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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

GPU-accelerated 3D neutron diffusion code based on finite difference method

Finite difference method, as a traditional numerical solution to neutron diffusion equation, although considered simpler and more precise than the coarse mesh nodal methods, has a bottle neck to be widely applied caused by the huge memory and unendurable computation time it requires. In recent years, the concept of General-Purpose computation on GPUs has provided us with a powerful computational engine for scientific research. In this study, a GPU-Accelerated multi-group 3D neutron diffusion code based on finite difference method was developed. First, a clean-sheet neutron diffusion code (3DFD-CPU) was written in C++ on the CPU architecture, and later ported to GPUs under NVIDIA's CUDA platform (3DFD-GPU). The IAEA 3D PWR benchmark problem was calculated in the numerical test, where three different codes, including the original CPU-based sequential code, the HYPRE (High Performance Pre-conditioners)-based diffusion code and CITATION, were used as counterpoints to test the efficiency and accuracy of the GPU-based program. The results demonstrate both high efficiency and adequate accuracy of the GPU implementation for neutron diffusion equation. A speedup factor of about 46 times was obtained, using NVIDIA's Geforce GTX470 GPU card against a 2.50 GHz Intel Quad Q9300 CPU processor. Compared with the HYPRE-based code performing in parallel on an 8-core tower server, the speedup of about 2 still could be observed. More encouragingly, without any mathematical acceleration technology, the GPU implementation ran about 5 times faster than CITATION which was speeded up by using the SOR method and Chebyshev extrapolation technique. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2016

Space-Dependent Wielandt Shift Methods for Multigroup Diffusion Eigenvalue Problems

In the calculation of neutronic eigenfunctions and eigenvalues, it is often desirable to approximate the neutron transport equation by a diffusion equation that requires significantly less computational resources to solve. Even when a full transport solution is needed, the solution of the diffusion system can be used to accelerate iterative methods for calculating the transport eigenfunction and eigenvalue. The convergence rate of the basic power iteration method for eigenvalue problems is governed by the dominance ratio; realistic reactor core problems often have high dominance ratios and the power iteration method can converge slowly as a result. A common technique for accelerating the convergence of power iteration is the Wielandt shift (WS) method, which reduces the dominance ratio by 'shifting' the eigenvalue spectrum using an estimate of the desired eigenvalue. WS provides a significant speedup over power iteration if the eigenvalue estimate is close to the true eigenvalue, but it can be difficult to obtain such an estimate. In practice, the shift in the WS method can be dynamically updated in the iteration scheme, in a way that depends on the eigenvalue estimates from the most recent iterations. We present a physically-motivated Space-Dependent Wielandt Shift (SDWS) for solving multigroup diffusion eigenvalue problems. The space-dependence of the eigenvalue shift in SDWS enables a more effective acceleration of the power iteration scheme than WS at the beginning of the iteration scheme, when the current estimate of the eigenvalue is not close to the actual eigenvalue. Several variants of SDWS are discussed here, including an SDWS method that combines the strengths of SDWS (in the early iterations) with those of standard WS (in the later iterations). Numerical results from a 1-D multigroup diffusion problem demonstrate an order of magnitude speedup over power iteration and a 41% speedup over a standard WS technique. This speedup is problem-dependent and appears to be greater for 'more difficult' problems. In this summary, the term 'speedup' refers to a reduction in the number of iterations required to converge to the correct eigenvalue and eigenfunction. Because the scattering is treated fully implicitly (i.e., it is part of the left-hand side of the diffusion system and directly inverted), the computational cost of a single iteration does not vary significantly among the methods discussed in this summary. The number of iterations is therefore representative of the total computational cost for a method. We acknowledge that certain iterative solvers for linear systems (e.g., GMRES) require more computational time to converge when the system is nearly singular (i.e., when a very aggressive Wielandt shift is applied). Nonetheless, when using SDWS in the manner described in this summary, we have seen that the reduction in computational time from reducing the number of power iterations generally outweighs the extra computational time required by iterative solvers such as GMRES. (authors)

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

On the Ill-Posed Nature of Inverse Problems: An Exactly Solvable Paradigm Inverse Neutron Diffusion Problem Illustrating the Solution's Non-Computability - Paper 48

In general, a physical system and/or the result of an indirect experimental measurement are modeled mathematically in terms of: (a) A system of linear and/or nonlinear equations that relate the system's independent variables and parameters to the system's state (i.e., dependent) variables; (b) Probability distributions, moments thereof, inequality and/or equality constraints that define the range of variations of the system's parameters; (c) One or several quantities, customarily referred to as system responses (or objective functions, or indices of performance), which are computed using the mathematical model. The 'direct problem' solves the 'parameter-to-output' mapping that describes the 'cause-to-effect' relationship in the respective physical process. In practice, the model parameters and/or responses are experimentally measured quantities and are therefore subject to uncertainties. Many measurement problems are 'inverse' to the direct problem in that they seek to determine the properties of the medium (i.e., various cross sections), or the properties of the source, and/or the size of the medium on its boundaries from measurements of quantities that depend on the neutron flux (or from direct measurements of the neutron flux). In general, two problems are called inverses of one another if the formulation of each involves all or part of the solution of the other. Some authors further group such inverse problems into 'invasive', when the interior flux is accessible for measurements as opposed to 'non-invasive', in which only fluxes on the boundary of (or exterior to) the medium can be measured. Examples of inverse radiative transfer problems have been reviewed by McCormick, while examples of inverse source problems for neutron transport have been provided by Sanchez and McCormick. More recently, Bledsoe et al., have presented two methods (the Levenberg-Marquard, and 'differential evolution method') for addressing numerically inverse transport problems. However, although these works (as well as other works cited therein) mentioned the detrimental effects of 'small perturbations in the input data', none of the works published thus far on inverse problems for particle transport or diffusion actually showed explicitly the effects that uncertainties in the measured responses (e.g., detector measurements) would actually have on the 'inverse' deduction (from such measurements) of the underlying medium's properties, sources, etc. This work highlights explicitly the nature of the difficulties encountered in so-called 'inverse problems' by presenting a paradigm inverse neutron diffusion problem admitting an exact solution, in which the 'inverse problem' is to determine the medium's diffusion coefficient from flux measurements. It will be shown explicitly (analytically) that this is not possible to achieve in a unique manner, even in the absence of measurement uncertainties, unless additional error-free information is provided about the quantity to be determined (in this case, the medium's diffusion coefficient). It if further shown that, even if this additional error-free information about the diffusion coefficient to be determined is available, the unavoidable experimental measurements errors, albeit it vanishingly small, in the measured flux render the 'exact solution' useless for computational purposes, since the exact solution turns out to be a divergent series. As discussed in this paper's ENDNOTES section, the paradigm problem presented in this work also underscores the need for 'regularization', which is a qualifier indicating that the inverse problem is to be solved in some approximate manner. (authors)

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

Value of Formalin Fixation for the Prolonged Preservation of Rodent Myocardial Microanatomical Organization: Evidence by MR Diffusion Tensor Imaging

Previous ex vivo diffusion tensor imaging (DTI) studies on formalin-fixed myocardial tissue assumed that, after some initial changes in the first 48 hr since the start of fixation, DTI parameters remain stable over time. Prolonged preservation of cardiac tissue in formalin prior to imaging has been seen many times in the DTI literature as it is considered orderly. Here, our objective is to define the effects of the prolonged cardiac tissue exposure to formalin on tissue microanatomical organization, as this is assessed by DTI parameters. DTI experiments were conducted on eight excised rodent hearts that were fixed by immersion in formalin. The samples were randomly divided into two equinumerous groups corresponding to shorter (~2 weeks) and more prolonged (~6–8 weeks) durations of tissue exposure to formalin prior to imaging. We found that when the duration of cardiac tissue exposure to formalin before imaging increased, water diffusion became less restricted, helix angle (HA) histograms flattened out and exhibited heavier tails (even though the classic HA transmural variation was preserved), and a significant loss of inter-voxel primary diffusion orientation integrity was introduced. The prolonged preservation of cardiac tissue in formalin profoundly affected its microstructural organization, as this was assessed by DTI parameters. The accurate interpretation of diffusivity profiles necessitates awareness of the pitfalls of prolonged cardiac tissue exposure duration to formalin. The acquired knowledge works to the advantage of a proper experimental design of DTI studies of fixed hearts.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Amino Acid Immobilization of Copper Surface Diffusion on Cu(111)

Surface diffusion and molecular self-assembly are two critically important processes in chemistry and nature. Amino acids deposited on a Cu(111) surface driving a separation at the 2D limit between self-assembling molecules and diffusing copper atoms is reported. Since the self-assembling amino acids prefer non-planar, tridentate bonding with neighboring adatoms, they attach to and immobilize diffusing copper adatoms on the surface. This chemical interaction freezes out the copper diffusion causing the condensation of solid copper adatom islands on the surface. Such separation and immobilization are observed for eight different amino acids, suggesting the generality of this phenomenon beyond a single amino acid species. Furthermore, at elevated temperatures, a disruption of the prototypical Ostwald ripening of adatom islands is also observed. Here, these results provide fundamental insight into chiral molecular self-assembly and its interplay with metal atom surface diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bulk and Short-Circuit Anion Diffusion in Epitaxial Fe 2 O 3 Films Quantified Using Buried Isotopic Tracer Layers

Self-diffusion is a fundamental physical process that, in solid materials, is intimately correlated with both microstructure and functional properties. In this work, a universal approach is presented to precisely characterize self-diffusion in ionic solids by isotopically enriching anions and/or cations at specific locations within an epitaxial film stack, and characterize their redistribution at high spatial resolution with atom probe tomography. Nanoscale anion diffusivity is quantified in epitaxial α-Fe 2 O 3 thin films deposited by molecular beam epitaxy with a thin (10 nm) buried layer highly enriched in 18 O. The isotopic sensitivity of the atom probe allows precise measurement of 18 O distribution across the sharp interfaces between this layer and the surrounding Fe 2 O 3 after annealing. Short-circuit anion diffusion through 1D and 2D structural defects in Fe 2 O 3 are also directly visualized in 3D. This versatile approach to study precisely tailored thin film samples at high spatial and mass fidelity will facilitate a deeper understanding of atomic-scale diffusion phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Distributions of Functionals of Switching Diffusions

The paper deals with methods of computation of distributions of functionals of switching diffusions. The switching between two collections of diffusion coefficients occurs at the Poisson time moments which are independent of the initial diffusions. One can also consider more general diffusions when the choice is made from three or more collections of diffusion coefficients.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2020

An Experimental Method to Measure Gaseous Diffusivity in Tight and Partially Saturated Porous Media via Continuously Monitored Mass Spectrometry

Detection of radioxenon and radioargon produced by underground nuclear explosions is one of the primary methods by which the Comprehensive Nuclear-Test–Ban Treaty (CTBT) monitors for nuclear activities. However, transport of these noble gases to the surface via barometric pumping is a complex process relying on advective and diffusive processes in a fractured porous medium to bring detectable levels to the surface. To better understand this process, experimental measurements of noble gas and chemical surrogate diffusivity in relevant lithologies are necessary. However, measurement of noble gas diffusivity in tight or partially saturated porous media is challenging due to the transparent nature of noble gases, the lengthy diffusion times, and difficulty maintaining consistent water saturation. Here, the quasi-steady-state Ney–Armistead method is modified to accommodate continuous gas sampling via effusive flow to a mass spectrometer. An analytical solution accounting for the cumulative sampling losses and induced advective flow is then derived. Experimental results appear in good agreement with the proposed theory, suggesting the presence of retained groundwater reduces the effective diffusivity of the gas tracers by 10–1000 times. Finally, by using a mass spectrometer, the method described herein is applicable to a broad range of gas species and porous media.

47 OTHER INSTRUMENTATION↗
DOE OSTI2012

Monte Carlo Modeling of Diffuse Scattering from Single Crystals: The Program ZMC

Diffuse scattering probes the local ordering in a crystal, whereas Bragg peaks are descriptive of the average long-range ordering. The population of local configurations can be explored by modeling the three-dimensional distribution of diffuse scattering. Local configurations are not constrained by the average crystallographic symmetry, so one way of modeling diffuse scattering is by modeling a disordered (short-range-ordered) structure and then calculating its diffuse scattering. The structure must contain enough unit cells to give a statistically valid model of the populations of local configurations, and so requirements for a program to model this ordering are very different from programs that model average crystal structures (used to fit the Bragg diffraction). ZMC is a program that has been developed to model diffuse scattering, particularly from molecular crystals. The strategies used to tackle the problem and the way in which they are implemented will be discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Application of IR Thermography with Thermal Diffusivity Analysis for Detection of Plies Displacement in CFRP Composites

This paper presents an efficient nondestructive testing (NDT) method for detection of plies displacement in carbon fiber reinforced plastic (CFRP) composites using the active thermography technique. To assess the capability of the proposed technique for detection of plies displacement, the CFRP composite specimens with simulated fault were designed and manufactured. The IR thermography measurements were conducted using two-sided experimental arrangement. From recorded sequences of thermal images, the temperature variations versus time plots were extracted to obtain the maximum thermal contrasts. For quantitative interpretation of the results, the thermal diffusivity values for each composite material were determined and related to the fiber volume fraction. The experimental results showed that the thermal diffusivity increases with an increase of fiber volume fraction; further, the values of thermal diffusivity are specimen thickness dependent. The obtained values of thermal diffusivity varied from 1.5 × 10{sup −7} m{sup 2}/s (for 3.2 mm composite with 8.5 vol.% carbon fiber) to 2.6 × 10{sup −7} m{sup 2}/s (for 7.7 mm composite with 34.1 vol.% carbon fiber). On the basis of these results, it was concluded that the active thermography combined with thermal diffusivity analysis can be considered as a reliable NDT technique for detection of plies displacement in CFRP composites.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Interdiffusion between Zr Diffusion Barrier and U-Mo Alloy

U-Mo alloys are being developed as low enrichment uranium fuels under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Significant reactions have been observed between U-Mo fuels and Al or Al alloy matrix. Refractory metal Zr has been proposed as barrier material to reduce the interactions. In order to investigate the compatibility and barrier effects between U-Mo alloy and Zr, solid-to-solid U-10wt.%Mo vs. Zr diffusion couples were assembled and annealed at 600, 700, 800, 900 and 1000 °C for various times. The microstructures and concentration profiles due to interdiffusion and reactions were examined via scanning electron microscopy and electron probe microanalysis, respectively. Intermetallic phase Mo2Zr was found at the interface and its population increased when annealing temperature decreased. Diffusion paths were also plotted on the U-Mo-Zr ternary phase diagrams with good consistency. The growth rate of interdiffusion zone between U-10wt.%Mo and Zr was also calculated under the assumption of parabolic diffusion, and was determined to be about 103 times lower than the growth rate of diffusional interaction layer found in diffusion couples U-10wt.%Mo vs. Al or Al-Si alloy. Other desirable physical properties of Zr as barrier material, such as neutron adsorption rate, melting point and thermal conductivity are presented as supplementary information to demonstrate the great potential of Zr as the diffusion barrier for U-Mo fuel systems in RERTR.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

A computational study of yttria-stabilized zirconia: II. Cation diffusion

Cubic yttria-stabilized zirconia is widely used in industrial electrochemical devices. While its fast oxygen ion diffusion is well understood, why cation diffusion is much slower—its activation energy (~5 eV) is 10 times that of anion diffusion—remains a mystery. Indeed, all previous computational studies predicted more than 5 eV is needed for forming a cation defect, and another 5 eV for moving one. In contrast, our ab initio calculations have correctly predicted the experimentally observed cation diffusivity. We found Schottky pairs are the dominant defects that provide cation vacancies, and their local environments and migrating path are dictated by packing preferences. As a cation exchanges position with a neighboring vacancy, it passes by an empty interstitial site and severely displaces two oxygen neighbors with shortened Zr-O distances. This causes a short-range repulsion against the migrating cation and a long-range disturbance of the surrounding, which explains why cation diffusion is relatively difficult. In comparison, cubic zirconia's migrating oxygen only minimally disturbs neighboring Zr, which explains why it is a fast oxygen conductor.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structural and chemical disorder enhance point defect diffusion and atomic transport in Ni 3 Al-based γ' phase

The ordered L1 2 γ' Ni 3 Al intermetallic inclusions are routinely used as strengthening constituents in Ni-based superalloys and recent high-entropy alloys. As there are different metallic elements relative to the alloy matrix, the composition of γ' phase inclusions usually suffers some variations. In this work, we studied the elemental partition and its influence on atomic transport properties in a model (NiCo) 3 Al γ' phase with chemical and structural disorder. Combining the Monte Carlo and molecular static techniques, we first determined the elemental distribution in such compositionally-complexed γ' phase. The results suggest that Co tends to segregate, which profoundly influences defect energetics and diffusion properties. By modeling thermally activated diffusion by molecular dynamics, we found that chemical disorder induced by Co can enhance vacancy diffusion while significantly suppress diffusion via interstitial atom migration, as compared to the stoichiometric γ' Ni 3 Al. We further reveal that these observations are closely related to the defect energetics in these systems. Furthermore, our results explain the effects of disorder, long- and short-range orders on the diffusion properties in multicomponent intermetallic alloys, which helps to understand the stability and, thus, γ'-phase-assisted strengthening at different conditions including annealing and irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Atomistic modeling of metal–nonmetal interphase boundary diffusion

Atomistic computer simulations are applied to investigate the atomic structure, thermal stability, and diffusion processes in Al–Si interphase boundaries as a prototype of metal–ceramic interfaces in composite materials. Some of the most stable orientation relationships between the phases found in this work were previously observed in epitaxy experiments. Here, a non-equilibrium interface can transform to a more stable state by a mechanism that we call interface-induced recrystallization. Diffusion of both Al and Si atoms in stable Al–Si interfaces is surprisingly slow compared with diffusion of both elements in Al grain boundaries but can be accelerated in the presence of interface disconnections. A qualitative explanation of the sluggish interphase boundary diffusion is proposed. Atomic mechanisms of interphase boundary diffusion are similar to those in metallic grain boundaries and are dominated by correlated atomic rearrangements in the form of strings and rings of collectively moving atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ultrafast molecular transport on carbon surfaces: The diffusion of ammonia on graphite

We present a combined experimental and theoretical study of the self-diffusion of ammonia on exfoliated graphite. Using neutron time-of-flight spectroscopy we are able to resolve the ultrafast diffusion process of adsorbed ammonia, NH 3 , on graphite. Together with van der Waals corrected density functional theory calculations we show that the diffusion of NH 3 follows a hopping motion on a weakly corrugated potential energy surface with an activation energy of about 4 meV which is particularly low for this type of diffusive motion. The hopping motion includes further a significant number of long jumps and the diffusion constant of ammonia adsorbed on graphite is determined with D = 3.9•10 -8 m 2 /s at 94 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Pore-scale lattice Boltzmann simulation of micro-gaseous flow considering surface diffusion effect

Some recent studies have shown that adsorbed gas and its surface diffusion have profound influence on micro-gaseous flow through organic pores in shale gas reservoirs. Here, a multiple-relaxation-time (MRT) LB model is adopted to estimate the apparent permeability of organic shale and a new boundary condition, which combines Langmuir adsorption theory with Maxwellian diffusive reflection boundary condition, is proposed to capture gas slip and surface diffusion of adsorbed gas. The simulation results match well with previous studies carried out using Molecular Dynamics (MD) and show that Maxwell slip boundary condition fails to characterize gas transport in the near wall region under the influence of the adsorbed gas. The total molar flux can be either enhanced or reduced depending on variations in adsorbed gas coverage and surface diffusion velocity. The effects of pore width, pressure as well as Langmuir properties on apparent permeability of methane transport in organic pores are further studied. It is found that the surface transport plays a significant role in determining the apparent permeability, and the variation of apparent permeability with pore size and pressure is affected by the adsorption and surface diffusion.

04 OIL SHALES AND TAR SANDS↗
DOE OSTI2013

Numerical analysis of the influence of scale effects and microstructure on hydrogen diffusion in polycrystalline aggregates

Predicting resistance to environmental degradation, especially hydrogen embrittlement (HE) has become a major concern for life assessment and risk analysis of structural materials. The microstructure of the materials plays a significant role in HE. Despite the large documentation about the subject, the contribution of hydrogen diffusion on this process stays unclear. In this work, we analyze the effects of the microstructure on hydrogen diffusion, especially the influence of grain boundaries considered as high diffusivity paths and possible sites of damage occurrence. Electrochemical permeation was simulated using finite elements method (FEM). Scale effects between the RVE (Representative Volume Element) and the size of the membrane are discussed. Domains of applicability for standard homogenization methods, especially Hashin Shtrikman model are studied using results from microstructural based FEM. Domains of invariance of diffusion behavior and concentration profiles for grain shapes and the size of the membrane are also analyzed. Thus, the difficulty to extract diffusion properties by permeation test for heterogeneous microstructures is highlighted and discussed.

hydrogen, permeation, diffusion, microstructure, g↗
DOE OSTI2016

Oxygen vacancy diffusion in bulk SrTiO3 from density functional theory calculations

Point defects and point defect diffusion contribute significantly to the properties of perovskite materials. However, even for the prototypical case of oxygen vacancies in SrTiO 3 (STO), predictions vary widely. Here we present a comprehensive and systematic study of the diffusion barriers for this material. We use density functional theory (DFT) and assess the role of different cell sizes, density functionals, and charge states. Our results show that vacancy-induced octahedral rotations, which are limited by the boundary conditions of the supercell, can significantly affect the computed oxygen vacancy diffusion energy barrier. The diffusion energy barrier of a charged oxygen vacancy is lower than that of a neutral one. Unexpectedly, we find that with increasing supercell size, the effects of the oxygen vacancy charge state, the type of DFT exchange and correlation functional on the energy barrier diminish, and the different DFT predictions asymptote to a value in the range of 0.39-0.49 eV. This work provides important insight and guidance that should be considered for investigations of point defect diffusion in other perovskite materials and in oxide superlattices.

36 MATERIALS SCIENCE↗