Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005. / Includes bibliographical references (p. 189-193). / This thesis describes the development and implementation of an algorithm for dramatically increasing the accuracy and reliability of multigroup radiation diffusion simulations at low group counts. This is achieved by allowing the energy group boundaries to move in energy space as the simulation evolves. This adaption in energy space effectively removes the sensitivity of multigroup diffusion to group boundary placement and makes the technique a viable option for large, computationally expensive computer simulations. Traditional multigroup radiation diffusion solvers break down at small group counts because of the coarse discretization of highly nonlinear material opacity. Small changes in the group boundary energies can lead to wildly different mean opacities and therefore significant changes in simulation output. This sensitivity has rendered the technique unpredictable and it is generally considered to be not worth the added computational expense. Unfortunately, multigroup diffusion is the only method available for adding frequency dependence to the radiation field in a standard diffusion solver. When attempting to model a system that includes a non-equilibrium radiation field, frequency-dependent effects become important. / (cont.) While running multigroup simulations with large numbers of groups is a reliable method for obtaining increased accuracy over grey diffusion, the computational expense scales linearly with the number of groups. For large simulations, running multigroup diffusion with increased group counts is infeasible and running with small group counts is unreliable. This has led to a tendency to use grey diffusion even in environments where the radiation field is known to be out of equilibrium with the material. This thesis includes a new derivation of the diffusion equation and an overview of traditional "static" multigroup radiation diffusion along with an analysis of its shortcomings. The sensitivity due to group boundary placement for small numbers of groups is shown. Data are presented which demonstrate that small group count multigroup calculations can actually provide a worse answer than grey diffusion. A system is developed and implemented for allowing the multigroup energy boundaries to "adapt," or move in energy space, as the simulation evolves as well as a method for determining where increased energy resolution is needed for an arbitrary set of material opacities. By adapting in energy space, the sensitivity of multigroup diffusion to group boundary placement is ameliorated. / (cont.) Data are presented that demonstrate a reliable increase in accuracy for adaptive multigroup diffusion as the number of groups is increased-even at very small group counts. Furthermore, the data show that the level of accuracy obtained with the adaptive multigroup approach is equivalent to or better than the best-case data obtained with the static multigroup approach. This result is more profound than a simple increase in accuracy-the increased reliability makes multigroup radiation diffusion a viable tool for large non-equilibrium simulations. Users are no longer forced to use the grey diffusion method which is known to be physically inadequate. Furthermore, users are finally free to incorporate a frequency-dependent treatment of radiation without the accuracy ambiguities of standard multigroup diffusion. / by Richard B. Williams. / Sc.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005. / Includes bibliographical references (leaves 111-115). / Many challenges exist when considering nuclear power to provide electricity for bases on the Moon or Mars, including launch safety, landing safety, deployment, control, and protecting the astronauts from radiation. Examples from the past provide guidance in these areas but surface operations on another body have never been attempted and rarely studied. This thesis discusses the risks and design considerations for launching, transporting, landing, and operating a nuclear fission reactor on the Moon or Mars. A reference mission and reactor were chosen to facilitate analysis in these areas. Launching a reactor involves meeting environmental and federal regulations. This includes an extensive safety analysis of launch to determine if launch accidents pose a serious risk to the public due to fuel release or inadvertent criticality. The reactor must also be able to survive the launch conditions without damage. Transport mainly involves protecting the reactor from damage from meteoroids. The reactor will then land through propulsive means on the Moon or Mars. Landing a reactor will also require a safety analysis to determine the consequences of a landing accident on the Moon or Mars. On the surface, the reactor must be at a location far enough away from the astronauts to limit radiation exposure to the astronauts from the reactor. / (cont.) Interaction from ground control and astronauts will be necessary to initiate startup, shutdown, and to change the power level of the reactor; however, startup and operation of the reactor must be autonomous due to the communications time lag between Earth and the Moon or Mars. These are significant challenges but all are feasible given the technology and experience in nuclear engineering and astronautics that exits today. / by Susan Christine Kane. / S.M.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1987. / Includes bibliographical references. / by Peter Andrew Donis. / B.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001. / Includes bibliographical references (p. 64-66). / In order to better understand the behavior of thorium breeder reactor fuel in a repository environment, the behavior of thorium oxide was investigated. The kinetics of crystalline thorium oxide dissolution were studied under argon and argon/10% CO2 over a broad pH range in 0.1 M ionic strength NaC1O4 solution. Samples were analyzed for thorium concentration using ICP-AES, ICP-MS, and NAA. Data was fit to a first order kinetics model. The kinetic rate constant under pure argon was 0.0107 ± 0.00123 h-1 for pH 2-3. Under 10% CO2, the rate constant was 0.00511 ± 0.000863 h-1. No kinetics results could be discerned from the data at higher pH. Under argon, solubility at pH greater than 3 was at nm/L levels, three orders of magnitude lower than at pH 3 and below. Under 10% CO2, solubilities decreased from 10 [mu]mol/L at pH 2.06 to 80 nm/Lat pH 3. Thorium oxide powder remnants from each experiment were analysed using XRD, FflR, and BET surface area analysis. Surface area analysis showed a 15% increase in surface area at pH 10 and higher under argon, and a uniform surface area increase of 15 to 30% under 10% CO2. XRD and FfIR spectra showed that the thorium oxide was otherwise identical to the untreated standards. / by Virginia Curran. / S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000. / Includes bibliographical references (p. 175-181). / Magnetohydrodynamic (MHD) activity has been analyzed primarily using electron cyclotron emission (ECE) diagnostics on Alcator C-Mod tokamak. The main results are that i) two MHD instabilities have been identified during current ramp-up discharges (resistive 'multiple' tearing mode and ideal interchange mode) and ii) a new approach to diagnose edge localized modes (ELMs) using ECE diagnostics was explored. Both MHD modes were accompanied by hollow pressure and current profiles. The associated q-profiles were also hollow with q0 >> 1, where q0 is the safety factor on the magnetic axis. In both cases, the electron temperature fluctuations observed on ECE diagnostics agreed reasonably well with the perturbed pressure fluctuations predicted in a resistive linear stability code (MARS). For the resistive 'multiple' tearing mode , the MHD fluctuations were peaked near the outer q=3 rational surface but had several other resonant layers, which affected the plasma globally. The predicted growth time was ~0.44 msec, which is within the typical range of tearing mode evolution times. For the ideal interchange mode, the MHD fluctuations were highly localized near the inner q=5 rational surface. According to ideal MHD stability theory, the q = 5 surface was found to be ideally unstable because of the reversed pressure gradient (dp/dr > 0) and q > 1 with moderate shear. When kinetic effects were added, the ideally unstable mode was finite ion Larmor radius (FLR) stabilized. However, considering that 1) electrons are collisional, 2) ions are collisionless, and 3) the thermal ion transit frequency is comparable to the ion diamagnetic drift frequency, ion Landau damping was found to be strong enough to drive a kinetic Mercier instability. As a result, a FLR modified kinetic Mercier instability has been identified, possibly for the first time since the Mercier criterion was formulated forty years ago. During 'Type III' ELMs, rather unusual signal changes were observed on two ECE diagnostics; signal drops of second harmonic X-mode on one diagnostic and signal spikes of fundamental harmonic 0-mode on another. These were explained in terms of refraction effects and found to be useful to infer the associated geometrical dimensions. For this investigation, a new ray tracing code, which can accommodate poloidal variations, has been developed. As a result, an ELM has been modeled successfully as a poloidally elongated density loss. Observations are consistent with the following dimensions; radial width of the affected region ([delta] r) ~ 1 - 3 cm, poloidal elongation ~1.5 (equivalent to a poloidal wave number ... minimum density 0.5 x I020m-3 at the mid plane ~~ 1cm inside the last closed flux surface (LCFS). This knowledge helps to assess the influence of the particle loss on the main plasma. Considering that ELMs challenge present diagnostic capabilities in terms of spatiotemporal resolution, such indirect measurement opens the door to improved physical understanding of ELMs. In particular, it is the first to reveal the poloidal structure of an ELM. / by Yongkyoon In. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1999. / Includes bibliographical references. / A preliminary comparative assessment is made of the spent fuel characteristics and disposal aspects between a high-temperature, gas cooled, reactor with a pebble bed core (PBR) and a pressurized water reactor (PWR). There are three significant differences which impact the disposal characteristics of PBR spent pebble fuel from PWR spent fuel assemblies. Pebble bed fuel has bum-up as high as 100,000 MWD(t)/MTHM and thus, there is significantly less activity and decay heat in the fuel when it is disposed. The large amount of graphite in the waste form leads to a low power density and more waste per unit volume than a typical PWR. Pebble Fuel contains a protective layer of Silicon Carbide. The theoretical spacing of waste packages of spent pebble fuel given its unique characteristics as applied to the conditions of Yucca Mountain is of major concern when determining the cost of disposing of the larger volumes of spent pebble fuel. Graphite is a unique waste form and atypical of waste designated for Yucca Mountain. The interactions of silicon carbide with uranium oxide fuel and its implications to long term storage at the repository are examined. There are three primary conclusions to this thesis. First, the area required to store pebble fuel is less than the area required to store light water reactor spent fuel. Second, graphite has excellent characteristics as a waste form. The waste form of the spent pebble fuel is more robust and will perform better than light water reactor fuel at the United States repository at Yucca Mountain. Third, a secondary phase forms between the layers of silicon carbide and the uranium oxide fuel. The secondary phase retards the release of radionuclides to the environment. / by Paul E. Owen. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1989. / Includes bibliographical references (leaves 174-178). / by Susan Spira Hakkarainen. / Ph.D.

Thesis (Nucl. E)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1988. / Includes bibliographical references. / by Guido Renato Solares Hernández. / Nucl.E

Thesis (Nucl. E.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1989. / Includes bibliographical references (leaves 184-187). / by Carlos Alexandre O. C. Lobo. / Nucl.E.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1987. / Title as it appears in M.I.T. Graduate List, Sept. 1987: Testing the predictive value of the one-dimensional transport equation with a diffuse light approximation in predicting light flux in tissue. / Bibliography: leaf 37. / by David Camus Zraket. / M.S.