Browsing by Author "Thomas, J.R."

Now showing 1 - 6 of 6
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    An economical method for the determination of group constants for reactor lattices
    Rogow, Ricardo (Virginia Polytechnic Institute and State University, 1984)
    The development of an economical method for determining accurately group constants of hexagonal and rectangular cells is considered in this dissertation. The mathematical model constructed for this purpose has the capability to characterize the group constants for the entire range of the neutron spectrum. Furthermore, this model is also rigorous enough to predict the group constants with the required accuracy for a specific range of interest in the energy spectrum and for a variety of energy group configurations. The model is implemented separately for the fast and thermal energy regions. These regions are subsequently coupled via the source term. The construction of the model for the fast energy range has been pursued by implementing the transport equation specialized in a two-region cell. The regions are coupled via the escape probability functions. The model for the thermal energy range has been attained by implementing the appropriate Nelkin and Honeck amplitude functions within the kernels of the transport equation. The Nelkin amplitude function is utilized for treating light water moderated systems, and the Honeck amplitude function relates to heavy water moderated systems. The group constants calculated with the economical model have been benchmarked with those computed by the VIM Monte Carlo code. The values obtained for the group constants agree within 1-2% with those computed by VIM for the fast energy region. The agreements for the thermal energy region are within 2-3%. The CPU running time of the implemented model is about 3 1/2 minutes for a four group configuration. On the other hand a typical VIM run comprising 25,000 neutron histories and a four-group structure expends about 30 minuts of CPU time for light water moderated systems. Moreover, similar VIM runs utilizing heavy water as moderator require over one hour of CPU time. Therefore, the implemented model makes utilization of computer resources with a cost advantage of a factor of 10 or better as compared to VIM. This economical benefit of the implemented model enables it to be coupled directly with fuel depletion codes, whereby the group constants and the fuel isotopics are updated at relatively short time intervals. On the other hand, the coupling of VIM with burnup codes would result in prohibitively expensive CPU costs.
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    An investigation of the excitation frequency dependent behavior of fiber reinforced epoxy composites during vibrothermographic inspection
    Russell, Samuel Stephen (Virginia Polytechnic Institute and State University, 1982)
    This investigation concerns the frequency related behavior of delaminations in fiber reinforced composites during vibrothermography, the use of active thermography with a mechanical excitation for the nondestructive evaluation of a structure or part. Two models, one where the size and geometry of the flaw control a local resonance and the other where the part or panel is undergoing structural resonance with the flaws dissipating the mechanical energy, are proposed for this frequency related behavior and tested on simulated and service produced delaminations in coupons, panels, and a machine part of complex geometry. The behavior predicted by the local resonance model is compared with experimental observations. The vibration state of the panels or coupons is determined during the vibrothermal tests, and compared with the frequencies which cause vibrothermographic heating of the flaws as a test of the structural resonance model. The usefulness of vibrothermography is demonstrated in glass and graphite reinforced epoxy components. Impact damage sites are located in graphite epoxy panels using vibrothermography. The size of the damage is indicated not only by the size of the hot region but also by the temperature rise in the center of the flawed region. A glass epoxy machine part, which was damaged during service, was subjected to interrogation by ultrasonic C-scans, X-ray radiography, and then compared with the vibrothermographic NDE.
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    Material flow simulation in a nuclear chemical process
    Mahgerefteh, Moussa (Virginia Polytechnic Institute and State University, 1984)
    At a nuclear fuel reprocessing plant the special nuclear materials (SNM) are received as constituents of spent fuel assemblies, are converted to liquid form, and undergo a series of chemical processes. Uncertainties in measurements of SNM at each stage of reprocessing limit the accuracy of simple material balance accounting as a safeguards method. To be effective, a formal safeguards program must take into account all sources of measurement error yet detect any diversion of SNM. The objective of this study is to demonstrate an analytical method for assessing the accountability of selected constituent SNM. A combined discrete-continuous, time-dependent model using the GASP IV simulation language is developed to simulate mass flow, material accountability and measurement error at each stage of the reprocessing plant. The study demonstrates that the simulation method may be utilized to estimate the magnitude of SNM loss in an operating reprocessing plant which could reasonably be detected. Thus, the simulation method provides a level of confidence for effective loss/no-loss decisions.
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    A Monte-Carlo optical workbench for radiometric imaging system design
    Walkup, Michael D. (Virginia Tech, 1996)
    The objective of this research is to study the field of optical engineering and to combine this knowledge with radiation heat transfer analysis to create a radiometric imaging design tool. In particular, the tool is aimed at design of reflective radiometric imaging telescopes, capable of highly accurate heat flux measurements, to be used in space-borne satellite applications. A concise reference guide is provided to aid in the design of somewhat generic reflective radiometric imaging systems. Also developed is a Monte-Carlo virtual optical workbench that models one- and two-mirror reflective radiometric telescopes. The virtual optical workbench is capable of performing optical analyses and generating radiometric images based on temperatures of scene components and telescope structures. The workbench model allows simulation of surfaces with different absorptivities, along with simulation of surfaces that have diffuse as well as specular components of reflectivity. Results from the virtual optical workbench are presented to show the myriad of functions incorporated in this powerful new tool.
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    Radiative characteristics of spherical cavities having partially or completely specular walls
    Kowsary, Farshad (Virginia Polytechnic Institute and State University, 1989)
    The radiant exchange problem for an isothermal spherical cavity having diffuse-specular walls is solved and the distribution of the local heat transfer for various opening angles and surface emissivities is obtained. Subsequently, the overall emission from the cavity (i. e., the apparent emissivity of the cavity) is calculated for various opening angles and surface conditions. In addition, the overall absorption characteristics of spherical cavities having purely specular walls is investigated analytically for the case of collimated radiation entering the cavity. Various opening angles and surface conditions are considered. The Monte Carlo method is utilized to support the results obtained from the analytical calculations. Results show that in spherical cavities the apparent emissivity is not very sensitive to the degree of specularity of the cavity wall. Also, there are situations in which the diffuse cavity is a more efficient emitter than a specular cavity. Absorption characteristic results show that for cavities having purely specular walls the absorption of collimated radiation is highly dependent on the angle of incidence of radiation on the opening for small opening angles.
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    A theoretical investigation of thermal waves
    Frankel, Jay Irwin (Virginia Polytechnic Institute and State University, 1986)
    A unified and systematic study of one-dimensional heat conduction based on thermal relaxation is presented. Thermal relaxation is introduced through the constitutive equation (modified Fourier's law) which relates this heat flux and temperature. The resulting temperature and flux field equations become hyperbolic rather than the usual classical parabolic equations encountered in heat conduction. In this formulation, heat propagates at a finite speed and removes one of the anomalies associated to parabolic heat conduction, i.e., heat propagating at an infinite speed. In situations involving very short times, high heat fluxes, and cryogenic temperatures, a more exact constitutive relation must be introduced to preserve a finite speed to a thermal disturbance. The general one-dimensional temperature and flux formulations for the three standard orthogonal coordinate systems are presented. The general solution, in the temperature domain, is developed by the finite integral transform technique. The basic physics and mathematics are demonstrated by reviewing Taitel's problem. Then attention is turned to the effects of radially dependent systems, such as the case of a cylinder and sphere. Various thermal disturbances are studied showing the unusual physics associated with dissipative wave equations. The flux formulation is shown to be a viable alternative domain to develop the flux distribution. Once the flux distribution has been established, the temperature distribution may be obtained through the conservation of energy. Linear one-dimensional composite regions are then investigated in detail. The general temperature and flux formulations are developed for the three standard orthogonal coordinate systems. The general solution for the flux and temperature distributions are obtained in the flux domain using a generalized integral transform technique. Additional features associated with hyperbolic heat conduction are displayed through examples with various thermal disturbances. A generalized expression for temperature dependent thermal conductivity is introduced and incorporated into the one-dimensional hyperbolic heat equation. An approximate analytical solution is obtained and compared with a standard numerical method. Finally, recommendations for future analytical and experimental investigations are suggested.