http://hdl.handle.net/1721.1/7853 Mon, 27 May 2013 02:54:00 GMT 2013-05-27T02:54:00Z http://hdl.handle.net/1721.1/78208 Separate effects of surface roughness, wettability and porosity on boiling heat transfer and critical heat flux and optimization of boiling surfaces O'Hanley, Harrison Fagan The separate effects of surface wettability, porosity, and roughness on critical heat flux (CHF) and heat transfer coefficient (HTC) were examined using carefully-engineered surfaces. All test surfaces were prepared on nanosmooth indium tin oxide - sapphire heaters and tested in a pool boiling facility in MIT's Reactor Thermal Hydraulics Laboratory. Roughness was controlled through fabrication of micro-posts of diameter 20[mu]m and height 15[mu]m; intrinsic wettability was controlled through deposition of thin compact coatings made of hydrophilic SiO₂ (typically, 20nm thick) and hydrophobic fluorosilane (monolayer thickness); porosity and pore size were controlled through deposition of layer-by-layer coatings made of SiO₂ nanoparticles. The ranges explored were: 0 - 15[mu] for roughness (Rz), 0 - 135 degrees for intrinsic wettability, and 0 - 50% and 50nm for porosity and pore size, respectively. During testing, the active heaters were imaged with an infrared camera to map the surface temperature profile and locate distinct nucleation sites. It was determined that wettability can play a large role on a porous surface, but has a limited effect on a smooth non-porous surface. Porosity had very pronounced effects on CHF. When coupled with hydrophilicity, a porous structure enhanced CHF by approximately 50% - 60%. However, when combined with a hydrophobic surface, porosity resulted in a reduction of CHF by 97% with respect to the reference surface. Surface roughness did not have an appreciable effect, regardless of the other surface parameters present. Hydrophilic porous surfaces realized a slight HTC enhancement, while the HTC of hydrophobic porous surfaces was greatly reduced. Roughness had little effect on HTC. A second investigation used spot patterning aimed at creating a surface with optimal characteristics for both CHF and HTC. Hydrophobic spots (meant to be preferential nucleation sites) were patterned on a porous hydrophilic surface. The spots indeed were activated as nucleation sites, as recognized via the IR signal. However, CHF and HTC were not enhanced by the spots. In some instances, CHF was actually decreased by the spots, when compared to a homogenous porous hydrophilic surface. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 157-161). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/78208 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/77084 Mitigation of RF sheaths through design and implementation of magnetic field-aligned ICRF antenna Garrett, Michael Lane In ITER and in eventual reactors, enhanced impurity confinement due to internal transport barriers (ITBs) and H-mode operation establishes a very low tolerance for high-Z impurities [1]. Experiments have shown that impurity accumulation increases as power in the ion cyclotron range of frequencies (ICRF) is increased [2]. As a result, one of the primary challenges of ICRF heating is the reduction or elimination of impurities introduced into the plasma during ICRF operation, particularly for tokamaks with high-Z plasma facing components (PFCs). Plasma impurities associated with ICRF auxiliary heating are universally observed [3, 4, 5, 6]. However, the underlying physics of ICRF-specific impurity generation is not well understood, and observations of impurity characteristics differ among various tokamak experiments. Several methods have been proposed to reduce ICRF-specific impurity characteristics: low-Z PFC coatings such as boronization [7]; toroidal phasing of antenna straps [3]; and alignment of antenna Faraday screen elements with the total magnetic field [8]. On Alcator C-Mod we have designed a new magnetic field-aligned ICRF antenna to minimize ICRF-specific impurity characteristics. The field-aligned antenna is rotated 100 from horizontal, such that the antenna straps are perpendicular to the total magnetic field at the edge for a typical plasma discharge (BT ~ 5.4 T, 1, ~ 1 MA). ICRFinduced E-parallel is a likely candidate for producing enhanced sheath voltages that lead to greatly increased sputtering of material surfaces and enhanced impurity edge transport. Initial simulations performed using both slab and cylindrical geometry suggested nearly complete cancellation of E-parallel in front of the antenna structure for certain toroidal phasings. Using toroidal models, the cancellation of E-parallel is more modest, suggesting 3-D geometrical effects are important. Multiple antenna phases were analyzed for the field-aligned antenna using finite element method with a 3-D toroidal cold plasma model. In each case, the field-aligned antenna had reduced integrated E-parallel relative to the existing non-aligned antenna geometry, with the greatest reduction for monopole [0, 0, 0, 0] phasing. Initial results suggest that the field-aligned antenna operation results in fewer impurities in the plasma than conventional antennas. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 133-138). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/77084 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/77070 Monitoring under the Plutonium Management and Disposition Agreement : the prospects of antineutrino detection as an IAEA verification metric for the disposition of weapons-grade plutonium in the United States Copeland, Christopher Michael, S.M. Massachusetts Institute of Technology After the end of World War II, the world entered an even more turbulent period as it faced the beginnings of the Cold War, during which the prospect of mutually assured destruction between the world's largest nuclear weapon states was ever-present, and often provoked tense confrontations. Although fears of a nuclear holocaust significantly subsided after the dissolution of the Soviet Union in 1991, the world faced a potentially more dangerous prospect: the proliferation risks associated with the insecurity and unauthorized acquisition of Soviet-era nuclear warheads. Although all Soviet-era weapons were eventually acquired by Russia, concerns about the excessively large weapons stockpiles of the United States and Russia, combined with the goal of nuclear disarmament, led to the Plutonium Management and Disposition Agreement (PMDA). During the Cold War, the US and the Soviet Union respectively produced approximately 100 and 150 metric tons of weapons-grade plutonium (WGPu). Under the terms of the PMDA, both nations formally each agreed to irradiate 34 MT of excess military plutonium in the form of mixed oxide fuel (MOX) in nuclear power reactors. One of the major issues of concern associated with this agreement relates to the verification measures that will be implemented to ensure actual WGPu disposition. Additionally, despite a commitment (Article VII.3 of the PMDA) to engage and consult with the International Atomic Energy Agency (IAEA) to establish arrangements to monitor its plutonium disposition process, a formalized IAEA role within a potential multilateral verification regime has yet to be determined. In this work, the ability of the US to achieve the goals of its plutonium disposition campaign by 2018 is assessed. The suitability of the IAEA as an objective party to a multilateral verification regime under the auspices of the PMDA is also analyzed. In an attempt to aid the IAEA with such expected verification procedures, the applicability of antineutrino detection as a potential monitoring technology which could significantly enhance current monitoring procedures is considered. Although there has not yet been a formal demonstration of this technology under the auspices of the PMDA, the technology has been successfully fielded and nonintrusively operated at US and Russian reactors for years at a time, with the explicit aim of demonstrating potential relevance to a range of safeguards and verification tasks. The sensitivity of an antineutrino detector to antineutrino count rate measurements was analyzed through a hypothesis testing procedure which sought to identify statistically significant differences between the count rate evolutions of a designated baseline and potential diversion scenarios. With a specified set of parameters, the test demonstrated that the detector was capable of identifying the replacement of 7 WGPu MOX fuel assemblies with conventional LEU fuel assemblies within 360 days of the fuel cycle operation at a >95% true positive rate and a 5% false positive rate limit. These results were essentially still maintained even with a nonreactor- based antineutrino event background signal as high as 25%. Although pitfalls with regard to systematic uncertainty and operator malfeasance were revealed, potential solutions to such issues are also presented and discussed. All in all, the results obtained in this work confirm the potential efficacy and viability of antineutrino rate based measurements for a range of reactor safeguards and verification tasks. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering; and, (S.M. in Technology and Policy)--Massachusetts Institute of Technology, Engineering Systems Division, Technology and Policy Program, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 104-111). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/77070 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/77069 Thermal hydraulic limits analysis for the MIT Research Reactor low enrichment uranium core conversion using statistical propagation of parametric uncertainties Chiang, Keng-Yen The MIT Research Reactor (MITR) is evaluating the conversion from highly enriched uranium (HEU) to low enrichment uranium (LEU) fuel. In addition to the fuel element re-design from 15 to 18 plates per element, a reactor power upgraded from 6 MW to 7 MW is proposed in order to maintain the same reactor performance of the HEU core. Previous approaches in analyzing the impact of engineering uncertainties on thermal hydraulic limits via the use of engineering hot channel factors (EHCFs) were unable to explicitly quantify the uncertainty and confidence level in reactor parameters. The objective of this study is to develop a methodology for MITR thermal hydraulic limits analysis by statistically combining engineering uncertainties in order to eliminate unnecessary conservatism inherent in traditional analyses. This methodology was employed to analyze the Limiting Safety System Settings (LSSS) for the MITR LEU core, based on the criterion of onset of nucleate boiling (ONB). Key parameters, such as coolant channel tolerances and heat transfer coefficients, were considered as normal distributions using Oracle Crystal Ball for the LSSS evaluation. The LSSS power is determined with 99.7% confidence level. The LSSS power calculated using this new methodology is 9.1 MW, based on core outlet coolant temperature of 60 'C, and primary coolant flow rate of 1800 gpm, compared to 8.3 MW obtained from the analytical method using the EHCFs with same operating conditions. The same methodology was also used to calculate the safety limit (SL) to ensure that adequate safety margin exists between LSSS and SL. The criterion used to calculate SL is the onset of flow instability. The calculated SL is 10.6 MW, which is 1.5 MW higher than LSSS, permitting sufficient margin between LSSS and SL. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references. Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/77069 2012-01-01T00:00:00Z