Interstitial Geometry Research Articles

DEVELOPMENT OF MPACT FOR FULL-CORE SIMULATIONS OF MAGNOX GAS-COOLED NUCLEAR REACTORS

The MPACT code, jointly developed by Oak Ridge National Laboratory and University of Michigan, is designed to perform high-fidelity light water reactor (LWR) analysis using wholecore pin-resolved neutron transport calculations on modern parallel-computing hardware. MPACT uses the subgroup method for resonance self-shielding, while the primary neutron transport solver uses a 2D/1D method that is based on the method of characteristics (MoC) for the x-y planes coupled with a 1D diffusion or transport solver in the axial dimension. Additional geometry capabilities are currently being developed in MPACT to support hexagonal-pitched lattices, as well as interstitial geometry (i.e., control rods at the corner of four adjacent pin cells). In this research, the MPACT method is tested on gas-cooled reactors by applying MPACT to full-core MAGNOX reactor test problems. MAGNOX test problems were chosen due to the availability of high-quality reactor design and validation data (available through an ongoing collaboration with the National Nuclear Laboratory in the United Kingdom) and the existence of a relatively complex axial power shape that is expected to challenge the MPACT method. MPACT’s convergence for partial- and full-core problems will be tested and verified. MPACT will be compared with high-fidelity continuous-energy Monte Carlo simulations to verify core reactivity, power distributions, and performance of the available cross section data libraries and energy group structures.

Mineral Fabric as a Hidden Variable in Fracture Formation in Layered Media

Two longstanding goals in subsurface science are to induce fractures with a desired geometry and to adaptively control the interstitial geometry of existing fractures in response to changing subsurface conditions. Here, we demonstrate that microscopic mineral fabric and structure interact with macroscopic strain fields to generate emergent meso-scale geometries of induced fractures. These geometries define preferential directions of flow. Using additively manufactured rock, we demonstrate that highly conductive flow paths can be formed in tensile fractures by creating corrugated surfaces. Generation, suppression and enhancement of corrugations depend on the relative orientation between mineral fabric and layering. These insights into the role of micro-scale structure on macro-scale flow provide a new method for designing subsurface strategies to maximize potential production or to inhibit flow.

Morphological and Structural Control of Organic Monolayer Colloidal Crystal Based on Plasma Etching and Its Application in Fabrication of Ordered Gold Nanostructured Arrays

The organic monolayer colloidal crystals, which are usually prepared by self-assembling, could be used as templates, due to their interstitial geometry, for the periodically arranged nanostructured arrays, which have important applications in many fields, such as photonic crystals, information storage, super-hydrophobicity, biological and chemical sensing. Obviously, the structures of the obtained arrays mainly depend on those of the templates. However, the self-assembled monolayer colloidal crystal is exclusive in structure and for its hexagonal close-packed colloidal arrangement, leading to the limitation of the monolayer colloidal crystal as the template for the nanostructured arrays. Therefore, structural diversity is important in order for colloidal crystals to be used as the templates for various nanostructured arrays. Recently, there have been some reports on the morphological and structural manipulation of the organic monolayer colloidal crystals. In this review article, we focus on the recent progress in morphological and structural manipulation of polystyrene monolayer colloidal crystals based on plasma etching, and its application in the fabrication of the ordered gold nanostructured arrays with different structures, mainly including close-packed monolayer colloidal crystal and its transferrable property; structural manipulation based on plasma etching; and fabrication of gold nanostructured arrays based on varied monolayer colloidal crystals as template.

Gas Solubility and Diffusion in Oxide Glasses – Implications for Nuclear Wasteforms

Gas solubility and diffusion in oxide glasses define the overall mechanisms of gas transport in these materials. There are numerous practical aspects of this transport with substantial commercial implications, e.g., gas separation, the outgassing and leak testing of vacuum systems, the thermal oxidation of silicon, and the focus of this conference: nuclear wasteforms. The fundamentals of gas transport are defined in regard to gas solubility and diffusion in rigid glasses below the glass transition temperature, including the case of nuclear wasteforms.Gas transport also provides an excellent tool for better understanding the atomic-scale structure of glasses, and, in turn, that understanding helps refine our understanding of the mechanisms of gas solubility and diffusion. This understanding has also had a significant influence on models of geological importance, such as volcanic eruptions and formation of the earth's atmosphere. Specifically, the statistical thermodynamics of gas solubility is analyzed in regard to the interstitial structure of the glass. The gas atoms or molecules serve as structural probes. The structural basis of gas solubility is best understood for vitreous silica, the most-studied noncrystalline solid that can be described reasonably well by the random network model. A detailed analysis of noble gas solubility has indicated that the distribution of interstitial size is log-normal in nature. This experimental result using gas solubility has been confirmed by the analysis of computer-generated models of vitreous silica. Also, the noble gas results have consequences for the transport of molecular species such as oxygen and water, e.g., during the thermal oxidation of silicon. The substantial amount of data for noble gas transport in vitreous silica also provides an especially useful comparison of the relative nature of solubility, diffusivity, and permeability of various sized noble gas atoms.There are also useful, empirical trends for the variation of gas solubility with modifier ion content in silicate glasses. The structural analysis of these data can provide general descriptions of interstitial site geometries. Structural modeling exercises have provided a more specific description of this interstitial geometry that is the basis for a more precise understanding of the atomic-scale mechanisms for solubility, diffusivity, and permeability of gases.Finally, the principles covered above will be discussed in terms of issues central to nuclear wasteforms, viz., the accommodation of helium produced by alpha decay and oxygen generated by radiolysis.

A 3-D Model of Ligand Transport in a Deforming Extracellular Space

Cells communicate through shed or secreted ligands that traffic through the interstitium. Force-induced changes in interstitial geometry can initiate mechanotransduction responses through changes in local ligand concentrations. To gain insight into the temporal and spatial evolution of such mechanotransduction responses, we developed a 3-D computational model that couples geometric changes observed in the lateral intercellular space (LIS) of mechanically loaded airway epithelial cells to the diffusion-convection equations that govern ligand transport. By solving the 3-D fluid field under changing boundary geometries, and then coupling the fluid velocities to the ligand transport equations, we calculated the temporal changes in the 3-D ligand concentration field. Our results illustrate the steady-state heterogeneities in ligand distribution that arise from local variations in interstitial geometry, and demonstrate that highly localized changes in ligand concentration can be induced by mechanical loading, depending on both local deformations and ligand convection effects. The occurrence of inhomogeneities at steady state and in response to mechanical loading suggest that local variations in ligand concentration may have important effects on cell-to-cell variations in basal signaling state and localized mechanotransduction responses.

GGA+Udescription of lithium intercalation into anataseTiO2

We have used density-functional theory [generalized gradient approximation (GGA)] to study lithium intercalation at low concentration into anatase ${\text{TiO}}_{2}$. To describe the defect states produced by Li doping a Hubbard ``$+U$'' correction is applied to the $\text{Ti}\text{ }d$ states $(\text{GGA}+U)$. Uncorrected GGA calculations predict ${\text{Li}}_{x}{\text{TiO}}_{2}$ to be metallic with the excess charge distributed over all Ti sites, whereas $\text{GGA}+U$ predicts a defect state 0.96 eV below the conduction band, in agreement with experimental photoelectron spectra. This occupied defect state corresponds to charge strongly localized at a single $\text{Ti}\text{ }3d$ site neighboring the intercalated lithium with a magnetization of $1\text{ }\ensuremath{\mu}\text{B}$. This polaronic state produces a redshifted optical absorption spectrum, which is compared to those for the native O-vacancy and Ti-interstitial defects. The strong localization of charge at a single Ti center lowers the symmetry of the interstitial geometry relative to that predicted by GGA. The intercalated lithium sits close to the center of the octahedral site, occupying a single potential energy minimum with respect to displacement along the [001] direction. This challenges the previous interpretation of neutron diffraction data that there exist two potential energy minima separated by $1.6\text{ }\text{\AA{}}$ along the [001] direction within each octahedron. Nudged elastic band calculations give barriers to interoctahedral diffusion of $\ensuremath{\sim}0.6\text{ }\text{eV}$, in good agreement with experimental data. These barrier heights are found to depend only weakly on the position of the donated electron. The intercalation energy is 2.14 eV with GGA and 1.88 eV with $\text{GGA}+U$, compared to the experimental value of $\ensuremath{\sim}1.9\text{ }\text{eV}$. Li-electron binding energies have also been calculated. The $[{\text{Li}}_{\text{i}}^{\ifmmode\bullet\else\textbullet\fi{}}{\text{-Ti}}_{\text{Ti}}^{\ensuremath{'}}]$ complex has a binding energy of 56 meV, and a second electron is predicted to be bound to give $[{\text{Li}}_{\text{i}}^{\ifmmode\bullet\else\textbullet\fi{}}\text{\ensuremath{-}}2{\text{Ti}}_{\text{Ti}}^{\ensuremath{'}}]$ with a stabilization energy of 30 meV, indicating that intercalated lithium will weakly trap excess electrons produced during photoillumination or introduced by additional $n$-type doping.

Sensitivity analysis of imaging geometries for prostate diffuse optical tomography.

Endoscopic and interstitial diffuse optical tomography have been studied in clinical investigations for imaging prostate tissues, yet, there is no comprehensive comparison of how these two imaging geometries affect the quality of the reconstruction images. In this study, the effect of imaging geometry is investigated by comparing the cross-section of the Jacobian sensitivity matrix and reconstructed images for three-dimensional mathematical phantoms. Next, the effect of source-detector configurations and number of measurements in both geometries is evaluated using singular value analysis. The amount of information contained for each source-detector configuration and different number of measurements are compared. Further, the effect of different measurements strategies for 3D endoscopic and interstitial tomography is examined. The pros and cons of using the in-plane measurements and off-plane measurements are discussed. Results showed that the reconstruction in the interstitial geometry outperforms the endoscopic geometry when deeper anomalies are present. Eight sources 8 detectors and 6 sources 12 detectors are sufficient for 2D reconstruction with endoscopic and interstitial geometry respectively. For a 3D problem, the quantitative accuracy in the interstitial geometry is significantly improved using off-plane measurements but only slightly in the endoscopic geometry.

Influence of image resolution and evaluation algorithm on estimates of the lacunarity of porous media

In recent years, experience has demonstrated that the classical fractal dimensions are not sufficient to describe uniquely the interstitial geometry of porous media. At least one additional index or dimension is necessary. Lacunarity, a measure of the degree to which a data set is translationally invariant, is a possible candidate. Unfortunately, several approaches exist to evaluate it on the basis of binary images of the object under study, and it is unclear to what extent the lacunarity estimates that these methods produce are dependent on the resolution of the images used. In the present work, the gliding-box algorithm of Allain and Cloitre [Phys. Rev. A 44, 3552 (1991)] and two variants of the sandbox algorithm of Chappard et al. [J. Pathol. 195, 515 (2001)], along with three additional algorithms, are used to evaluate the lacunarity of images of a textbook fractal, the Sierpinski carpet, of scanning electron micrographs of a thin section of a European soil, and of light transmission photographs of a Togolese soil. The results suggest that lacunarity estimates, as well as the ranking of the three tested systems according to their lacunarity, are affected strongly by the algorithm used, by the resolution of the images to which these algorithms are applied, and, at least for three of the algorithms (producing scale-dependent lacunarity estimates), by the scale at which the images are observed. Depending on the conditions under which the estimation of the lacunarity is carried out, lacunarity values range from 1.02 to 2.14 for the three systems tested, and all three of the systems used can be viewed alternatively as the most or the least "lacunar." Some of this indeterminacy and dependence on image resolution is alleviated in the averaged lacunarity estimates yielded by Chappard et al.'s algorithm. Further research will be needed to determine if these lacunarity estimates allow an improved, unique characterization of porous media.

Interatomic potential for vanadium suitable for radiation damage simulations

The ability to predict the behavior of point defects in metals, particularly interstitial defects, is central to accurate modeling of the microstructural evolution in environments with high radiation fluxes. Existing interatomic potentials of embedded atom method type predict disparate stable interstitial defect configurations in vanadium. This is not surprising since accurate first-principles interstitial data were not available when these potentials were fitted. In order to provide the input information required to fit a vanadium potential appropriate for radiation damage studies, we perform a series of first-principles calculations on six different interstitial geometries and vacancies. These calculations identify the 〈111〉 dumbbell as the most stable interstitial with a formation energy of approximately 3.1 eV, at variance with predictions based upon existing potentials. Our potential is of Finnis–Sinclair type and is fitted exactly to the experimental equilibrium lattice parameter, cohesive energy, elastic constants and a calculated unrelaxed vacancy formation energy. Two additional potential parameters were used to obtain the best fit to the set of interstitial formation energies determined from the first-principles calculations. The resulting potential was found to accurately predict both the magnitude and ordering of the formation energies of six interstitial configurations and the unrelaxed vacancy ground state, in addition to accurately describing the migration characteristics of the stable interstitial and vacancy. This vanadium potential is capable of describing the point defect properties appropriate for radiation damage simulations as well as for simulations of more common crystal and simple defect properties.

The electronic properties of native interstitials in ZnO

To investigate the role of interstitial geometry, we reported ab initio calculations of octahedrally and tetrahedrally native interstitials in ZnO, using the full-potential linear muffin-tin orbital method. The results show that both zinc interstitials could contribute to the native n-type conduction in ZnO, meanwhile, the zinc vacancy and octahedral oxygen interstitial may contribute to the p-type conduction in ZnO. We also suggest that the 31 and 61 meV donors found by temperature-dependent Hall experiment originate from tetrahedral- and octahedral zinc interstitials, respectively, based on their charge distribution. It is noticeable that tetrahedral interstitials cause stronger interaction between interstitials and its neighboring atoms than that of octahedral interstitials.

Pulmonary microvascular and perivascular interstitial geometry during development of mild hydraulic edema

To study pulmonary arteriolar vasomotion in control conditions and in the transition to hydraulic edema, changes in subpleural pulmonary arteriolar diameter and perivascular interstitial volume were evaluated in anesthetized spontaneously breathing rabbits. Images of subpleural pulmonary microvessels were recorded in control conditions and for up to 180 min during a 0.5 ml x kg(-1) x min(-1) intravenous saline infusion through an intact parietal pleural window. Images were digitized and analyzed with a semiautomatic procedure to determine vessel diameter and perivascular interstitial thickness from which interstitial fluid volume was derived. In control vessels, the diameter of approximately 30-, approximately 50-, and approximately 80-microm arterioles and the perivascular interstitial thickness were fairly stable. During infusion, the diameter increased maximally by 20% in approximately 30 microm vessels, was unchanged in approximately 50 microm vessels, and decreased by 25% in approximately 80-microm arterioles; the perivascular interstitial volume increased by 54% only around 30-microm microvessels. In papaverine-treated rabbits, all arterioles dilated and a larger increase in perivascular interstitial thickness was observed. The data suggest that the opposite vasomotor behavior of 30- and 80-microm arterioles during development of mild edema may represent a local specific response of the pulmonary microcirculation to reduce capillary pressure in the face of an increased transendothelial fluid filtration, thus counteracting progression toward severe edema.

Local Mass Transfer Correlations for Nonaqueous Phase Liquid Pool Dissolution in Saturated Porous Media

Local mass transfer correlations are developed to describe the rate of interface mass transfer of single component nonaqueous phase liquid (NAPL) pools in saturated subsurface formations. A three-dimensional solute transport model is employed to compute local mass transfer coefficients from concentration gradients at the NAPL-water interface, assuming that the aqueous phase concentration along the NAPL-water interface is constant and equal to the solubility concentration. Furthermore, it is assumed that the porous medium is homogeneous, the interstitial fluid velocity steady and the dissolved solute may undergo first-order decay or may sorb under local equilibrium conditions. Power-law expressions relating the local Sherwood number to appropriate local Peclet numbers are developed for both rectangular and elliptic/circular source geometries. The proposed power law correlations are fitted to numerically generated data and the correlation coefficients are determined using nonlinear least squares regression. The estimated correlation coefficients are found to be direct functions of the interstitial fluid velocity, pool dimensions, and pool geometry.

Intercomparative studies of brachytherapy techniques combined with 252CF Boron neutron capture therapy

In this work the depth dose distribution for γ-rays, slow and fast neutrons, nuclear recoils and α-particles inside a tumor phantom filled with 100 ppm 10B in distilled water was measured. The depth dose distributions were measured and represented on three-dimensional plots when a 252Cf neutron source was situated in interstitial, surface and intracavitary brachytherapy techniques. Comparison of the three brachytherapy techniques when combined with BNCT revealed that the damage induced in the tumor by the α-particles as nuclear recoils in the interstitial geometry is much more than the other two techniques.

The thermodynamics of liquid Nb-N solutions

Solubility isotherms for liquid Nb-N solutions exhibit large departures from Sieverts' Law. It is shown that this behavior is a consequence of the interstitial geometry of such solutions and a small repulsive interaction between N-atoms.

The possibility of a split interstitial silver ion in AgCl

Hartree-Fock calculations treating the normal and split interstitial geometries for the interstitial silver ion at low temperature in AgCl have been performed. Several different embedded cluster models having different sizes, basis sets, and treatments of lattice polarization and electron correlation were studied. We found that the split interstitial geometry was favoured by up to a few tenths of an electronvolt, with the greatest relative stability occurring with the most extensive basis sets and with treatment of electron correlation. This species has a calculated bond length of 2.36 AA and is not a deep electron trap in AgCl.