http://hdl.handle.net/1721.1/7687 2013-05-22T02:12:21Z http://hdl.handle.net/1721.1/77061 Rotation generation and transport in tokamak plasmas Podpaly, Yuri Anatoly Plasma toroidal rotation is a factor important for plasma stability and transport, but it is still a fairly poorly understood area of physics. This thesis focuses on three aspects of rotation: momentum transport, Ohmic rotation reversals, and LHCD induced rotation. Momentum transport is approached in a semi-empirical method through the development of the "Toy Model." The "Toy Model" assumes that the toroidal momentum is transported via diffusive and convective profiles, and, using assumptions about the diffusive and convective terms, it can generate the profiles of the residual stress or source as a function of space and time. Several resultant source profile calculations are shown for SSEP sweeps, rotation reversals, H-modes, and I-modes. Generally, it is observed that the convective profiles do not greatly improve the fits to the data, and that source profiles have peaks around the steep core rotation gradient region of the plasma. Rotation reversals, spontaneous reversals of the rotation direction during the Ohmic phase, are also described in this work. It is seen that they are related to the Linear Ohmic Confinement (LOC) to Saturated Ohmic Confinement (SOC) regime changeover. This relation is supported through linear gyrokinetic simulations that show that the co- to counter- reversal coincides with a change from marginally electron to ion diamagnetic direction most unstable modes which is believed to play a role in the LOC to SOC explanation as well. Lower Hybrid Current Drive (LHCD) induced rotation is also described, including the first experimental observations of bi-directional rotation on a single tokamak. These observations help to explain differences in rotation seen among the various devices running lower hybrid. The LHCD rotation reverses direction as a function of plasma current, and this occurs in a similar parameter space as the Ohmic rotation reversal; it also has turbulence changes that are reminiscent of the Ohmic reversal as well. This suggests that LHCD is, in fact, causing the plasma to transition from the ITG dominated regime to the TEM dominated regime, which explains the rotation differences. These experiments and models provide new tools to understand rotation transport and generation in tokamaks. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 187-201). 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/77059 Energy system transformation : an evaluation of innovation requirements and policy options Finan, Ashley (Ashley E.) The U.S. government and others around the world have been exploring strategies to respond to climate change for nearly two decades. Consideration of these efforts as well as the 2010 oil spill in the Gulf of Mexico, the 2011 nuclear accident at Fukushima Daichi, and improved shale gas recovery methods are spurring debate on energy policy options. An important focus of this debate is the role of innovation in reducing carbon emissions while also maintaining the affordability of energy supplies. The scale of the required transition to a low-carbon energy system is large. A simple calculation scheme based on the Kaya identity is used to evaluate this transition and to estimate the magnitude of the changes that would be required. The recent performance of the U.S. economy with respect to decarbonization and energy intensity is shown to fall far short of future needs in low-carbon scenarios. The MARKAL model is used to estimate the magnitude of the capital investment required to transform the U.S. electric power sector. A comprehensive treatment of the innovation process must consider not only research and development but also the 'downstream' stages of demonstration, early adoption, and evolutionary post-commercialization improvements. Under greenhouse gas reduction scenarios, investments will be needed in low-carbon technologies when there is still considerable uncertainty and risk associated with their performance, and when they may not be competitive with incumbent energy systems. No less than investments in research and development, these are investments in innovation. A two-stage model of the innovation process is used to estimate the investment needed to bring a new technology to a competitive cost level. The model is used to explore the contributions of early-stage and later-stage investments in innovation, and illustrates the importance of the technological learning process. A case study of innovation in the nuclear energy industry is used to evaluate the effectiveness of alternative policies for driving investment in energy technologies more generally. The case study reveals a pattern of erratic policy that discouraged private investment. The use of technology-push rather than market-pull policy tools is found to have encouraged technology lock-in and discouraged market-driven innovation. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 311-326). 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/76976 Conceptual design of an annular-fueled superheat boiling water reactor Ko, Yu-Chih, Ph. D. Massachusetts Institute of Technology The conceptual design of an annular-fueled superheat boiling water reactor (ASBWR) is outlined. The proposed design, ASBWR, combines the boiler and superheater regions into one fuel assembly. This ensures good neutron moderation throughout the reactor core. A single fuel design is used in the core. Each annular fuel element, or fuel tube, is cooled externally by boiling water and internally by steam. Fuel pellets are made of low enrichment U0 2, somewhat higher than the traditional BWR fuel enrichment. T91 and Inconel 718 are selected as candidates for the cladding material in view of their excellent physical properties and corrosion resistance. The fuel-cladding gap is filled with pressurized helium gas, like the existing lighter water reactor fuels. The ASBWR fuel assembly contains sixty annular fuel elements and one square water rod (occupying a space of four fuel elements) in an 8 by 8 square array. Annular separators and steam dryers are utilized and located above the core in the reactor vessel. Reactor internal pumps are used to adjust the core flow rate. Cruciform control rods are used to control the reactivity of the core, but more of them may be needed than a traditional BWR in view of the harder spectrum. The major design constraints have been identified and evaluated in this work. The ASBWR is found promising to achieve a power density of 50 kW/L and meet all the main safety requirements. This includes a limit on the minimum critical heat flux ratio, maximum fuel and cladding operating temperatures, and appropriate stability margin against density wave oscillations. At the expected superheated steam of 520 °C, the plant efficiency is above 40%, which is substantially greater than the efficiency of 33 to 35% that today's generation of LWRs can achieve. In addition to generating electricity, the ASBWR may also be useful for liquid fuel production or other applications that require high temperature superheated steam. The uncertainties about this design include the performance of cladding materials under irradiation, the attainment of desirable heat transfer ratio between the external and internal coolant channels throughout the fuel cycle, and the response to the traditional transients prescribed as design basis events. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, February 2011.; "September 2010." Cataloged from PDF version of thesis.; Includes bibliographical references (p. 219-225). 2011-01-01T00:00:00Z http://hdl.handle.net/1721.1/76975 An assessment of silicon carbide as a cladding material for light water reactors Carpenter, David Michael An investigation into the properties and performance of a novel silicon carbide-based fuel rod cladding under PWR conditions was conducted. The novel design is a triplex, with the inner and outermost layers consisting of monolithic SiC, while the middle layer consists of a SiC fiberwound composite. The goal of this work was evaluation of the suitability of this design for use as a fuel rod cladding material in PWRs and the identification of the effects of design alternatives on the cladding performance. An in-core loop at the MITR-II was used to irradiate prototype triplex SiC cladding specimens under typical PWR temperature, pressure, and neutron flux conditions. The irradiation involved about 70 specimens, of monolithic as well as of triplex constitution, manufactured using several different processes to form the monolith, composite, and coating layers. Post-irradiation examination found some SiC specimens had acceptably low irradiation-enhanced corrosion rates and predictable swelling behavior. However, other specimens did not fare as well and showed excessive corrosion and cracking. Therefore, the performance of the SiC cladding will depend on appropriate selection of manufacturing techniques. Hoop strength testing found wide variations in tensile strength, but patterns or performance similar to the corrosion tests. The computer code FRAPCON, which is widely used for today's fuel assessment, modified properly to account for SiC properties, was applied to simulate effects of steady-state irradiation in an LWR core. The results demonstrated that utilizing SiC cladding in a 17x17 fuel assembly for existing PWRs may allow fuel to be run to somewhat higher burnup. However, due to lack of early gap closure by creep as well as the lower conductivity of the cladding, the fuel will experience higher temperatures than with zircaloy cladding. Several options were explored to reduce the fuel temperature, and it was concluded that annular fuel pellets were a solution with industrial experience that could improve the performance sufficiently to allow reaching 40% higher burnup. Management of the fuel-cladding gap was identified as essential for control of fuel temperature and PCMI. SiC cladding performance may be limited unless cladding/fuel conductivity or gap conductance is improved. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, February 2011.; "October 2010." Cataloged from PDF version of thesis.; Includes bibliographical references (p. 194-201). 2011-01-01T00:00:00Z