Several computer codes based on one and two-group diffusion theory models were developed for SHUFFLE. The programs were developed to calculate power distributions in a two-dimensional quarter core geometry of a pressurized power reactor. The various coarse-mesh numerical computations for the power calculations yield the following: the Borresen's scheme applied to the modified one-group power calculation came up with an improved power distribution, the modified Borresen's method yielded a more accurate power calculations than the Borresen's scheme, the face dependent discontinuity factor method have a better prediction of the power distribution than the node averaged discontinuity factor method, Both the face dependent discontinuity factor method and the modified Borresen's methods for the two-group model have quite attractive features. / Graduation date: 1992

We analyze experimental evidence of transport through evanescent waves in graphene, reconciling existing experimental data with theory. We propose novel experimental geometries that provide even more compelling evidence of evanescent waves. We investigate the shot noise generated by evanescent modes in graphene for several experimental setups. For two impurity-free graphene strips kept at the Dirac point by gate potentials, separated by a long highly doped region, we find that the Fano factor takes the universal value F=1/4. For a large superlattice consisting of many strips gated to the Dirac point, interspersed among doped regions, we find F=1/(8ln2). These results differ from the value F=1/3 predicted for a disordered metal, providing an unambiguous experimental signature of evanescent mode transport in graphene. For a graphene nano-ribbon transistor geometry, we explain that the experimentally observed anomalous voltage scale of the shot noise can arise from doping by the contacts to the electrical circuit. These observations provide strong evidence of evanescent mode transport in graphene.

This work presents a whole-core benchmark problem based on a 2-loop pressurized water reactor with both UO₂and MOX fuel assemblies. The specification includes heterogeneity at both the assembly and core level. The geometry and material compositions are fully described and multi-group cross section libraries are provided in 2, 4, and 8 group formats. Simplifications made to the benchmark specification include a Cartesian boundary, to facilitate the use of transport codes that may have trouble with cylindrical boundaries, and control rod homogenization, to reduce the geometric complexity of the problem. These modifications were carefully chosen to preserve the physics of the problem and a justification of these modifications is given. Detailed Monte Carlo reference solutions including core eigenvalue, assembly averaged fission densities and selected fuel pin fission densities are presented for benchmarking diffusion and transport methods. Three different core configurations are presented in the paper namely all-rods-out, all-rods-in, and some-rods-in.

<p>This dissertation explores strategies for reducing HIV-virion transport to mucosal surfaces to prevent HIV infection. Infection requires contact between HIV and an infectable cell, so any means of inhibiting this step could contribute to HIV prevention. Our goals were to quantify the effects of strategies that reduce transport of HIV virions and to evaluate them in the context of HIV prevention. We used fundamental transport theory to design two basic strategies: (1) modifying the effective radius of virions; and (2) modifying the native medium through which virions diffuse. We proposed to implement these strategies using (1) anti-HIV antibodies that would bind and aggregate virions and (2) topically-applied semi-solid gels that coat vaginal epithelial surfaces. </p> <p>We measured diffusion coefficients of HIV virions and HIV-like particles in the presence of antibodies and within semi-solid gels. In experiments with antibodies, we did not observe reductions in the diffusion coefficients. In experiments using particle tracking to measure the diffusion coefficients of virions in vaginal gels, we found that the diffusion coefficients in gels were approximately 10,000 times lower than those in water. </p> <p>We proceeded to evaluate the potential for semi-solid gels to prevent HIV transmission at mucosal surfaces. From previous experiments in our lab that characterized the topical deployment of vaginal gels <italic>in vivo</italic>, we know that vaginal gels form an uneven coating on the epithelium with gel layer thicknesses of the order of hundreds of microns. Thus, we determined whether semi-solid gels could function as physical barriers to HIV when deployed as thin, incomplete layers on the epithelium. </p> <p>We developed an experimental system to test the barrier functioning of thin gel layers. We applied thin gel layers to the porous membrane of a Transwell system, and added a solution of HIV to the top compartment. After incubation, samples were assayed for levels of HIV. We found that thin gel layers reduced levels of HIV in the bottom compartment compared to controls where no gel had been applied: There was a log reduction in levels of HIV in conditions where gel layers of approximately 150 &mu;m thickness had been applied to the membrane after 0-, 4-, and 8-hour incubation. Thus, it appears possible for gel layers of thicknesses found <italic>in vivo</italic> to function as physical barriers to HIV over biologically-relevant time scales. </p> <p>We studied how nonuniform deployment of semi-solid gels affects accumulation of virions in tissue using a mathematical model. We used transport theory to develop a model of HIV diffusing from semen, through gel layers where present, to tissue. Our findings suggest that comprehensive coating of over 80% of the tissue surface area and gel layer thicknesses over 100 &mu;m are crucial to the barrier functioning of topical gels. Under these conditions, the level of viral restriction makes a significant contribution to increasing the time required for virions to reach tissue. </p> <p>Overall, the work presented here applies transport theory in the context of HIV transmission and prevention. Results contribute to theoretical and experimental frameworks that can help understand events in HIV transmission and to design and evaluate new technologies for HIV prevention.</p> / Dissertation

The generation of multigroup neutron cross sections is usually the first step in the solution of reactor physics problems. This typically includes generating condensed cross section sets, collapsing the scattering kernel, and within the context of diffusion theory, computing diffusion coefficients that capture transport effects as accurately possible. Although the calculation of multigroup parameters has historically been done via deterministic methods, it is natural to think of using the Monte Carlo method due to its geometric flexibility and robust computational capabilities such as continuous energy transport. For this reason, a stochastic cross section generation method has been implemented in the Mont Carlo code MCNP5 (Brown et al, 2003) that is capable of computing macroscopic material cross sections (including angular expansions of the scattering kernel) for transport or diffusion applications. This methodology includes the capability of tallying arbitrary-order Legendre expansions of the scattering kernel. Furthermore, several approximations of the diffusion coefficient have been developed and implemented. The accuracy of these stochastic diffusion coefficients within the multigroup framework is investigated by examining a series of simple reactor problems.

ZnO is one of the most important materials for electronics, optoelectronics, piezoelectricity and optics. With a wide band gap of 3.37eV and an exiton binding energy of 60meV, ZnO 1D nanostructures exhibit promising properties in a lot of optical device applications. It is also an important piezoelectric material and has applications in a new category of nanodevices, nano-piezotronics. Demonstrated prototype of devices includes nanogenerators, piezoelectric-FET, and a series of evolutive devices based on the concept of nanogenerator. This is based on working principle of a semiconductor and piezoelectric coupled property. This thesis is about the growth, characterization and device fabrication of ZnO nanowires and nanobelts for sensors and UV detectors. First, the fundamental synthesis of ZnO nanostructurs is investigated, particularly polar surface dominated nanostructues, to illustrate the unique growth configurations of ZnO. Detail study in this part includes nanobelts, nanorings, nanocombs, nanonetworks, and nanodiskettes synthesis. Important factors in driving the nanostructure synthesis mechanism are analyzed, such as the chemical activities of different surface of ZnO and the polar surface dominated effects. Then, the devices fabricated methods using individual nanowires/nanobelts and their electrical transport properties were carefully characterized. In this part, dominant factors which are critical for nanobelt device performance are investigated, such as the contact properties, interface effects, and durability testing. Also, a metal doping method is studied to explore the controlling and modification of nanowire electric and optical properties. Further more, I will present the surface functionalization of nanobelt for largely improving its electrical, optoelectronic and chemical performance. Surface functionalization of nanobelts is proven to be an effective method in enhancing the semiconductor and metal contact. Piezoelectric field-effect transistors will be demonstrated as a powerful approach as chemical sensors. Finally, a technique is illustrated for functionalizing the surfaces of ZnO nanobelts for enhancing its UV sensitivity by over five orders of magnitude. This demonstrates an effective approach for fabricatiing ultrasensitive UV detectors. The research results presented in this thesis have made great contribution to the growth, device fabrication and novel applications of ZnO nanostructures for photonics, optoelectronics and sensors.

A generalization of multigroup energy condensation theory has been developed. The new method generates a solution within the few-group framework which exhibits the energy spectrum characteristic of a many-group transport solution, without the computational time usually associated with such solutions. This is accomplished by expanding the energy dependence of the angular flux in a set of general orthogonal functions. The expansion leads to a set of equations for the angular flux moments in the few-group framework. The 0th moment generates the standard few-group equation while the higher moment equations generate the detailed spectral resolution within the few-group structure. It is shown that by carefully choosing the orthogonal function set (e.g., Legendre polynomials), the higher moment equations are only coupled to the 0th-order equation and not to each other. The decoupling makes the new method highly competitive with the standard few-group method since the computation time associated with determining the higher moments become negligible as a result of the decoupling. The method is verified in several 1-D benchmark problems typical of BWR configurations with mild to high heterogeneity.

Transport of amorphous and semi-crystalline, oriented, annealed and non-annealed PET films has been studied using pure and mixed gas/vapor feeds to understand the influence of flavor molecules on the efficacy of the barrier material. Methanol has been used as the flavor molecule simulant, and pure methanol vapor sorption studies show swelling and relaxation effects in the polymer. Multi-component transport of O2/methanol and O2/CO¬2/methanol mixtures, performed at different activities of methanol, shows that vapor induced plasticization leads to increases in O2 and CO2 permeability. Annealed, semi-crystalline PET is shown to be most resistant to plasticization effects. It has been shown that the non-annealed film is less stable despite similar crystallinity as the annealed film due to the presence of orientation related stress in the material. Presence of crystals also restricts the chain motion, and helps suppress the plasticization effects. The results have been compared with the predictions of the dual mode model for multi-component mixtures. Plasticization effects at the high activities have been analyzed within the framework of the free volume theory. It has been proposed that only the densified domains of a glassy polymer be considered when evaluating fractional free volume change due to swelling in the polymer-penetrant system. The free volume parameter- BA has been evaluated for O2 and CO2 in PET and is found to be different from that for other high permeability polymers.

Originally presented as the author's thesis--Universität Rostock, 2007. / Includes bibliographical references.

Thesis (M.S.)--Mississippi State University. Department of Computational Engineering. / Title from title screen. Includes bibliographical references.