Darrin D. Byler

Characterization of Incipient Spall Damage in Shocked Copper Multicrystals

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.

Qualitative comparison of bremsstrahlung X-rays and 800 MeV protons for tomography of urania fuel pellets.

We present an assessment of x-rays and proton tomography as tools for studying the time dependence of the development of damage in fuel rods. We also show data taken with existing facilities at Los Alamos National Laboratory that support this assessment. Data on surrogate fuel rods have been taken using the 800 MeV proton radiography (pRad) facility at the Los Alamos Neutron Science Center (LANSCE), and with a 450 keV bremsstrahlung X-ray tomography facility. The proton radiography pRad facility at LANSCE can provide good position resolution (<70 μm has been demonstrate, 20 μm seems feasible with minor changes) for tomography on activated fuel rods. Bremsstrahlung x-rays may be able to provide better than 100 μm resolution but further development of sources, collimation, and detectors is necessary for x-rays to deal with the background radiation for tomography of activated fuel rods.

Possible Demonstration of a Polaronic Bose-Einstein(-Mott) Condensate in UO2(+x) by Ultrafast THz Spectroscopy and Microwave Dissipation

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.

Coherent band excitations in CePd3: A comparison of neutron scattering and ab initio theory

Neutrons peek into f-electron bands Neutron scattering can be used to tease out the details of collective magnetic excitations that yield well-defined peaks in the data. In principle, it could also be used to look into single-electron band excitations, but collecting enough data to capture broad distributions of intensity is tricky. Goremychkin et al. used neutron spectrometers that could efficiently capture a large amount of data by rotating the sample, a crystal of the intermediatevalence compound CePd3 (see the Perspective by Georges). The measured dynamical magnetic susceptibility, in combination with detailed ab initio calculations, showed the formation of coherent f-electron bands at low temperatures. Science, this issue p. 186; see also p. 162 Inelastic neutron scattering with a rotating sample is used to reveal coherent f-electron bands at low temperatures. 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 CePd3 provides a benchmark for ab initio calculations based on dynamical mean field theory. The magnetic response is strongly momentum dependent thanks to the formation of coherent f-electron bands at low temperature, with an amplitude that is strongly enhanced by local particle-hole interactions. The agreement between experiment and theory shows that we have a robust first-principles understanding of the temperature dependence of f-electron coherence.

Demonstration of near Field High Energy X-Ray Diffraction Microscopy on High-Z Ceramic Nuclear Fuel Material

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 UO2 nuclear 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.

Three-Dimensional Characterization of Sintered UO2+x: Effects of Oxygen Content on Microstructure and Its Evolution

The oxygen content during the intermediate and final stages of sintering can have a strong effect on the microstructural evolution of oxide fuels. Two depleted urania (d-UO2.0 and d-UO2.14) samples, sintered up to a theoretical density of 90%, were serial sectioned using a focused ion beam and characterized with electron backscatter diffraction (EBSD). The EBSD data were used to make three-dimensional reconstructions of the microstructures to evaluate their characteristics at an intermediate stage of sintering. The oxygen content was found to affect grain shape and grain boundary (GB) mobility, as curved and elongated grains were observed in UO2.0, as well as stronger pore-GB interactions, which is an indication that microstructure was less evolved in UO2.0. Both samples presented a similar fraction ([approximate]20%) of special, coincident site lattice boundaries, with larger amounts of Σ3n GBs, and a rather large fraction of Σ11 GBs for UO2.14. Crystallographic GB planes were also determined to study the distributions of all GB parameters. The UO2.0 sample had a large fraction of GB planes close to the Σ3 twinning planes, which suggests that lower-energy interfaces are used to minimize energy in this sample, potentially due to lower overall GB mobility as compared to UO2.14.

Microstructurally explicit simulation of intergranular mass transport in oxide nuclear fuels

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, when solid-state diffusion is likely to be the controlling mechanism that sets the stage for subsequent phenomena, e.g., fission gas bubble formation and linkage, we created a three-dimensional (3-D) finite element model based on the real microstructure of a depleted UO2 sample. The model couples grain bulk, grain boundary (GB), and triple junction (TJ) diffusion by using 3-D elements for grain bulks, two-dimensional elements for GBs, and one-dimensional elements for TJs. Grain boundary percolation theory is applied in one case study, and the result shows that the presence of high-diffusivity TJs reduces the effect of GB percolation. The model is also used with mass generation from grain bulks, and it is found that localized regions with a high concentration of FPs can form in the presence of a dominant GB percolation path. The work introduces an approach to model diffusion through GBs and TJs at a fair computational cost that can be applied to study the effects of microstructure on FP transport.

Long pulse laser driven shock wave loading for dynamic materials experiments

We present two laser driven shock wave loading techniques utilizing long pulse lasers, laser-launched flyer plate and confined laser ablation, and their applications to shock physics. The full width at half maximum of the drive laser pulse ranges from 100 ns to 10 μs, and its energy, from 10 J to 1000 J. The drive pulse is smoothed with a holographic optical element to achieve spatial homogeneity in loading. We characterize the flyer plate during flight and dynamically loaded target with temporally and spatially resolved diagnostics. The long duration and high energy of the drive pulse allow for shockless acceleration of thick flyer plates with 8 mm diameter and 0.1-2 mm thickness. With transient imaging displacement interferometry and line-imaging velocimetry, we demonstrate that the planarity (bow and tilt) of the loading is within 2-7 mrad (with an average of 4±1 mrad), similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible in particular when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved. Temporal shaping of the drive pulse in confined laser ablation enables flexible loading, e.g., quasi-isentropic, Taylor-wave, and off-Hugoniot loading. These dynamic loading techniques using long pulse lasers (0.1-10 μs) along with short pulse lasers (1-10 ns) can be an accurate, versatile and efficient complement to conventional shock 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-recovered samples, in a wide range of strain rates and pressures at meso- and macroscopic scales.

DYNAMICS OF THE ONSET OF DAMAGE IN METALS UNDER SHOCK LOADING

This paper presents results on the dynamics of damage in copper under incipient spall conditions for multicrystalline specimens. Specimens were annealed from polycrystalline material to reduce the number of grain boundaries the shock wave traverses in passing through the specimen. The specimens incorporated unique fiducials that permit accurate correlation of pre‐shot characterization of the grain orientations, grain size etc. with the location of the dynamic diagnostics and with post‐shot metallography. The dynamic diagnostics—Transient Imaging Displacement Interferometry (TIDI), point VISAR, line VISAR—were all precisely aligned on the specimen surface to view regions of interest revealed in the pre‐shot characterization. Initial analyses demonstrate the wealth of information obtained from this experimental approach. Examples include observation that regions in the surface microstructure most likely to be damaged based on pre‐shot characterization show some of the largest displacements during the shock‐...

Non-Destructive Characterization of UO2+x Nuclear Fuels

This article describes the effect of fabrication conditions on as-sintered microstructures of various stoichiometric ratios of uranium dioxide, UO 2+x, with the aim of enhancing the understanding of fabrication process and developing and validating a predictive microstructurebased model for fuel performance. We demonstrate the ability of novel, non-destructive methods such as near-field high-energy X-ray diffraction microscopy (nf-HEDM) and micro-computed tomography (μ-CT) to probe bulk samples of high-Z materials by non-destructively characterizing three samples: UO 2.00, UO 2.11, and UO 2.16, which were sintered at 1450°C for 4 hours. The measured 3D microstructures revealed that grain size and porosity were influenced by deviation from stoichiometry.