Dissertations and Theses - Nuclear, Plasma, and Radiological Engineering
http://hdl.handle.net/2142/14837
Wed, 27 May 2015 16:05:22 GMT2015-05-27T16:05:22ZIrradiation damage in neutron-irradiated Fe-Cr model alloys
http://hdl.handle.net/2142/72981
Irradiation damage in neutron-irradiated Fe-Cr model alloys
Ferritic-martensitic (F-M) steels are considered as lead candidate structural materials for Generation IV fission reactors and future fusion reactors. Compared to austenitic stainless steels, these steels have superior properties in thermal conductivities and thermal expansion coefficients. In addition, they have better resistance to swelling, helium embrittlement and irradiation creep at elevated temperature (T/Tm > 0.4). However, F-M steels exhibit low-temperature irradiation-induced embrittlement that leads to a substantial decrease in toughness at lower irradiation temperature (T < 500°C) even at very low doses. The underlying microstructral mechanisms and their dependence on the irradiation temperatures and chromium contents are not well understood.
Body-centered cubic iron (Fe) and iron-chromium (Fe-Cr) (Cr = 10-16 at%) are used as model to study the irradiation-induced microstructural evolution and their relationship to mechanical properties. The irradiation effects as a function of Cr contents and irradiation temperatures were systematically investigated. Through using model materials, the effects of other substitutional alloying elements (e.g. nickel, tungsten and manganese), interstitial impurities (e.g. carbon and nitrogen) and secondary phases (e.g. carbides, nitrides and G-phase precipitates) commonly seen in commercial F-M steels can be reduced.
Neutron irradiations were carried out at Advanced Test Reactor with target doses of 0.01, 0.1 and 1 dpa and target irradiation temperatures of 300 and 450°C. Following irradiations, the resulting microstructure were investigated with transmission electron microscopy (TEM), atom probe tomography (APT) and electron backscatter diffraction (EBSD). TEM was used to observe the crystallographic defect structures caused by irradiation damage, including dislocation loops and voids. APT was used to study the precipitation of Cr-rich α' phase under irradiation-enhanced diffusion process. EBSD is used to examine the grain size distribution and possible grain growth. The corresponding mechanical properties were evaluated with the hardness measurements. Both Vicker hardness test (microhardness) and high-load nanoindentation were used.
The results of mechanical properties and microstructures were compared and related through Orowan model. In general, the increase in hardness in irradiated specimens can be attributed primarily to the formation of dislocation loops and α' precipitates. The dislocation loops appeared to results hardening at the lowest dose of 0.01 dpa. The addition of Cr in Fe greatly reduced the mobility of interstitials and small a/2<111> dislocation loops, leading to smaller loop size and more uniform distribution. The Cr effect on loop density is not clear in this study.
Increasing irradiation temperature increased the mobility of point defects and small a/2<111> dislocation loops, resulting in larger loop size and lower loop density. In Fe, a/2<111> loops were sufficiently mobile at 300°C, leading to a dislocation decoration structure. Irradiation at 450°C predominantly produced immobile a<100> dislocation loops in Fe, leading to a relatively uniform loop distribution. The addition of Cr caused a retention of a/2<111> loops in Fe-Cr irradiated at 450°C. The enhanced mobility of a/2<111> loops at higher temperature is related to the formation of some dislocation decoration in Fe-Cr.
α' precipitate effects on hardening in Fe-Cr appeared at higher dose of 1 dpa. The precipitate density is higher with increasing Cr contents and decreasing irradiation temperatures. On contract, the size of α' precipitates was relatively invariant as ~2 nm (radii) to irradiation conditions. α' precipitates were identified to be the major reason that resulted in higher hardening in Fe-Cr with higher Cr contents at 1 dpa.
Voids formation was observed in Fe irradiated at both 300 and 450°C to 1 dpa. The void size is larger in 450°C than in 300°C specimen. No voids formation was evidently observed in Fe-Cr. No grain growth can be detected in Fe irradiated at 450°C.
Iron-chromium (FeCr); Radiation Damage; Neutron Irradiation; Dislocations; Voids; Alpha prime; transmission electron microscopy (TEM); scanning electron microscopy (SEM); electron backscatter diffraction (EBSD); nanoindentation; Vickers Hardness; Atom Probe
Wed, 21 Jan 2015 00:00:00 GMThttp://hdl.handle.net/2142/729812015-01-21T00:00:00ZModern Rossi alpha measurements
http://hdl.handle.net/2142/72834
Modern Rossi alpha measurements
The Rossi-α method determines the prompt neutron decay constant in a nuclear fissioning system at or near delayed critical. Knowledge of the prompt neutron decay constant is important for a critical system as it is a major contributor to the dynamic system behavior. The classical method for the Rossi experiment used gated circuitry to track the time when a neutron was incident upon the detector. The downside of this method is that the circuitry was complex and only one single fission chain could be measured at a time. The modern method allows many chains to be measured simultaneously by a pulse time tagging system such as the LANL custom designed List-mode module.
This thesis examines the implementation of the modern Rossi-α method on the all highly enriched uranium, HEU, Zeus experiment. Measurements are taken at several subcritical configurations, at critical in the presence of a source, and at one supercritical point. During the experiment, the List-mode module generates time tags of incoming neutron pulses. After the experiment, this list of neutron pulses is complied using custom software into a histogram. This histogram is fit using off the shelf graphing software to determine the value of α.
The subcritical measurements of α are used to extrapolate α at delayed critical. The extrapolation determined the value of α at delayed critical to be α = 89910 s-1. This value is compared to the measured value of α at delayed critical which is determined to be α = 90408.4 s-1. These values differ by 0.55% which is remarkably good agreement. This thesis also examines the expected value of α using a Monte Carlo transport code, MCNP. MCNP determined the value of α at delayed critical to be 100048 ± 0.584 s-1. This result differs by 11.3% from the extrapolated value of α determined experimentally. When compared to systems with similar neutron spectra, the measured value of α fits well in comparison to historical measurements.
Reactor Physics; Rossi Alpha; Zeus Experiment; Neutron Measurements
Wed, 21 Jan 2015 00:00:00 GMThttp://hdl.handle.net/2142/728342015-01-21T00:00:00ZTwo-phase two-fluid model solver based on a high-resolution total variation diminishing scheme
http://hdl.handle.net/2142/72749
Two-phase two-fluid model solver based on a high-resolution total variation diminishing scheme
A new numerical method and a solver for the two-phase two-fluid model are developed using an innovative high-resolution, Total Variation Diminishing (TVD) scheme. The new scheme is derived first for scalar hyperbolic problems using the method of flux limiters, then extended to the two-phase two-fluid model.
A hybridization of the monotone 1st-order upwind scheme and the Quadratic Upstream Interpolation scheme (QUICK) is implemented using a new flux limiter function. The new function is derived in a systematic manner by imposing conditions necessary to ensure the TVD properties of the resulting scheme. For temporal discretization, the theta method is used, and values for the parameter theta are chosen such that the scheme is unconditionally stable (1/2≤theta≤1). Finite volume techniques with staggered mesh are then used to develop a solver for the one-dimensional two-phase two-fluid model based on different numerical schemes including the new scheme developed here. Linearized equations of state are used as closure relations for the model, with linearization derivatives calculated numerically using water properties based on the IAPWS IF-97 standard.
Numerical convergence studies were conducted to verify, first, the new numerical scheme and then, the two-phase two-fluid solver. Numerical scheme results are presented for one-dimensional pure advection problem with smooth and discontinuous initial conditions and compared to those of other classical and high-resolution numerical schemes. Convergence rates for the new scheme are examined and shown to be higher compared to those of other schemes. For smooth solutions, the new scheme was found to exhibit a convergence rate of 1.3 and a convergence rate of 0.82 for discontinuous solutions. The two-phase two-fluid model solver is implemented to analyze numerical benchmark problems for verification and testing its abilities to handle discontinuities and fast transients with phase change. Convergence rates are investigated by comparing numerical results to analytical solutions available in literature for the case of the faucet flow problem. The new solver based on the new TVD scheme is shown to exhibit higher-order accuracy compared to other numerical schemes with convergence rate of 0.8. Mass errors are also examined when phase change occurs for the shock tube problem, and compared to those of the 1st-order upwind scheme implemented in common nuclear thermal-hydraulics codes like TRACE and RELAP5. The solver is shown to exhibit numerical stability when implemented to problems with discontinuous solutions and results of the new solver were free of spurious oscillations.
Two-fluid model; Hyperbolic equations; Finite volume schemes; High resolution schemes; Total variation diminishing schemes; Flux limiters
Wed, 21 Jan 2015 00:00:00 GMThttp://hdl.handle.net/2142/727492015-01-21T00:00:00ZDifferential and Integral Invariance of the Relativistic Vlasov-Boltzmann Equation and the Associated Invariant Variational Problem
http://hdl.handle.net/2142/72450
Differential and Integral Invariance of the Relativistic Vlasov-Boltzmann Equation and the Associated Invariant Variational Problem
Relativistic Vlasov-Boltzmann kinetic equations are constructed from quantum field theory for scalar bosons and Dirac fermion fields. Lie's principle of differential invariance is extended to the case of non-linear integro-differential equations for both scalar and matrix distribution functions, corresponding to scalar and fermion kinetic equations respectively, and then is used to demonstrate the invariance of these equations under the action of the Poincare group. The invariant variational problem is then constructed for these equations, and is shown to yield the kinetic equations as Euler-Lagrange equations for the variational principle, as well as first integrals for the Euler-Lagrange, which are interpreted as conservation laws, for energy-linear momentum and angular momentum under the action of the Poincare group. Finally, macroscopic balance equations for particle and energy density, linear and angular momentum, and entropy production, appropriate for the description of non-ideal relativistic magnetohydrodynamics are obtained from moments of the relativistic kinetic equation.
Engineering, Nuclear; Physics, Fluid and Plasma; Physics, Elementary Particles and High Energy
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/2142/724501993-01-01T00:00:00ZInformation Theory for Arm Visuo-Motor Coordination
http://hdl.handle.net/2142/72451
Information Theory for Arm Visuo-Motor Coordination
The present work addresses the information processing of visuo-motor coordination. The aim of this research is to develop an adaptive model for coordinating mechanical action of an arm according to visual information. As a result, a large neural map has been generated which has been used to guide a pneumatically driven robot arm through a vision system consisting of image boards and two stereo cameras. This engineering approach has been employed to test the algorithm which might share close features with the way that biological beings solve the same sensory-motor task.; Basically, arm postures are represented through their projections onto a set of image planes. Based on the link orientations and lengths as visual primitives extracted from these images, a topological state-space is characterized. Arm kinematics is defined as transformations of topological hypersurfaces, the intersections of which represent all possible postures which any redundant arm possesses in reaching an arbitrary target position. The self-organizing feature map has learned how the topological hypersurfaces transform in the state-space during arbitrary movements of the arm. The analyses of these transformations helped in idealizing a connectionistic model for kinematics.; A model for the collision-free motion of a redundant arm manipulator moving in a workspace with obstacles is presented. A mapping of the arm surface onto a set of lattices, in which visual, motor information, and surface location are encoded, is adaptively developed through a learning procedure fed by trial movements. The map, which carries topographical features of the arm surface, is then used to guide the arm avoiding collisions with obstacles in unpredictable positions.; The main achievements of this research are the topographical neural model for obstacle-avoidance and the connectionist model for kinematics for redundant arms. Both models have been developed based on the analyses of a dynamic geometry, induced by the arm movements, embedded in a denoted visuo-motor space. The topographical neural model presents similar features with the motor cortex which might provide some insight for understanding biological visuo-motor control.
Biology, Neuroscience; Mathematics; Engineering, Mechanical
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/2142/724511993-01-01T00:00:00ZElemental Characterization of Municipal Solid Waste Incinerator Ash and Its Leachates Using Neutron Activation Analysis and Inductively Coupled Plasma Atomic Emission Spectroscopy
http://hdl.handle.net/2142/72449
Elemental Characterization of Municipal Solid Waste Incinerator Ash and Its Leachates Using Neutron Activation Analysis and Inductively Coupled Plasma Atomic Emission Spectroscopy
Incineration of municipal solid waste (MSW) is an increasingly attractive option for dealing with the burgeoning quantity of solid waste generated in the West. A combination of neutron activation analysis (NAA), inductively coupled plasma (ICP), and x-ray fluorescence (XRF) are used to analyze a wide range of elements in a variety of ash and extraction fluids, facilitating a detailed assessment of the hazards posed by the ash produced from MSW incineration.; Samples of bottom, fly and combined ash were analyzed using NAA and XRF. Epithermal NAA in conjunction with a Compton suppression system were especially helpful in reducing the detection limits of As, Au, Cd, Mo and Sb. The distribution of elements varied with the size of fly ash particles, with most volatile heavy metals except Hg being concentrated on the smallest ash particles. Since the smallest particles are those most likely to escape in emissions, elements occurring at concentrations 100-10000 times those typically found in soil (Ag, Au, Br, Ce, Cd, Cl, Hg, In, Pb, Sb, Ta and Zn) can be used to develop an aerosol source signature for MSW incinerators.; In addition to the regulatory TCLP test, water batch and sequential extractions were completed on each ash and analyzed by ICP and NAA. The smallest particles in the fly ash, which account for 75% of its mass, have the greatest buffering capacity and maintain the alkaline nature of the ash. The pH of the leaching solution, rather than the concentration of the metals in the ash, governs the metal concentrations in the leachates. Based on the results of the water batch and sequential extractions, elements easily leached from the ash with the first washing create a concentration spike which is more important than the slow release of more tightly bound forms attacked in TCLP. Liquid-to-solid ratios of 200 in the water batch extraction have no effect on the chemical behavior of the ash, implying that fears of the acidification of ash monofills by rain are grossly exaggerated.
Engineering, Nuclear; Chemistry, Nuclear
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/2142/724491993-01-01T00:00:00ZTransition Flow in Edge Plasmas: Solution of the Two-Fluid Region With Sources
http://hdl.handle.net/2142/72448
Transition Flow in Edge Plasmas: Solution of the Two-Fluid Region With Sources
In the context of the transition flow between highly collisional plasma and collisionless sheath, the linearized Fokker-Planck equation for the ions, in an intermediate collisionality region, is generalized to include an arbitrary source function. Nonisentropic effects due to the source function impact the isotropic part of the distribution function. A Legendre polynomial decomposition is performed up to an arbitrary order, and the problem for the first Legendre component is treated. Analytic expansions around zero and infinite velocities are performed, and a general solution for the distribution function is found. The treatment of the weakly collisional region, up to second order in the ratio of the mean free path over the scale height of the system, is discussed. The governing equations for this problem are laid out, and the methods of solving them are presented in detail.
Engineering, Nuclear; Physics, Fluid and Plasma
Wed, 01 Jan 1992 00:00:00 GMThttp://hdl.handle.net/2142/724481992-01-01T00:00:00ZMethods for Testing Transport Theories
http://hdl.handle.net/2142/72447
Methods for Testing Transport Theories
Classical transport based on simple particle collisions theory underestimates the rates of energy losses. The main sources of energy transport are instabilities due to fluctuations in magnetic fields, electric fields, and plasma density and/or temperature gradient. Although many theoretical models have been introduced to describe anomalous transport mechanisms, none of the proposed models, alone, provides a satisfying description of the experimental data. In recent years, there has been a great interest in the theoretical multiple mode tokamak transport model. In this study, we develop methods for testing models which contain a linear combination of several theoretical transport fluxes, namely, drift waves, resistive ballooning, and rippling modes added to a fixed level of neoclassical transport. The purpose of this work is to calibrate the proposed model against existing L-mode energy confinement time data.
Engineering, Nuclear
Wed, 01 Jan 1992 00:00:00 GMThttp://hdl.handle.net/2142/724471992-01-01T00:00:00ZX Ray Analysis of a Dense Plasma Focus
http://hdl.handle.net/2142/72446
X Ray Analysis of a Dense Plasma Focus
Both the experimental and theoretical approaches to the X-ray analysis of a dense plasma focus are explored in this research. Diagnostic techniques and analytical tools were developed to analyze the X-ray production of the dense plasma focus and to optimize its output. Time-resolved temperatures were measured with dispersive rather than filtering X-ray analysis. Densities were measured with a nitrogen interferometer. A radiative-collisional one-dimensional simulation of the plasma focus pinch was constructed with emphasis on the radiation emission from the plasma. The occurrence of radiative collapse was anticipated with the computer simulation and observed experimentally in the case of heavily seeded neon discharges. The addition of an axial magnetic field to stabilize the focus pinch was found to have beneficial effects on the X-ray emission in terms of reliability and improved source geometry.
Physics, Fluid and Plasma
Fri, 01 Jan 1988 00:00:00 GMThttp://hdl.handle.net/2142/724461988-01-01T00:00:00ZDevelopment of Advanced Nodal Diffusion Methods for Modern Computer Architectures
http://hdl.handle.net/2142/70916
Development of Advanced Nodal Diffusion Methods for Modern Computer Architectures
A family of highly efficient multidimensional multigroup advanced neutron diffusion nodal methods, ILLICO, have been implemented on sequential, vector, and vector-concurrent computers. Three dimensional realistic benchmark problems can be solved in vectorized mode in less than 0.73 s (33.86 Mflops) on a Cray X-MP/48. Vector-concurrent implementations yield speedups as high as 9.19 on an Alliant FX/8. These results show that the ILLICO method preserves essentially all of its speed advantage (previously demonstrated on scalar computers) over finite difference methods.; A self-consistent higher order nodal diffusion method has been developed and implemented. The method is shown to yield results nearly as accurate (0.02% maximum relative assembly power errors) as those of very fine mesh finite difference methods, using an assembly size coarse mesh. The method was applied to a numerical study of the transverse leakage approximation.; Nodal methods for global nuclear reactor multigroup diffusion calculations which account explicitly for heterogeneities in the assembly nuclear properties have been developed and evaluated. A systematic analysis of the zero order variable cross section nodal method has been conducted. Analyzing the KWU PWR depletion benchmark problem, it is shown that when burnup heterogeneities arise, ordinary nodal methods, which do not explicitly treat the heterogeneities, suffer a significant systematic error that accumulates. It has been recognized that the use of in-node spatially variable material properties is incompatible with the current homogenization methods. A procedure for generating generalized spatially variable nodal cross sections has therefore been devised.; A nodal method which treats explicitly the space dependence of diffusion coefficients has been developed and implemented. It is shown to be effective in problems where very strong heterogeneities occur.; A consistent burnup correction method for nodal microscopic depletion analysis has been developed. The new method makes extremely accurate and computationally highly efficient one node per assembly depletion computations possible. The nodal depletion method is benchmarked on the KWU 2-cycle PWR depletion problem. The results compare very well to the published results obtained with expensive commercially available depletion codes like VENTURE, SIMULATE-3, and NEM-BC.
Mathematics; Engineering, Nuclear; Computer Science
Fri, 01 Jan 1988 00:00:00 GMThttp://hdl.handle.net/2142/709161988-01-01T00:00:00Z