Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003. / Includes bibliographical references (p. 155-161). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / This dissertation reports on the development of a diagnostic visible imaging system on the Alcator C-Mod tokamak and the results from that system. The dissertation asserts the value of this system as a qualitative and quantitative diagnostic for magnetically confined plasmas. The visible imaging system consists of six CCD cameras, absolutely calibrated and filtered for specific spectral ranges. Two of these cameras view the divertor region tangentially, two view RF antenna structures and two are used for a wide-angle survey of the vacuum vessel. The divertor viewing cameras are used to generate two-dimensional emissivity profiles using tomography. Three physics issues have been addressed using the visible imaging system: 1) Using two-dimensional emissivity profiles of Da, volumetric recombination rate profiles have been measured and found to have a structure that depends on a poloidal temperature gradient in the outer scrape-off-layer. 2) A camera viewing the inner wall tangentially was used to measure Da emission profiles. A sharp break in slope of the radial density profile was found at the location of the secondary separatrix near the inner wall by using these profiles and a kinetic model of the neutrals. 3) Two-dimensional emissivity profiles of visible continuum (420-430nm) have been measured and found to be an order of magnitude too large when compared to expected levels from electron-ion bremsstrahlung and radiative recombination. Several atomic and molecular processes have been considered to explain the enhanced continuum. However, none of the considered processes could explain the continuum level without particle densities inconsistent with current modeling efforts. / (cont.) The visible imaging system was also used in identifying the causes of impurity injections during discharges, in identifying the failure of invessel components, and as a monitor of vessel and plasma conditions. Both the physics results and the operational benefits of the visible imaging system show that the system is a valuable quantitative and qualitative diagnostic. / by Christopher James Boswell. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003. / Includes bibliographical references (p. 199-211). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Multiscale materials modeling has emerged in recent years as a significant concept and the only viable approach to understand the mechanical response of materials by linking modeling research at different length scales and time scales, including quantum mechanics, atomistics modeling, mesoscale modeling and continuum modeling together. The role that atomistic modeling plays is important and indispensable in that it can generate understanding of the physics and pass this mechanistic understanding as well as important parameters to higher level simulations. This thesis presents theories and simulations of defect nucleation and mobility in BCC transition metal molybdenum using atomistic methods, with the two primary defects of interest being dislocations and deformation twinning, and emphasis on the atomistic mechanisms and measures. The contributions presented in the thesis may be regarded as advances in both methods development and mechanistic understanding of dislocation mobility and twinning nucleation. For atomistic studies of dislocations in a simulation cell of finite size, new methods have been derived to (1) calculate the atomic displacement field under PBC and generate atomic dislocation configuration from this displacement field; (2) quantify artificial image effect for a dislocation moving within the periodic cell and optimize geometry of simulation cell to minimize such effects. These methodological breakthroughs serve as the basis for atomistic studies of dislocations and are used repeatedly in this thesis. To understand the complex plasticity behavior of BCC metal molybdenum, we need to first understand the most fundamental problem of core structure / (cont.) and lattice resistance of straight dislocations before considering more complicated structures including kinks, jogs, junctions, etc. In this thesis, we examine the two major types of dislocations that are of primary interest in BCC molybdenum: screw dislocations, which is the major plasticity carrier at low temperatures, and edge dislocations, which become important at elevated temperatures. For screw dislocations, the issue of whether core is polarized or not is explored by a static calculation of core structure for the first time by a tight-binding potential. The screw core structure is determined to be non-polarized; however, the magnitude of Peierls stress, which is a measure of the lattice resistance to dislocation motion and is calculated accurately using a local-driving-force method in this thesis, remains rather high despite of the non-polarized core, suggesting that core polarization is a secondary and less important effect compared to core planarity. For edge dislocations in molybdenum, the core structure and Peierls stress is calculated using a Finnis-Sinclair potential and it is shown that edge dislocation in Mo has a planar core structure and a core width of approximately 11[angstroms], is indeed very mobile and has a Peierls stress of only 25 MPa, after correction for image effects due to boundary conditions. This new result is consistent with experimental observations of high mobility of edge dislocations and clearly rejects the proposal of kink mechanisms operating on edge dislocations that arose in the literature as a result of previous inaccurate Peierls stress calculations yielding an unrealistically high value of 700 MPa ... / by Jinpeng Chang. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003. / Includes bibliographical references. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / A comparative analysis of nuclear fuel cycles was carried out. Fuel cycles reviewed include: once-through fuel cycles in LWRs, PHWRs, HTGRs, and fast gas cooled breed and burn reactors; single-pass recycle schemes: plutonium recycle in LWRs and direct-use of spent PWR fuel in CANDU reactors (DUPIC); multi-pass recycle schemes: transmutation of transuranics in LWRs, fast reactors, double strata systems, and molten salt reactors. Mass flow calculations for the fuel cycles at equilibrium were carried out based on data available in the open literature, and results were used to compare the performance of the fuel cycles with respect to uranium utilization, waste management, proliferation resistance, and economics. Potential for mid-century deployment was assessed based on these results. Once-through fuel cycles based on solid fuel thermal reactors are found to be the best candidates for mid-century deployment because the substantial increase in electricity costs entailed by reprocessing schemes is unlikely to be justified by the afforded reductions in long-term proliferation and waste management risks. Furthermore, once-through cycles present lower proliferation and waste management risks in the short-term and their inefficient use of uranium is not likely to become an important issue before the middle of the century even under a high growth scenario. / by Etienne Parent. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003. / Includes bibliographical references (p. 40). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Functional magnetic resonance imaging (fMRI) has been very useful in helping neuroscientists map the brain. One tool to investigate the interactions between brain regions is to disable a small region in the brain, and look at the functional consequences of this (reversible) inactivation upon regions anatomically connected to the inactivated site. A number of issues need to be resolved before the reversible cooling technique can be used in fMRI studies. The solutions to a number of problems directly related to using reversible inactivation by cooling in conjunction with fMRI experiments on monkey brains are presented in this thesis. Specifically, these include (1) designing a cooling system and cooling probe capable of reversibly cooling the surface cortex of the monkey's brain, (2) develop or use an existing method to measure the temperature distribution with the MR-scanner, and (3) design and construct a coil (phase array) that will be used to obtain temperature and fMRI data at the highest resolution possible. A cooling system and coolant probe were designed capable of changing the temperature of the surface cortex from 37 oC to 20 oC. The Proton Resonance Frequency Shift method, which calculates the temperature based on the phase change between two images, was used to measure the temperature distribution inside an object using an fMRI sequence similar to the one that will be used in the actual experiment. The method was tested and showed an accuracy of ± 0.6 oC as compared with concurrent thermocouple measurements when adjusted for phase drift. A precision of ± 0.15 oC was found at a resolution of 2.1 x 2.1 x 1.0 mm3. A phase array head coil was designed with superior imaging qualities to the current single coil. An increase of SNR from 40 to 52 was observed in the image (30% increase) as compared with the theoretical calculated increase of 70%. / by Mark Haig Khachaturian. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1987. / Bibliography: v. 2, leaves 429-436. / by Ethel Sherry Gordon. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (p. 143). / The first measurements of the nuclear burn region of OMEGA implosions have been made with the Proton Core Imaging Spectroscope (PCIS). Using CR-39 nuclear track detectors, PCIS applies the technique of penumbral imaging to measure the radial profile of D-D and D-3He protons produced by implosions of D2-3He-filled capsules. For capsules with 20 [mu]m-CH shells, images of D-3He protons resulted in Gaussian profiles with an average l/e radius of [approx.]35 [mu]m. Gaussian profiles inferred from the D-3He protons and D-D protons produced by implosions of 2 [mu]tm SiO2-shell capsules had average l/e radii of 60 [mu]m and 94 [mu]m, respectively. [mu]m and 94 [mu]m, respectively. / by Brook-Eden Schwartz. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (p. 89). / To facilitate the mission to Mars initiative, the current work has focused on conceptual designs for transformational and enabling space nuclear reactor technologies. A matrix of design alternatives for both the reactor core and the power conversion unit were considered. Based on a preliminary screening of technologies using simplified analyses, a conceptual design was established for more detailed design work. The boiling sodium Rankine cycle was selected for the power conversion unit, and the reactor core is an ultra high power density core with highly enriched uranium fuel. The sodium Rankine cycle has many advantages, lending to a highly efficient radiator and compact reactor core. The sodium- cooled, epithermal long-term exploration nuclear engine (SELENE) is designed to be scalable to meet many differing mission requirements. The SELENE core is a comprised of Nb-lZr clad, lead bonded, UC plates in a honeycomb pattern. The fuel plates are arranged into a rectangular grid, roughly 25cm on each end. The fuel is in two zones, one is 97 a/o enriched in ²³⁵U and the other is 70 a/o enriched in ²³⁵U. The core is a fast spectrum reactor, BeO reflected, and ex-core controlled. / (cont.) Three designs are proposed, the first is for a 10 MWth / 1.0 MWe Low Temperature (1300 IK) system (SELENE-10-LT) and the second for a 10 MWth / 1.2 MWe High Temperature (1500 K) system (SELENE-10-HT) and the third for a 27 MWth / 2.6 MWe system (SELENE-30). All of these designs utilize essentially the same system architecture. Three designs are proposed so that low power variants can be used to verify the technology and develop experience. The reactor systems may then by uprated to a higher power level. The system lifetime is 9 effective full power months, corresponding roughly to a single trip from Earth to Mars and back. / by Peter Yarsky. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (p. 110-113). / The future of new nuclear power plant construction will depend in large part on the ability of designers to reduce capital, operations, and maintenance costs. One of the methods proposed, is to enhance the modularity of the designs in which the basic plant is broken down into modules, each of which is built offsite at a "factory", transported to the plant site, and assembled into a working plant using a minimum amount of labor and tooling. The value of this approach is that it improves overall quality, reduces site field work and rework, and speeds the construction of the plant. This approach also takes advantage of economies of mass-production, rather than relying on economy of scale to reduce costs. The ability to remove and replace modules for repair and maintenance also leads to reduced costs and downtime. Past work at MIT has resulted in a reference design that takes into account modularity. Based on this reference cycle design, a detailed analysis and modularity design of a power producing plant was performed. This design takes into account the goals for the modularity and construction approach. / (cont.) The plant takes the factory approach to its logical conclusion, resulting in a "virtual factory" where each major component is integrated into a modular space-frame by its manufacturer. Each module is designed to be transported by truck (although, if the necessary infrastructure is available the modules could be transported at potentially lower cost by rail or barge) to the construction site. The plant site itself requires only simple excavation and the plant containment building is a simple, reinforced poured concrete structure. Assembly of the plant is simple by comparison to conventional facilities as the modularity approach requires that each space-frame module be stacked together, pipe flanged bolted together, a self-test performed, and the plant started. In order to make this assembly method possible a pressure-backed insulation system was designed. This insulation system reduces the temperature of the pressure boundary, enabling the use of lower-cost alloys for system construction. The metallic liner and insulation layer also reduce the temperature swings of the piping and vessel walls, reducing thermal expansion loads that must be borne by the structure of the system. / (cont. ) To enable flange-joints between component modules a scavenged flange design was created. This flange design enables redundant sealing of the pressure boundary, and also prevents Helium loss and contamination of the plant. This design also enables monitoring of the flange integrity. This design should enable the MIT pebble bed reactor to meet the cost, operations and maintenance goals of an advanced reactor. / by Marc Vincent Berte. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (leaves 117-119). / Deep boreholes, 3 to 5 km into igneous rock, such as granite, are evaluated for next- generation repository use in the disposal of spent nuclear fuel and other high level waste. The primary focus is on the stability and solubility of waste species, waste forms, and canister materials in saline, anoxic water, which is the most severe potential downhole environment given the sparse data available. Pourbaix (Eh-pH) diagrams and solubility products were calculated for 20 materials of interest. In general, extremely low dissolved concentrations were estimated. Copper was identified as the best canister material. Wall-to-far-field temperature increases were estimated to be about 20⁰ C for canisters containing two PWR assemblies, which is quite tolerable. Aspects requiring further work in the near term are detailed canister interior design to withstand crushing under a 1 km stack of same, and development of a borehole plug concept having a comparable or better impermeability and radionuclide holdup than the surrounding granite bedrock. / by Victoria Katherine Anderson. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, February 2005. / Includes bibliographical references (leaf 102). / This work developed an approach to diagnose problems and identify good practices in the operation of nuclear power plants using the system dynamics technique. The research began with construction of the ORSIM (Nuclear Power Plant Operations and Risk Simulator) model, with its operational component modified from an existing model, and its risk component newly created in this research. A matrix of high-level performance indices was also created to measure plant performance as a function of continuous operation. The research continued with development of an interface program to provide a user-friendly environment, and then culminated with a report of the results obtained from a pilot project, and a demonstration of using ORSIM to investigate EPRI (Electric Power Research Institute) good practices. Both problem diagnosis and policy investigations were conducted in the pilot project. It was found that the performance matrix developed in this work was able to measure stability, reliability, and economic performance in a concise and clear manner. With this matrix, ORSIM was able to pinpoint bottlenecks of current operations and to project into the future what would be the implications of various policy changes. / (cont.) It was also found that the tracking capability of ORSIM makes it easy to locate the root causes of problems and aids in identifying places where operational improvements might be implemented. In the EPRI good practices investigation, it was demonstrated that ORSIM is a good tool that assists plant managers in evaluating the benefits and risks of applying new practices. The pilot project and EPRI practices study showed that ORSIM is a good tool for plant managers to identify potential problems and to better understand the implications of their policy changes. It therefore can help improve the quality of decision-making and help achieve more stable, reliable, and economic operations of nuclear power plants. / by Haibo Chen. / Ph.D.