the hazards of conducting experimental work on radioactive substances, a robust theoretical understanding of ThO2 and uranium-doped ThO2 fuel is needed. We have developed a new Th–O interatomic potential, which we have used to model ThO2, U-doped thoria surfaces and grain boundaries, and which we will use to investigate the effect of defects on these materials.

Thorium dioxide is used industrially in high temperature applications, but more insight is needed into the behavior of the material as part of a mixed-oxide (MOX) nuclear fuel, incorporating uranium. We have developed a new interatomic potential model including polarizability via a shell model, and commensurate with a prominent existing UO2 potential, to conduct configurational analyses and to investigate the thermophysical properties of uranium-doped ThO2. Using the GULP and Site Occupancy Disorder (SOD) computational codes, we have analyzed the distribution of low concentrations of uranium in the bulk material, where we have not observed the formation of uranium clusters or the dominance of a single preferred configuration. We have calculated thermophysical properties of pure thorium dioxide and Th(1−x)UxO2 which generated values in very good agreement with experimental data.

We establish that routine B3LYP and MP2 methods give qualitatively wrong conformations for flexible organic systems containing pi systems and that recently developed methods can overcome the known inadequacies of these methods. This is illustrated for a molecule (a conformer of the Tyr-Gly dipeptide) for which B3LYP/6-31+G(d) and MP2/6-31+G(d) geometry optimizations yield strikingly different structures [Mol. Phys. 2006, 104, 559-570]: MP2 predicts a folded "closed-book" conformer with the glycine residue located above the tyrosine ring, whereas B3LYP predicts a more open conformation. By employing different levels of theory, including the local electron correlation methods LMP2 (local MP2) and LCCSD(T0) (local coupled cluster with single, double, and noniterative local triple excitations) and large basis sets (aug-cc-pVnZ, n=D, T, Q), it is shown that the folded MP2 minimum is an artifact caused by large intramolecular BSSE (basis set superposition error) effects in the MP2/6-31+G(d) calculations. The B3LYP functional gives the correct minimum, but the potential energy apparently rises too steeply when the glycine and tyrosine residues approach each other, presumably due to missing dispersion effects in the B3LYP calculations. The PWB6K and M05-2X functionals, designed to give good results for weak interactions, remedy this to some extent. The reduced BSSE in the LMP2 calculations leads to faster convergence with increasing basis set quality, and accurate results can be obtained with smaller basis sets as compared to canonical MP2. We propose LMP2 as a suitable method to study interactions with pi-electron clouds.

While thorium dioxide is already used industrially in high temperature applications, more insight is needed about the behaviour of the material as part of a mixed-oxide (MOX) nuclear fuel, incorporating uranium. We have developed a new interatomic potential model, commensurate with a prominent existing UO2 potential, to conduct configurational analyses of uranium-doped ThO2 supercells. Using the GULP and Site Occupancy Disorder (SOD) computational codes, we have analysed the distribution of low concentrations of uranium in the bulk material, but have not observed the formation of uranium clusters or a single dominant configuration.