Search

Gadolinium titanate (Gd2Ti2O7, or GTO) and other lanthanide pyrochlores are interesting candidates for actinide waste disposal and fast ion conduction because the relevant material properties are intimately dependent on local cation structure. Therefore, a deep understanding of the kinetics associated with cation ordering and disordering is required if such material properties are to be tuned for specific device designs. To this end, single crystals of GTO were irradiated with 190 keV helium ions to a total fluence of 1 × 1017 ions/cm2, amorphizing the sample surface to a depth of ~1 μm and resulting in significant He bubble accumulation. FIB lamellae lifted out from the irradiated sections were examined during heat treatment in the (scanning) transmission electron microscope. Two distinct stages of the re-crystallization of the amorphized material were observed. The material near the end of the ions’ range transformed first and with the same orientation as the pristine material. This was due to the close proximity of the pristine material and the presence of small defect fluorite seeds, but the propagation of this growth front was frustrated by the large pores (formerly bubbles) in the He accumulation layer. This was followed by heterogeneous nucleation of new crystallites at random orientations at the top of the He accumulation layer, which is attributed to the high surface area associated with the many small He bubbles in that region. It is inferred that the kinetics of grain growth in this material are significantly faster than the kinetics of grain nucleation.

The in situ response of stoichiometric and non-stoichiometric uranium dioxide during flash sintering is examined using high energy X-ray diffraction. Our results show that the onset of flash is driven by an increase in temperature and controlled by the applied field with no evidence of an accumulation of defects. The incubation time, that is the time after the application of the field before the flash occurs, is found to be material specific with hyper-stoichiometric samples requiring lower fields to flash. For low current/voltage fields we quantify very little change in the atomic and microstructure of the different uranium dioxide samples post-flash. Microstructural changes are identified for high fields and currents, where joule heating and sample temperatures are high, resulting in the complete transformation of the U4O9 phase. Our results highlight the usefulness of high energy X-ray characterization in understanding the subtle structural changes that occur during the flash sintering process.

Flash sintering (FS), a novel fabrication technique belonging to the family of field assisted sintering (FAS) techniques, has been utilized in this study to fabricate uranium dioxide (UO 2 ) pellets. Stoichiometric (UO 2.00 ) and hyper-stoichiometric (UO 2.16 ) pellets were flash sintered at 600 °C within a few (2–3) minutes. This is in sharp contrast to conventional sintering where temperatures hundreds of degrees higher are necessary and the sintering time extends to hours. Relating this in terms of the homologous temperature ratio (T H ) for both conditions shows that in the case of flash sintering at 600 °C, T H = 0.3 versus T H = 0.6 for conventional sintering at 1600 °C. The highest density achieved for a UO 2.00 pellet was 81% theoretical density (TD) when flash sintered at 600 °C for 185 s at a field of 188 V/cm and a current density of 442 mA/mm 2 . For the UO 2.16 pellet, the highest achieved density was 91% TD when flash sintered at 600 °C for 123 s at a field of 188 V/cm and a current density of 632 mA/mm 2 . X-ray diffraction (XRD) characterization of the sintered pellets showed the final sintered material to be single cubic fluorite phase. Scanning electron microscopy (SEM) of longitudinal sections revealed non-uniform microstructures with regions of high density where the grain size ranged from 1 to 15 μm. Comparisons between conventionally and flash sintered pellets that achieved equivalent shrinkage strains were also conducted. In all cases, the flash sintered pellets achieved similar densification to the conventionally sintered pellets at much lower furnace temperatures and shorter times.

In this work, flash sintering was demonstrated on stoichiometric and non-stoichiometric uranium dioxide pellets at temperatures ranging from room temperature ( 26 ° C ) up to 600 ° C . The onset conditions for flash sintering were determined for three stoichiometries (UO2.00, UO2.08, and UO2.16) and analyzed against an established thermal runaway model. The presence of excess oxygen was found to enhance the flash sintering onset behavior of uranium dioxide, lowering the field required to flash and shortening the time required for a flash to occur. The results from this study highlight the effect of stoichiometry on the flash sintering behavior of uranium dioxide and will serve as the foundation for future studies on this material.

Single crystals of U3Si2 and U3Si5 were grown using the modified Czochralski tri-arc growth technique in a gettered argon atmosphere. A variety of growth parameters including seed pull rate, seed rotation rate, crystal size, charge size and stoichiometry were evaluated to determine the best growth conditions in an effort to produce high quality single crystals for property measurements and basic studies. The crystals were characterized for quality, phase purity, density, and axial variations in chemistry. Near phase-pure, single crystals 5 mm in diameter and 35 mm in length were grown with minimal variation in stoichiometry and limited cracking using a crystal rotation rate of 30 rpm, a hearth rotation rate of 22 rpm, and a pull rate of 16 mm/h up to solidified fraction of g = 0.75.

Uranium dioxide (UO2) composites with uranium diboride (UB2) and uranium tetraboride (UB4) have been proposed as advanced fuel candidates due to their high thermal conductivity, high melting point, high fissile density and their ability to incorporate a built-in burnable poison by tailoring the targeted 10B/11B ratio. As such, it is important to assess the fabrication, and thermal and micromechanical properties of such composites. In this work, UO2-UB2 and UO2-UB4 samples with boride phase fractions of 5, 15 and 30 wt% were fabricated to high densities (above 95% theoretical density) via spark plasma sintering (SPS). This enabled sintering at relatively low temperatures and short timescales. SPS also aided in maintaining the target phase fractions of the samples as reactions between the constituent phases were suppressed due to the short timescales and reducing environment during sintering. Thermal diffusivity measurements from 299 to 1273 K were conducted through laser flash analysis (LFA). The diffusivity increased as a function of boride weight fraction, and UB2 additions increased the thermal diffusivity of the composites more than UB4 additions. Assessment of the LFA results indicated that in-situ reactions between the UO2 and boride phases that suppress the thermal diffusivity occur above 800 K for all samples. Oxidation of the boride phase was proposed as the underlying reaction. This was supported by thermodynamic assessments from the literature, as well as microstructural, crystallographic, and nanoindentation characterization performed on these samples.

Three-dimensional x-ray tomography (XRT) provides a nondestructive technique to characterize the size, shape, and location of damage in dynamically loaded metals. A shape-fitting method comprising the inertia tensors of individual damage sites was applied to study differences of spall damage development in face-centered-cubic (FCC) and hexagonal-closed-packed (HCP) multicrystals and for a suite of experiments on high-purity copper to examine the influence of loading kinetics on the spall damage process. Applying a volume-weighted average to the best-fit ellipsoidal aspect-ratios allows a quantitative assessment for determining the extent of damage coalescence present in a shocked metal. It was found that incipient transgranular HCP spall damage nucleates in a lenticular shape and is heavily oriented along particular crystallographic slip directions. In polycrystalline materials, shape distributions indicate that a decrease in the tensile loading rate leads to a transition to coalesced damage dominance and that the plastic processes driving void growth are time dependent.

Hypervalent UO2, UO2(+x) formed by both addition of excess O and photoexcitation, exhibits a number of unusual or often unique properties that point to it hosting a polaronic Bose–Einstein(-Mott) condensate. A more thorough analysis of the O X-ray absorption spectra of UO2, U4O9, and U3O7 shows that the anomalous increase in the width of the spectral features assigned to predominantly U 5f and 6d final states that points to increased dispersion of these bands occurs on the low energy side corresponding to the upper edge of the gap bordered by the conduction or upper Hubbard band. The closing of the gap by 1.5 eV is more than twice as much as predicted by calculations, consistent with the dynamical polaron found by structural measurements. In addition to fostering the excitation that is the proposed mechanism for the coherence, the likely mirroring of this effect on the occupied, valence side of the gap below the Fermi level points to increased complexity of the electronic structure that could be associated with the Fermi topology of BEC–BCS crossover and two band superconductivity.

Consideration of uranium silicide compounds as candidate nuclear reactor fuels requires the accurate knowledge of their thermophysical properties as a function of temperature. Stoichiometric USi has received little attention in the literature with regard to property characterization. This absence of data prevents modeling and simulation communities from predicting performance of uranium silicide fuels that are either nominally USi itself, or are designed with other uranium silicide phases but may include appreciable fractions of USi introduced as a result of the fuel synthesis or fabrication process. This study was undertaken to quantify the thermal expansion coefficient, specific heat capacity, thermal diffusivity, and thermal conductivity of USi from ambient conditions to just below the peritectic decomposition of the compound. Stoichiometric samples that were prepared by arc melting and powder metallurgical routes for thermophysical property measurements exhibited 94% USi phase, with the balance being U 3 Si 5 phase that likely formed during the solidification process. An energetic phase transformation was observed at 723 K, which is attributed to the inclusion of the secondary U 3 Si 5 phase.

Uranium diboride (UB2) and uranium tetraboride (UB4) are candidate constituents for multi-phase accident tolerant fuel due to their anticipated high thermal conductivity. These fuels have high uranium density that contributes to fission, and by tailoring the ratio of 10B/11B, can also act as an integrated burnable poison. Understanding the thermophysical and mechanical properties of uranium borides, for which only limited data are available in the literature, is of importance to determine their accident tolerance. In this work UB2 and UB4 have been synthesized via arc melting and sintered to high densities via spark plasma sintering (SPS). High density samples, >90% theoretical density, were used to measure the thermal diffusivity and thermal expansion of UB2 and UB4 and, in conjunction with specific heat literature data, their thermal conductivities were calculated from 298 to 1773 K. Additionally, resonance ultrasound spectroscopy (RUS) and nanoindentation were performed to investigate the mechanical properties of the uranium borides. Our results are discussed in the context of available literature. Both UB2 and UB4 exhibit thermal conductivities higher than that of UO2, with UB2 having the highest. The thermal conductivity of UB2 increases with temperature above 874 K, while for UB4 there is a linear increase over the entire measured range. X-ray diffraction (XRD) results indicate that impurity phases were present in the fabricated materials, which could explain why literature density functional theory (DFT) results predict higher values. This suggests that if impurity phases or any microstructural defects can be eliminated then the thermal conductivity can be further increased.

Author(s): Janish, Matthew T; Schneider, Matthew M; Ophus, Colin; Ciston, Jim; Valdez, James A; McClellan, Kenneth J; Byler, Darrin D; Chen, Di; Wang, Yongqiang; Holesinger, Terry G; Uberuaga, Blas P

An application of nuclear physics, a facility for using protons for flash radiography, has been developed at the Los Alamos Neutron Science Center (LANSCE). Protons have proven far superior to high energy x-rays for flash radiography because of their long mean free path, good position resolution, and low scatter background. Although this facility is primarily used for studying very fast phenomena such as high explosive driven experiments, it is finding increasing application to other fields, such as tomography of static objects, phase changes in materials and the dynamics of chemical reactions. The advantages of protons are discussed, data from some recent experiments will be reviewed and concepts for new techniques are introduced.

Nondestructive characterization techniques are essential for maximizing the amount of high fidelity data obtained from post mortem mechanically tested materials, particularly for the complex experimental techniques used for shock loading. Polycrystalline copper samples of varying thermomechanical histories were shock loaded to induce incipient spall damage conditions via plate impacts at low pressures. Electron backscattering diffraction (EBSD) and 3-D X-ray tomography (XRT) were utilized to obtain microstructural and void volume data, respectively. Voids were fit to ellipsoids using their inertia tensors obtained from the 3-D XRT data, and the results compared to EBSD data. This, in turn, enabled establishing quantitative correlations between void shapes and preferential damage mode, either inter- or trans-granular, for each thermomechanical history. Greater than 60 % of voids present in heat-treated samples were spherical, indicative of transgranular damage preference. Greater than 90 % of voids present in as-received rolled samples were flat sheet-like voids or unsymmetrical oblate ellipsoids, indicative of intergranular and coalesced damage modes. Analyzing grain boundary damage statistics in conjunction with global damage mode distributions indicate that boundaries become less dominant as preferential void nucleation sites within the spall plane as the bulk strength of the material decreases from heat treatments.

Possible use of U 3 Si 5 as a nuclear reactor fuel requires knowledge of its thermophysical properties as a function of temperature. While limited data is available for U–Si compounds containing higher uranium densities, no investigations of U 3 Si 5 have been presented in the literature to date. High purity U 3 Si 5 was fabricated to facilitate a set of experiments to determine the coefficient of thermal expansion, heat capacity, thermal diffusivity, and thermal conductivity from room temperature to 1773 K. Each measurement on nearly stoichiometric U 3 Si 5 showed the existence of a phase transformation at 723 K, which is not consistent with the most recently published phase diagram.

Mixed valence O-doped $\mathrm{U}{\mathrm{O}}_{2+x}$ and photoexcited $\mathrm{U}{\mathrm{O}}_{2}$ containing transitory ${\mathrm{U}}^{3+}$ and ${\mathrm{U}}^{5+}$ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Fr\"ohlich-type, nonequilibrium, phonon-coupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in $\mathrm{U}{\mathrm{O}}_{2}$, ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$, and ${\mathrm{U}}_{3}{\mathrm{O}}_{7}$ with O XAS and NIXS and $\mathrm{U}\phantom{\rule{0.16em}{0ex}}{\mathrm{M}}_{5}$ RIXS shows that O doping raises the peak of the $\mathrm{U}\phantom{\rule{0.16em}{0ex}}5f$ states of the valence band by \ensuremath{\sim}0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2--2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied $\mathrm{U}\phantom{\rule{0.16em}{0ex}}5f$ states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of $\mathrm{U}{\mathrm{O}}_{2}$, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied $\mathrm{U}\phantom{\rule{0.16em}{0ex}}5f$ states of $\mathrm{U}{\mathrm{O}}_{2}$ and no hot electrons. 3.1 eV excitation, however, complements the O-doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited $\mathrm{U}{\mathrm{O}}_{2}$ therefore fulfills the criteria for a nonequilibrium condensate. The electron distributions resulting from both excitations persist for 5--10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: (1) the direct formation of both of these long lived (g3--10 ps) excited states without the short lived nonthermal intermediate; (2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and (3) the absence of hot electrons accompanying the insulating $\mathrm{U}{\mathrm{O}}_{2}$ excited state. This heterogeneous, nonequilibrium, Fr\"ohlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.

In common with many strongly correlated electron systems, intermediate valence compounds are believed to display a crossover from a high-temperature regime of incoherently fluctuating local moments to a low-temperature regime of coherent hybridized bands. We show that inelastic neutron scattering measurements of the dynamic magnetic susceptibility of CePd

Local microstructural weak links for spall damage were investigated using three-dimensional (3-D) characterization in multicrystalline copper samples (grain size ≈ 450 µm) shocked with laser-driven plates at low pressures (2 to 4 GPa). The thickness of samples and flyer plates, approximately 1000 and 500 µm respectively, led to short pressure pulses that allowed isolating microstructure effects on local damage characteristics. Electron Backscattering Diffraction and optical microscopy were used to relate the presence, size, and shape of porosity to local microstructure. The experiments were complemented with 3-D finite element simulations of individual grain boundaries (GBs) that resulted in large damage volumes using crystal plasticity coupled with a void nucleation and growth model. Results from analysis of these damage sites show that the presence of a GB-affected zone, where strain concentration occurs next to a GB, correlates strongly with damage localization at these sites, most likely due to the inability of maintaining strain compatibility across these interfaces, with additional effects due to the inclination of the GB with respect to the shock. Results indicate that strain compatibility plays an important role on intergranular spall damage in metallic materials.

Electron beam evaporation and ion beam assisted deposition (IBAD) methods were utilized to fabricate depleted UO 2 films and UO 2 films with embedded Xe atoms, respectively. The films were fabricated at elevated temperature of 700 °C and also subsequently annealed at 1000 °C to induce grain growth and Xe atom redistribution. The goal of this work was to synthesize reference UO 2 samples with controlled microstructures and Xe-filled bubble morphologies, without the effects attendant to rector irradiation-induced fission. Transmission electron microscopy (TEM) microstructural characterization revealed that fine Xe-filled bubbles nucleated in the as grown films and subsequent annealing resulted in noticeable bubble size increase. Reported results demonstrate the great potential IBAD techniques and UO 2 films have for various areas of nuclear materials studies.

U3Si has the highest uranium density of the U–Si compounds, making it an attractive option for specialized reactor applications. Although the compound has been used in research and test reactors for nearly six decades, little data has been published in the literature which characterizes the thermophysical properties at elevated temperatures. A systematic study was conducted on U3Si to characterize thermal expansion, heat capacity, thermal diffusivity, and thermal conductivity as a function of temperature to 1150 K. Thermophysical properties were also tabulated for the high temperature δ ′ -U3Si phase as a function of temperature, which has not been reported previously.

Polycrystalline copper samples with varying thermomechanical histories were shock loaded to induce spall via laser-driven plate impacts at low shock stress (

Nickel ferrite (NiFe2O4) is a major constituent of the corrosion deposits formed on the exterior of nuclear fuel cladding tubes during operation. NiFe2O4 has attracted much recent interest, mainly due to the impact of these deposits, known as CRUD, on the operation of commercial nuclear reactors. Although advances have been made in modeling CRUD nucleation and growth under a wide range of conditions, the thermophysical properties of NiFe2O4 at high temperatures have only been approximated, thereby limiting the accuracy of such models. In this study, samples of NiFe2O4 were synthesized to provide the thermal diffusivity, specific heat capacity, and thermal expansion data from room temperature to 1300 K. These results were then used to determine thermal conductivity. Numerical fits are provided to facilitate ongoing modeling efforts. The Curie temperature determined through these measurements was in slight disagreement with literature values. Transmission electron microscopy investigation of multiple NiFe2O4 samples revealed that minor nonstoichiometry was likely responsible for variations in the Curie temperature. However, these small changes in composition did not impact the thermal conductivity of NiFe2O4, and thus are not expected to play a large role in governing reactor performance.

Near-field high energy x-ray diffraction microscopy (nf-HEDM) and high energy x-ray micro-tomography (μT) have been utilized to characterize the pore structure and grain morphology in sintered ceramic UO2nuclear fuel material. μT successfully images pores to 2-3μm diameters and is analyzed to produce a pore size distribution. It is apparent that the largest number of pores and pore volume in the sintered ceramic are below the current resolution of the technique, which might be more appropriate to image cracks in the same ceramics. Grain orientation maps of slices determined by nf-HEDM at 25 μm intervals are presented and analyzed in terms of grain boundary misorientation angle. The benefit of these two techniques is that they are non-destructive and thus could be performed before and after processes (such as time at temperature or in-reactor) or even in-situ.

Transport of fission products (FPs) inside fuel pellets is an important mechanism that affects microstructure evolution as well as fuel performance. To study this phenomenon for low fuel burnups, w...

Intermediate valence compounds containing rare earth or actinide ions are archetypal systems for the investigation of strong electron correlations. Their effective electron masses of 10 to 50 times the free electron mass result from a hybridization of the highly localized $f$-electrons with the more itinerant $d$-electrons, which is strong enough that their properties are dominated by on-site electron correlations. To a remarkable degree, this can be modeled by the Anderson Impurity Model, even though the $f$-electrons are situated on a periodic lattice. However, in recent years, there has been increasing evidence that the dynamic magnetic susceptibility of intermediate valence compounds is not purely local, but shows variations across the Brillouin zone that have been ascribed to $f$-band coherence. So far, this has been based on simplified qualitative models. In this article, we present a quantitative comparison of inelastic neutron scattering from a single crystal of CePd$_3$, measured in four-dimensional (Q,$\omega$)-space, with ab initio calculations, which are in excellent agreement on an absolute scale. Our results establish that the Q-dependence of the scattering is caused by particle-hole excitations within $f$-$d$ hybridized bands that grow in coherence with decreasing temperature., Comment: 8 page article with 6 figures, 4 page supplementary information with 8 figures

The electricity from a light-water nuclear reactor ultimately comes from the heat emitted by fuel rods made of UO${}_{2}$. However, as fission products and defects build up in the urania, its effective thermal conductivity---and power output---changes, with potentially serious implications for reactor efficiency and safety. The authors develop a methodology to account for spin-phonon scattering, which is usually neglected in multiscale modeling of nuclear fuel. Their results are of interest both for reactor operation and for studying a much wider class of paramagnetic materials with antiferromagnetic transitions at low temperatures, including transition-metal oxides.

The distribution and atomic structure of grain boundaries has been investigated in UO 2 . Our scanning electron microscopic/ electron backscatter diffraction experiments on a depleted UO 2 sample showed real nuclear fuels contain a combination of special coincident site lattice (CSL) and general boundaries. The experimental data indicated that ∼ 16% of the boundaries were CSL boundaries and the CSL distribution was dominated by low Σ boundaries; namely E9, E3, and Σ5 Based on our experimental observations, the structures of select low E (E5 tilt, Σ5 twist, Σ3 tilt) and a random boundary were analyzed in greater detail using empirical potential atomic-scale calculations. Our calculations indicate that the boundaries have very different structures and each CSL boundary had multiple minima on the γ-surface. The presence of a significant fraction of CSL boundaries and the differences in their structures are expected to have important consequences on fuel properties.

Correlations between spall damage and local microstructure were investigated using polycrystalline copper samples via laser-driven plate impacts at low pressures. Electron backscattering diffraction was used to relate the presence of porosity to microstructural features such as grain boundaries and triple points. Preferred void-nucleation sites were identified in terms of their crystallography via statistical sampling in serial sectioned specimens. Results indicate that terminated twins and grain boundaries with misorientations between 25° and 50° are the preferred locations for intergranular damage localization.

Correlations between spall damage and local microstructure were investigated in multicrystalline copper samples via impact tests conducted with laser-driven plates at low pressures (2—6 GPa). The copper samples had a large grain size as compared to the thickness, which was either 200 or 1000 μm, to isolate the effects of microstructure on the local response. Velocity interferometry was used to measure the bulk response of the free-surface velocity of the samples to monitor traditional spall tensile failure and to examine heterogeneities on the shock response due to microstructure variability from sample to sample. The shock pressure, dynamic yield strength and spall strength were determined from the measured velocity history via standard hydrodynamic approximations, while the effect of strength was explored via 1D hydrocode calculations. Electron Backscattering Diffraction, both in-plane and through-thickness, was used to relate crystallography to the presence of porosity around microstructural features such as grain boundaries and triple points. It was found that the dynamic yield strength measured from velocity histories in different samples correlated well with the crystallographic dependence reported for the dynamic yield strength in single crystals. Transgranular damage dominated in thin specimens with 230 μm grain size, where porosity appeared close to, but not exactly at, grain boundaries. However, a transition to dominant intergranular damage was observed as the grain size was reduced to 150 μm. Thick specimens (450 μm grain size) showed both modes, with intergranular damage found mostly where grains were smaller than average and the sites for preferred damage nucleation in these samples included grain boundaries and triple points. In particular, twin boundaries, especially tips of terminated twins, showed a large mismatch in surface displacements on the diagnostic surface as compared to the surrounding grains as well as a tendency for damage localization on the through-thickness sections.

Bose-Einstein condensates (BECs) composed of polarons would be an advance because they would combine coherently charge, spin and a crystal lattice. Following our earlier report of unique structural and spectroscopic properties, we now identify potentially definitive evidence for polaronic BECs in photo- and chemically doped UO2(+x) on the basis of exceptional coherence in the ultrafast time dependent terahertz absorption and microwave spectroscopy results that show collective behavior including dissipation patterns whose precedents are condensate vortex and defect disorder and condensate excitations. That some of these signatures of coherence in an atom-based system extend to ambient temperature suggests a novel mechanism that could be a synchronized, dynamical, disproportionation excitation, possibly via the solid state analog of a Feshbach resonance that promotes the coherence. Such a mechanism would demonstrate that the use of ultra-low temperatures to establish the BEC energy distribution is a convenience rather than a necessity, with the actual requirement for the particles being in the same state that is not necessarily the ground state attainable by other means. A macroscopic quantum object created by chemical doping that can persist to ambient temperature and resides in a bulk solid would be revolutionary in a number of scientific and technological fields.

The importance of the thermal transport in nuclear fuel has motivated a wide range of experimental and modelling studies. In this report, the reduction of thermal transport in UO2 due to defects and fission products has been investigated using non-equilibrium MD simulations, with two sets of empirical potentials for studying the degregation of UO2 thermal conductivity including a Buckingham type interatomic potential and a recently developed EAM type interatomic potential. Additional parameters for U5+ and Zr4+ in UO2 have been developed for the EAM potential. The thermal conductivity results from MD simulations are then corrected for the spin-phonon scattering through Callaway model formulations. To validate the modelling results, comparison was made with experimental measurements on single crystal hyper-stoichiometric UO2+x samples.

With the increasing concentration volume of carbon dioxide in the atmosphere, there has been an increasing interest in carbon dioxide sequestration. One method is to store the carbon dioxide in mineral form, reacting solution dissolved CO2 to precipitate carbonates. In order to understand whether or not such an endeavor is feasible, the carbonation reaction must first be understood. In this study, the surface of ground serpentine, untreated, heat treated and following a carbonation experiment, has been characterized using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The results indicate that the mechanism for the reaction involves dissolution of the serpentine through the formation of an amorphous phase and subsequent precipitation of magnesite. The rate limiting step appears to be the diffusion of Mg out of the amorphous phase.

Transport mechanisms, such as mass and heat transfer, are critical to the efficiency and the reliability of nuclear fuels such as uranium oxide. These properties can be significantly affected by the microstructure of materials. This paper looks into the effects of grain boundary (GB) Kapitza resistance on the overall thermal conductivity and fission gas transport of UO2 using a 3-D finite element model with microstructurally explicit information. The model developed is created with a reconstruction of the microstructure of depleted uranium samples performed using serial sectioning techniques with Focused Ion Beam (FIB) and Electron Backscattering Diffraction (EBSD). The properties of these microstructural entities are characterized by misorientation angles and Coincident Site Lattice (CSL) models, which provide a framework to assign spatially dependent thermal and mass transfer properties based on the location and connectivity of these entities in actual microstructures. The key feature of this model is the coupling between heat transfer and mass transfer of fission products which makes it a multi-physics model capable of following the evolution of thermal performance as fission products are produced.

Global and local microstructural weak links for spall damage were investigated using 3-D characterization in polycrystalline (PC) and multicrystalline (MC) copper samples, respectively. All samples were shocked via flyer-target plate experiments using a laser drive at low pressures (2–6 GPa). The flyer plates measured approximately 500 μm thick and 8 mm in diameter and the target plates measured approximately 1000 μm thick and 10 mm in diameter. Electron Backscattering Diffraction (EBSD) and optical microscopy were used to determine to presence of voids and relate them to the surrounding microstructure. Statistics on the strength of grain boundaries (GBs) was conducted by analyzing PC samples and collecting the misorientation across GBs with damage present, and it was found that a misorientation range of 25–50° is favorable for damage. Statistics were also taken of copper PC samples that had undergone different heat treatments and it was found that although the 25–50° range is less dominant, it is still favorable for damage nucleation. Removal of initial plastic strain via heat treatments and an increase in Σ3 CSL boundaries, indicative of strong annealing twins, also led to an increased amount of transgranular damage. 3-D X-ray tomography data were used to investigate the shape of the voids present in untreated, as received and heat treated samples. It was found that the as received sample contained a higher amount of “disk”, or, “sheet-like” voids indicative of intergranular damage, whereas the heat treated samples had a higher fraction of spherical shaped voids, indicative of transgranular damage. MC samples were used to study microstructural weak links for spall damage because the overall grain size is much larger than the average void size, making it possible to determine which GBs nucleated damage. Simulations and experimental analysis of damage sites with large volumes indicate that high Taylor factor mismatches with respect to the crystallographic grain GB normal is the primary cause for the nucleation of damage at a GB interface and a low Taylor factor along the shock direction in either grain drives void growth perpendicular to the GB. Cases where experimental results show damage and simulation results show no damage are attributed to the presence of an intrinsic microstructural weak link, such as an incoherent twin boundary.Copyright © 2013 by ASME

Transport mechanisms, such as mass and heat transfer, are critical to the efficiency and the reliability of nuclear fuels such as uranium oxide. These properties can be significantly affected by the microstructure of the material. This paper looks into the effects of grain boundary (GB) Kapitza resistance on the overall heat conductivity of UO2 using a 3-D finite element model with microstructurally explicit information. The model developed is created with a 3-D reconstruction of the microstructure of depleted uranium samples performed using serial sectioning techniques with Focused Ion Beam (FIB) and Electron Backscattering Diffraction (EBSD). The model treats grain bulks, GBs and triple junctions using elements of different dimensionalities, and it is thus capable of incorporating information of all three entities in one model while keeping a manageable computational cost. Furthermore, the properties of these microstructural entities are characterized by misorientation angles and Coincident Site Lattice (CSL) models, which provide a framework to assign spatially dependent thermal and mass transfer properties based on the location and connectivity of these entities in actual microstructures. Coupling between heat transfer and mass transfer of fission products is also taken into account in the study, to make it a multi-physics model capable of following the evolution of thermal performance as fission products are produced. These simulations can provide input and insight into the fuel pellet behaviors at the initial stage of power generation when burnups are low.Copyright © 2013 by ASME

X-ray pair distribution function (pdf) and U L${}_{3}$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_{2}$ show U(VI)-oxo moieties with x rays for mixed valence U${}_{4}$O${}_{9}$ and U${}_{3}$O${}_{7}$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_{2}$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_{4}$O${}_{9}$ does not behave as a linear combination of the UO${}_{2}$ and U${}_{3}$O${}_{7}$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump--1.57 eV probe reflectivity on UO${}_{2}$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-\ensuremath{\mu}sec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-\ensuremath{\mu}sec peak but also a plateau bracketing this peak that ends in a cusp at 50--60 K and an abrupt change in the hardening rate of a novel 12--15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5--1.1 THz range with a decrease in temperature from \ensuremath{\sim}30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer--phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) \ensuremath{\leftrightarrow} U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

Neutron resonance absorption imaging is a non-destructive technique that can characterize the elemental composition of a sample by measuring nuclear resonances in the spectrum of a transmitted beam. Recent developments in pixelated time-of-flight imaging detectors coupled with pulsed neutron sources pose new opportunities for energy-resolved imaging. In this paper we demonstrate non-contact measurements of the partial pressure of xenon and krypton gases encapsulated in a steel pipe while simultaneously passing the neutron beam through high-Z materials. The configuration was chosen as a proof of principle demonstration of the potential to make non-destructive measurement of gas composition in nuclear fuel rods. The pressure measured from neutron transmission spectra (∼739 ± 98 kPa and ∼751 ± 154 kPa for two Xe resonances) is in relatively good agreement with the pressure value of ∼758 ± 21 kPa measured by a pressure gauge. This type of imaging has been performed previously for solids with a spatial resolution of ∼ 100 μm. In the present study it is demonstrated that the high penetration capability of epithermal neutrons enables quantitative mapping of gases encapsulate within high-Z materials such as steel, tungsten, urania and others. This technique may be beneficial for the non-destructive testing of bulk composition of objects (such as spent nuclear fuel assemblies and others) containing various elements opaque to other more conventional imaging techniques. The ability to image the gaseous substances concealed within solid materials also allows non-destructive leak testing of various containers and ultimately measurement of gas partial pressures with sub-mm spatial resolution.

This report documents the testing of a urania (UO{sub 2.00}) sample as a baseline and the attempt to determine the melt point associated with 4 compositions of urania-ceria and urania-neodymia pseudo binaries provided by ORNL, with compositions of 95/5, and 80/20 and of (U/Ce)O{sub 2.00} and (U/Nd)O{sub 2.00} in the newly developed ceramic melt point determination system. A redesign of the system using parts fabricated from tungsten was undertaken in order to help prevent contamination and tungsten carbide formation in the crucibles. The previously developed system employed mostly graphite parts that were shown to react with the sample containment black-body crucible leading to unstable temperature readings and crucible failure, thus the redesign. Measured melt point values of UO{sub 2.00} and U{sub 0.95}Ce{sub 0.05}O{sub 2.00}, U{sub 0.80}Ce{sub 0.20}O{sub 2.00}, U{sub 0.95}Nd{sub 0.05}O{sub 2.00} and U{sub 0.80}Nd{sub 0.20}O{sub 2.00} were measured using a 2-color pyrometer. The value measured for UO{sub 2.00} was consistent with the published accepted value 2845 C {+-} 25 C, although a wide range of values has been published by researchers and will be discussed later in the text. For comparison, values obtained from a published binary phase diagram of UO{sub 2}-Nd{sub 2}O{sub 3} were used for comparison withmore » our measure values. No literature melt point values for comparison with the measurements performed in this study were found for (U/Ce)O{sub 2.00} in our stoichiometry range.« less

Ion irradiation experiments and atomistic simulations were used to demonstrate that irradiation-induced lattice swelling in a complex oxide, ${\mathrm{Lu}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, is due initially to the formation of cation antisite defects. X-ray diffraction revealed that cation antisite formation correlates directly with lattice swelling and indicates that the volume per antisite pair is approximately $12\text{ }\text{ }{\AA{}}^{3}$. First principles calculations revealed that lattice swelling is best explained by cation antisite defects. Temperature accelerated dynamics simulations indicate that cation Frenkel defects are metastable and decay to form antisite defects.

We have used a suite of diffraction, thermodynamic, and transport measurements to study the localized rattling of Al guest atoms in VAl${}_{10.1}$. The mean-square displacement of the rattling atom shows a concave-down temperature dependence. This is characteristic of an anharmonic vibration, the frequency of which increases with amplitude, akin to a particle in a box. We find that the rattling is best described in terms of a sixth-order interatomic potential, with negligible contributions from harmonic and quartic terms. The rattler has a characteristic temperature of ${\ensuremath{\theta}}_{RM}=21$ K and couples strongly to both the acoustic phonons and conduction electrons. The coupling to the phonons is evident from the large value of the Gr\"uneisen parameter, which increases with decreasing temperature to $\ensuremath{\Gamma}\ensuremath{\approx}43$ at 5 K. Below 6 K, the electrical resistivity varies as ${T}^{3}$, which can be explained in terms of electron scattering from damped sixth-order vibrations. VAl${}_{10.1}$ is a superconductor below ${T}_{c}=1.53$ K, with an upper critical field of $\ensuremath{\sim}$1 kG.

We present two laser driven shock wave loading techniques utilizing long pulse lasers, laser-launched ”yer plateand con“ned laser ablation, and their applications to shock physics. The full width at half maximum of the drivelaser pulse ranges from 100 ns to 10 µ s, and its energy, from 10 J to 1000 J. The drive pulse is smoothed witha holographic optical element to achieve spatial homogeneity in loading. We characterize the ”yer plate during”ight and dynamically loaded target with temporally and spatially resolved diagnostics. The long durationand high energy of the drive pulse allow for shockles s acceleration of thick ”yer plates with 8 mm diameterand 0.1 2 mm thickness. With transient imaging displa cement interferometry and lin e-imaging velocimetry, wedemonstrate that the planarity (bow and tilt) of the loading is within 2 Š 7 mrad (with an average of 4 ± 1mrad),similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible inparticular when a plasma shield is adopted. For ”yer plate loading, supported shock waves can be achieved.Temporal shaping of the drive pulse in con“ned laser ablation enables ”exible loading, e.g., quasi-isentropic,Taylor-wave, and o- Hugoniot loading. These dynamic loading techniques using long pulse lasers (0.1 10 µ s)along with short pulse lasers (1 10 ns) can be an accurate, versatile and ec ient complement to conventionalshock wave loading for investigating such dynamic responses of materials as Hugoniot elastic limit, plasticity,spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recoveredsamples, in a wide range of strain rates and pressures at meso- and macroscopic scales.Keywords: long pulse laser, shock waves, ”yer plate, con“ned ablation, planarity, o- Hugoniot, quasi-isentropic

We present an investigation of defects acting as electron traps in ${\text{Lu}}_{2}{\text{SiO}}_{5}$ (LSO) and ${\text{Lu}}_{x}{\text{Y}}_{2\ensuremath{-}x}{\text{SiO}}_{5}$ (LYSO) performed by wavelength-resolved thermally stimulated luminescence (TSL) measurements from 20 to $400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ after room-temperature (RT) x-ray irradiation. Single crystals doped with several rare-earth ions such as Ce, Tb, Tm, and Sm were considered. A comparison between TSL and RT radio-luminescence (RL) emission spectra is also presented. The glow curves for both LSO and LYSO are similar, showing a series of TSL peaks at 78, 135, 181, and $236\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. In addition, a further peak at about $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ is observed only in LYSO. Our results confirm the role of oxygen vacancies as electron traps in the material; the presence of several glow peaks with a unique trap depth $(0.99\text{ }\text{eV}\ifmmode\pm\else\textpm\fi{}0.07\text{ }\text{eV})$ for the 78, 135, 181, and $236\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ peaks is explained by suggesting that electrons stored in oxygen vacancies recombine through a thermally assisted tunneling mechanism with holes localized at ${\text{Ce}}^{3+}$ or ${\text{Tb}}^{3+}$ centers residing on Lu sites at different crystallographic distances from the traps. This model is supported by the very good correlation among O-Lu distances in the monoclinic $C2/c$ structure of LSO and LYSO and the frequency factors of the traps containing the transmission coefficients of the potential barriers between traps and centers, evaluated in the framework of the thermally assisted tunneling process. Tm and Sm ions do not act as TSL recombination centers possibly due to their tendency to trap electrons during irradiation with ionizing radiation.

The aim of this work was to explore the limits of polycrystalline ceramic scintillator in countering the nuclear threat. The goal was to develop a polycrystalline LaBr 3 :Ce, which can be processed from ceramic forming techniques and can be produced in large size scintillator panels with lower cost and high production rate. Three high purity raw powders were used as the starting materials including LaBr 3 , LaCl 3 , and CeBr 3 . Powder characteristics were measured. A melt spinning method was used to synthesize the nanoparticle LaBr 3 :Ce with stoichiometric compositions. The synthesized nanoparticles were characterized and the average particle size of the synthesized nanoparticle LaBr 3 :Ce was about 50 nm. The melt spun powders were consolidated using a "Nanosintering" method to achieve a high density while maintaining the stoichiometric composition. The grain size of the sintered polycrystalline is about 50 nm, which shows no grain growth during the densification process.