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Studies of coaxial multipactor in the presence of a magnetic field

Becerra, Gabriel E. (Becerra Toledo)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 107 p.
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Multipactor discharges consists of electron multiplication between two surfaces by secondary electron emission in resonance with an alternating electric field. They are detrimental to the performance of radio frequency (RF) systems, such as the ICRF (ion cyclotron range of frequencies) antennas for heating of plasmas in the Alcator C-Mod tokamak and other nuclear fusion devices. This work investigates multipactor discharges in the coaxial geometry in the presence of a constant and uniform magnetic field transverse to the direction of electromagnetic wave propagation. Studies on the Coaxial Multipactor Experiment (CMX) show that the magnetic field decreases the degree to which the discharge detunes the RF circuit. However, it enhances the susceptibility of the system to multipactor-induced gas breakdown at low pressures, which appears to cause the observed neutral pressure limits on antenna performance in Alcator C-Mod. Different surface treatment methods involving roughening and in-situ cleaning failed to suppress the multipactor discharges in a consistent and reliable way in experiments on CMX, despite the success of similar techniques in the parallel-plate geometry. Electron trajectories are significantly more complicated in the presence of magnetic fields of different strengths...

Steady state thermal hydraulic analysis of hydride fueled BWRs

Ferroni, Paolo, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 208 p.
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(cont.) Since the results obtained in the main body of the analysis account only for thermal-hydraulic constraints, an estimate of the power reduction due to the application of neutronic constraints is also performed. This investigation, focused only on the "New Core" cases, is coupled with an increase of the thickness of the gap separating adjacent bundles from 2 to 5 mm. Under these more conservative conditions, the power gain percentages are lower, ranging between 24% and 43% (depending on the discharge burnup considered acceptable) for the upper pressure drop limit, and between 17% and 32% for the lower pressure drop limit.; (cont.) The benefits of the latter approach are evident since the space occupied by the bypass channel for cruciform control rod insertion becomes available for new fuel and a higher power can be achieved. The core power is constrained by applying thermal-hydraulic limits that, if exceeded, may induce failure mechanisms. These limits concern Minimum Critical Power Ratio (MCPR), core pressure drop, fuel average and centerline temperature, cladding outer temperature and flow-induced vibrations. To limit thermal-hydraulic instability phenomena, core power and coolant flow are constrained by fixing their ratio to a constant value. In particular...

Assessment of innovative fuel designs for high performance light water reactors

Carpenter, David Michael
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 189 p.
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To increase the power density and maximum allowable fuel burnup in light water reactors, new fuel rod designs are investigated. Such fuel is desirable for improving the economic performance light water reactors loaded with transuranic-bearing fuel for transmutation, as well as those using UO2 fuel. A proposal for using silicon carbide duplex as fuel cladding is investigated. The cladding consists of a monolithic inner layer surrounded by a tightly wound fiber-matrix composite. The monolith layer retains the volatile fission products while the composite adds strength. The FRAPCON steady-state thermo-mechanical fuel rod modeling code is used to examine the performance of SiC cladding at high fuel burnup and high power density. Empirical models are developed to describe the physical properties of the composite as a function of operating temperature and neutron fluence. A comparison of the behavior of the SiC cladding to the conventional Zircaloy cladding demonstrates that the SiC has superior resistance to creep and mechanical degradation due to radiation or oxidation. However, the lower thermal conductivity of the SiC is a major issue, which results in significantly increased peak fuel temperatures. Mixed U02-PuO2 fuel is also examined in place of traditional UO2 pellets...

Actinide minimization using pressurized water reactors

Visosky, Mark Michael
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 253 p.
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Transuranic actinides dominate the long-term radiotoxity in spent LWR fuel. In an open fuel cycle, they impose a long-term burden on geologic repositories. Transmuting these materials in reactor systems is one way to ease the long-term burden on the repository. Examining the maximum possible burning of trans-uranic elements in Combined Non-Fertile and U02 (CONFU) PWR assemblies is evaluated. These assemblies are composed of a mix of standard U02 fuel pins and pins made of recycled trans-uranics (TRU) in an inert matrix, and are designed to fit in current or future PWRs. Applying appropriate limits on the neutronic and thermal safety parameters, a CONFU-Burndown (CONFU-B) assembly design is shown to attain net TRU destruction in each fuel batch through at least 9 recycles. This represents a time span of nearly 100 years of in-core residence and out-of-core storage time. In this way, when the TRU is multi-recycled, only fission products and separation/reprocessing losses are sent to the repository, and the initial inventory of TRU is reduced over time. Thus, LWRs are able to eventually operate in a fuel cycle system with an inventory of transuranic actinides much lower than that accumulated to date. Three recycling strategies are considered...

Experimental investigation of electron multipactor discharges at very high frequency

Graves, Timothy P. (Timothy Paul)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 166 p.
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Multipactor discharges are a resonant condition in which electrons impact a surface in phase with an alternating electric field. The discharge is sustained by electron multiplication from secondary emission. As motivation, multipactor discharges can adversely affect many different rf systems in vacuum, and this work provides an improved general understanding of multipactor and gives promising results for improved performance and reliability of these systems. The Coaxial Multipactor Experiment (CMX) creates and investigates multipactor discharges in both parallel plate and coaxial geometries at very high frequency (VHF). CMX provides the first detailed investigation of multipactor energy distribution functions for both coaxial and parallel plate geometries with the use of retarding potential analyzers (RPA). A 1-D particle tracking simulation supports these experimental distributions and yields the underlying physics behind the distribution shape. Experimental and simulation energy distributions have a low energy population of defocused electrons due to space charge effects and RPA emission, and a high energy population responsible for sustaining the discharge. Results show a higher energy distribution for the coaxial geometry as compared to the parallel plate geometry with the same electrode spacing...

Collisionless ion collection by a sphere in a weakly magnetized plasma

Patacchini, Leonardo
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 135 p.
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The interaction between a probe and a plasma has been studied since the 1920s and the pioneering work of Mott-Smith and Langmuir [1], and is still today an active topic of experimental and theoretical research. Indeed an understanding of the current collection process by an electrode is relevant to diverse matters such as Langmuir and Mach-probes calibrations, dusty plasma physics, or spacecraft charging. Recent simulations relying on the ad hoc designed code SCEPTIC have fully addressed the collisionless and unmagnetized problem for a drifting collector idealized as a sphere. SCEPTIC is a 2d/3v hybrid Particle In Cell (PIC) code, in which the ion motion is fully resolved, while the electrons are treated as a Boltzmann distributed fluid [2, 3]. In the present work we tackle the transition between the unmagnetized and the weakly magnetized regime of ion collection by a spherical probe (The mean ion Larmor radius rL > rp) in a collisionless plasma (The ion mean free path Am,fp > rp). When the sphere is at space potential, we demonstrate that the ion current dependence on the background magnetic field B is linear for low B, and provide analytical expressions for this dependence. When the probe potential can not be neglected, the problem shows two distinct scale lengths: A collisionless layer of a few rp close to the probe...

An evolutionary fuel assembly design for high power density BWRs; Evolutionary BWR fuel assembly design

Karahan, Aydin
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 140 p.
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An evolutionary BWR fuel assembly design was studied as a means to increase the power density of current and future BWR cores. The new assembly concept is based on replacing four traditional assemblies and large water gap regions with a single large assembly. The traditional BWR cylindrical UO2-fuelled Zr-clad fuel pin design is retained, but the pins are arranged on a 22x22 square lattice. There are 384 fuel pins with 9.6 mm diameter within a large assembly. Twenty-five water rods with 27 mm diameter maintain the moderating power and accommodate as many finger-type control rods. The total number and positions of the control rod drive mechanisms are not changed, so existing BWRs can be retrofitted with the new fuel assembly. The technical characteristics of the large fuel assembly were evaluated through a systematic comparison with a traditional 9x9 fuel assembly. The pressure, inlet subcooling and average exit quality of the new core were kept equal to the reference values. Thus the power uprate is accommodated by an increase of the core mass flow rate. The findings are as follows: - VIPRE subchannel analysis suggests that, due to its higher fuel to coolant heat transfer area and coolant flow area, the large assembly can operate at a power density 20% higher than the traditional assembly while maintaining the same margin to dryout. - CASMO 2D neutronic analysis indicates that the large assembly can sustain an 18-month irradiation cycle (at uprated power) with 3-batch refueling...

Pool boiling heat transfer characteristics of nanofluids

Kim, Sung Joong, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 83 leaves
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Nanofluids are engineered colloidal suspensions of nanoparticles in water, and exhibit a very significant enhancement (up to 200%) of the boiling Critical Heat Flux (CHF) at modest nanoparticle concentrations (50.1% by volume). Since CHF is the upper limit of nucleate boiling, such enhancement offers the potential for major performance improvement in many practical applications that use nucleate boiling as their prevalent heat transfer mode. The nuclear applications considered are main reactor coolant for PWR, coolant for the Emergency Core Cooling System (ECCS) of both PWR and BWR, and coolant for in-vessel retention of the molten core during severe accidents in high-power-density LWR. To implement such applications it is necessary to understand the fundamental boiling heat transfer characteristics of nanofluids. The nanofluids considered in this study are dilute dispersions of alumina, zirconia, and silica nanoparticles in water. Several key parameters affecting heat transfer (i.e., boiling point, viscosity, thermal conductivity, and surface tension) were measured and, consistently with other nanofluid studies, were found to be similar to those of pure water. However, pool boiling experiments showed significant enhancements of CHF in the nanofluids. Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometry (EDS) analyses revealed that buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability...

Blind benchmark predictions of the NACOK air ingress tests using the CFD code FLUENT

Brudieu, Marie-Anne V
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 127, [84] p.
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The JAERI and NACOK experiments examine the combined effects of natural convection during an air ingress event: diffusion, onset of natural circulation, graphite oxidation and multicomponent chemical reactions. MIT has been benchmarking JAERI tests using the FLUENT code for approximately three years [1]. This work demonstrated that the three fundamental physical phenomena of diffusion, natural circulation and then chemical reactions can be effectively modeled using computational fluid dynamics. The latest series of tests conducted at the NACOK facility were two graphite corrosion experiments: The first test consisted of an open chimney configuration heated to 650C with a pebble bed zones of graphite pebbles and graphite reflectors. The second test is a similar test with a cold leg adjacent to the hot channel with an open return duct below the hot channel. Natural circulation, diffusion and graphite corrosion were studied for both tests. Using and adapting previous computational methods, the FLUENT code is used to blind benchmark these experiments. The objective is to assess the adequacy of the modeling method used in this blind bench-marking analysis by comparing these blind test predictions to the actual data and then modify the model to improve predictive capability. Ultimately...

Minor actinide waste disposal in deep geological boreholes

Sizer, Calvin Gregory
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 65 leaves
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The purpose of this investigation was to evaluate a waste canister design suitable for the disposal of vitrified minor actinide waste in deep geological boreholes using conventional oil/gas/geothermal drilling technology. The nature of minor actinide waste was considered, paying particular attention to nuclides whose decay energy and half lives were of relative significance to the minor actinide waste as a whole. Thermal Analysis was performed based on a reference borehole design, by Ian C. Hoag. The strategy of the thermal analysis is aimed at finding peak temperatures within the configuration, paying particular attention to the heat transfer under deep geological conditions in the air gap between the canister and the borehole. A first order economic analysis was made to compare the designed canister emplacement costs to that of intact spent fuel. The results of this analysis show that three minor actinide nuclides dominate heat generation after ten years cooling: Cm-244, Am-241, and Am-243 account for 97.5% of minor actinide decay heat. These three nuclides plus Np-237 account for 99% of the minor actinide mass. The thermal analysis was based on an irretrievable canister design, consisting of a 5 meter long synroc waste form, with minor actinides loaded to 1% wt...

Cluster-state creation in liquid-state NMR

Choy, Jennifer T
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 60 p.
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The subject of this thesis is devoted to a class of multiparticle entangled states known as the cluster-states. In particular, we focused on a system of four spins and studied the entanglement properties of a four-qubit cluster-state, using a set of entanglement measures for quantifying multipartite entanglement. We then experimentally prepared the linear cluster-state in a liquid NMR sample of crotonic acid, by applying a set of pulses generated by the Gradient Ascent Pulse Engineering (GRAPE) algorithm on a temporally averaged pseudo-pure state of four carbon spins. While our spectral results were consistent with the creation of a linear cluster-state, the reconstruction of the experimental density matrix via a full state tomography of the system revealed additional challenges in the detection of certain desired spin terms. These problems must be overcome before the system could be studied quantitatively.; by Jennifer T. Choy.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007.; Includes bibliographical references (p. 57-60).

Dependence of transuranic content in spent fuel on fuel burnup

Reese, Drew A. (Drew Amelia)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 53 p.
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As the increasing demand for nuclear energy results in larger spent fuel volume, implementation of longer fuel cycles incorporating higher burnup are becoming common. Understanding the effect of higher burnup on the spent fuel composition and radioactive properties is essential to ensure that spent fuel receives proper cooling in storage before it is sent to a disposal site or proper treatment and reprocessing if its useful content is to be extracted prior to disposal. Using CASMO-4, a standard Westinghouse 4-loop pressurized water reactor model was created and simulated with a three batch fuel cycle. U-235 enrichment was adjusted to achieve fuel burnups of 30, 50, 70 and 100 MWD per kg of initial uranium. These burnups demanded reload enrichments of 3.15%, 4.63%, 6.26% and 9.01% U-235 w/o respectively. The resultant spent fuel transuranic isotopic compositions were then provided as input into ORIGEN to study the decay behavior of the spent fuel. It was found that when burnup increased from 30 MWD/kg to 100 MWD/kg, the activity more than doubled due to the decreased Pu-241 content and the increased Np-239 presence. More importantly, the activity per MWD significantly decreased despite absolute increases in unit mass. The net result is that the half-life of high burnup fuels is greatly increased in comparison to low burnup fuels for the first decade of life. Beginning from day 14 after shutdown and until 10 years later...

Towards the development of an explosives detection system using Neutron Resonance Radiography

Raas, Whitney
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 188 leaves
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Detection of conventional explosives remains a challenge to air security, as indicated by recent reports detailing lapses in security screening and new requirements that mandate screening 100% of checked luggage. Neutron Resonance Radiography (NRR) has been under investigation as a supplement to conventional x-ray systems as a non-invasive, non-destructive means of detecting explosive material in checked luggage. Using fast (1-6 MeV) neutrons produced by an accelerator-based D(d,n)3He reaction and a scintillator-coupled CCD camera, NRR provides both an imaging capability and the ability to determine the chemical composition of materials in baggage or cargo. Theoretical studies and simulations have shown the potential of NRR. This thesis takes the first step towards experimental implementation using a deuterium target for multiple-element discrimination. A new neutron source has been developed to provide the high-flux neutron beam required for NRR while simultaneously minimizing gamma ray production. The gas target incorporates a 4 atm D2 gas chamber, separated from the accelerator beamline with thin, 5 [tm tungsten or 7 [im molybdenum foils supported by a honeycomb lattice structure to increase structural integrity and provide a heat removal pathway. An argon gas cooling system is incorporated to cool the target and thus increase the neutron flux. The gas target has been shown to withstand 3.0 MeV deuteron beam currents in excess of 35 ýLA for extended periods without failure...

Radiobiological evaluation of new boron delivery agents for boron neutron capture therapy

Chung, Yoonsun
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 132 p.
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This thesis evaluates the radiobiological effectiveness of three new boron compounds namely a boronated porphyrin (BOPP) and two liposome formulations for neutron capture therapy (BNCT). The methodology utilizes in vitro and in vivo comparisons that characterize compounds relative to boric acid and boronophenylalanine (BPA). In vitro evaluations utilized a colorimetric assay and 96-well plates to minimize the quantities of compound required for testing. The assay was optimized for the murine SCCVII, squamous cell carcinoma to determine the chemical toxicity and relative cellular uptake of a compound. BOPP was toxic at low concentrations and comparisons between the different compounds for thermal neutron irradiations were performed with approximately 5 [mu]g 10B/ml in the culture medium to allow radiation induced effects to govern the observed response. Using less than 300 [mu]g of compound and 250 kVp X-rays as control irradiations, a compound biological effectiveness (CBE) of 3.3 ± 0.7 was determined for BOPP that is comparable to the result for boric acid (3.5 ± 0.5) indicating a non-selective intracellular accumulation of 10B. BPA has a significantly higher CBE of 6.1 + 0.7. Boronated liposomes (MAC-16 and MAC+TAC) were evaluated with the EMT-6 murine mammary carcinoma. Biodistribution studies showed high 10B uptake in tumor (20-40 [mu]g 10B/g) 30 hours after a single i.v. injection (dose 6-20 [mu]g 10B per gram of body weight). Tumor control experiments were performed using thermal neutrons to study the efficacy of the boron delivered by liposomes and BPA. The MAC-16 produced a 16 % tumor control and BPA (dose 43 [mu]g 10B/gbw) 63 % for tumor boron concentrations of approximately 20 [mu]g 10B/g and the same neutron fluence.; (cont.) Liposome doses were limited by injection volume and so two injections were tried 2-hours apart that doubled the boron concentration in tumor compared to a single administration. This improved the therapeutic response to 67 % with less apparent skin damage than with BPA. Microscopic studies using fluorescent labeled liposomes revealed 10B was nonuniformly distributed and concentrated at the edge of the tumor. Based on these studies in the tumor cell lines chosen neither of the compounds appear superior to BPA.; by Yoonsun Chung.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

Selective irradiation of the vascular endothelium

Schuller, Bradley W
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 181 p.
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We developed a unique methodology to selectively irradiate the vascular endothelium in vivo to better understand the role of vascular damage in causing normal tissue radiation side-effects.The relationship between vascular endothelial cell apoptosis and intestinal crypt stem cell death was evaluated using uniform whole-body and selective vascular irradiation techniques. Mice received whole-body epithermal neutron beam irradiation. Additional dose was selectively targeted to endothelial cells from the short-ranged (5-9 [mu]m) particles released from neutron capture reactions in 10B confined to the blood by incorporation into 70-90 nm-diameter liposomes. Mice also received uniform photon doses above and below the threshold for death from the gastrointestinal (GI) syndrome. When plotted versus neutron beam dose, the crypt microcolony assay showed the same dose response for both the neutron beam-only and neutron beam plus boronated liposome groups. The added dose selectively delivered to the microvasculature did not cause any additional crypt loss. Jejunal cross-sections were prepared 4 hrs after irradiation and stained with TUNEL to observe and score apoptotic cells in the villus lamina propria. To uniquely identify the type of cell undergoing apoptosis in the lamina propria...

Food irradiation as a method of limiting crop loss in developing nations

Parmar, Nishaal Jitendra
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 34 leaves
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Introduction: Introduction: The world today contains an estimated 6.7 billion humans, and our population is growing at an unprecedented rate, consuming an ever-increasing amount of global resources. According to United Nations projections, the majority of this growth will occur in the third-world nations of Africa, and, to a lesser extent, Asia, among those peoples least able to afford the increasing burden on their resources. Clearly, what is needed in these African nations in the near future are more efficient, low-cost methods of using those resources they already have. Foremost among the problems faced by African developing nations is a lack of a reliable, sufficient, and nutritious food supply. Much of the African population survives on malnourished diets irregularly supplied by subsistence agriculture. In addition, crop loss due to both pests and post-harvest spoilage is much higher than in first world nations, with cold-storage technologies and modem pesticides. Equally important are the lives lost each year to food-borne disease. In the United States alone, food-borne infections cause an estimated 76 million cases of illness and 323,000 hospitalizations annually, for an estimated annual treatment cost of $6.7 billion and a death toll of thousands. In developing nations...

Wire wrapped fuel pin hexagonal arrays for PWR service

Diller, Peter Ray
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 258 p.
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This work contributes to the Hydride Fuels Project, a collaborative effort between UC Berkeley and MIT aimed at investigating the potential benefits of hydride fuel use in light water reactors (LWRs). Core design is accomplished for both hydride and oxide-fueled cores over a range of geometries via steady-state and transient thermal hydraulic analyses, which yield the maximum power, and fuel performance and neutronics studies, which provide the achievable discharge burnup. The final optimization integrates the outputs from these separate studies into an economics model to identify geometries offering the lowest cost of electricity, and provide a fair basis for comparing the performance of hydride and oxide fuels. This work focuses on the steady-state and transient thermal hydraulic as well as economic analyses for PWR cores utilizing wire wraps in a hexagonal array with UZrH1.6 and UO2. It was previously verified that square and hexagonal arrays with matching rod diameters and H/HM ratio have the same thermal hydraulic performance. In this work, this equivalence is extended to hexagonal wire wrap arrays, and verified by comparing the thermal hydraulic performance of a single hexagonal wire wrap core with its equivalent square array core with grid spacers. A separate neutronics equivalence is developed...

Empirical aspects of a Mini-Helicon Plasma Thruster Experiment (mHTX@MIT); mHTX@MIT

Palaia, Joseph Eugene, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 137 leaves
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A helicon plasma source experiment has been developed and then constructed in the MIT Space Propulsion Laboratory (SPL) vacuum chamber. This experiment allows study of the intrinsic advantages of efficient helicon plasma production for use in space electric propulsion. Historical helicon experiment data have been applied to help size the experiment. The goal was to create a robust and flexible experimental system which would allow optimization of the source and system parameters for efficient thrust generation, and would permit for correlation between helicon theory and experiment. This effort may lead to the development of a new electric propulsion device, the Mini-Helicon Plasma Thruster. A design process was undertaken for the creation of this experimental setup, with the aid of a number of students and researchers at the MIT SPL. This thesis will focus on the author's specific contributions to this larger effort, which included the following elements. A survey of past helicon experiment parameters was completed, made possible largely due to the wealth of data provided by helicon plasma use in academia and for research and development. An analysis of the flow of propellant through the thruster was completed, utilizing low Mach number flow theory. A metallic structure was designed...

Neoclassical polarization

Xiao, Yong, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 129 p.
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Sheared zonal flow is known to be the predominant saturation mechanism of plasma turbulence. Rosenbluth and Hinton(R-H) have shown that the zonal flow level is inversely proportional to the plasma radial polarizability due to magnetic drift departure from a flux surface. In another calculation, Hinton and Rosenbluth (H-R) considered the weakly collisional case in the banana regime and calculated the neoclassical polarization and associated zonal flow damping in the high frequency and low frequency limits. The work presented here extends R-H's calculation in several aspects. We calculate the neoclassical polarization for arbitrary radial wavelength zonal flows so that finite ion banana width and ion gyroradius are retained. We also add plasma shape effects into the R-H collisionless calculation and find the influence of elongation and triangularity on neoclassical polarization and zonal flow damping. In addition, we extend the H-R collisional calculation using an exact eigenfunction expansion of the collision operator to calculate neoclassical polarization for the entire range of frequencies. A semi-analytical fit of the exact results is obtained that gives the polarization to within 15% and allows the collisional zonal flow damping rate to be evaluated for arbitrary collisionality.; by Yong Xiao.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

Radiobiology of normal rat lung in Boron Neutron Capture Therapy

Kiger, Jingli Liu
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 207 p.
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Boron Neutron Capture Therapy (BNCT) is a binary cancer radiation therapy that utilizes biochemical tumor cell targeting and provides a mixed field of high and low Linear Energy Transfer (LET) radiation with differing biological effectiveness. This project investigated the radiobiology of normal rat lung in BNCT and measured the relative biological effectiveness factors for the lung. Rat thorax irradiations were carried out with x-rays and neutrons with or without the boron compound boronophenylalanine-fructose (BPA-F). Monte Carlo radiation transport simulations were used to design the rat lung neutron irradiations. Among the neutron beam facilities available for BNCT at the MIT Research Reactor, the thermal neutron beam facility was found to provide a suitable dose distribution for this project. A delimiter was designed and constructed for the rat lung irradiations as a lithiated-polyethylene plate of 1.5 cm thickness with an aperture tapered from 4 to 3 cm in width to expose the lung to the beam and shield adjacent radiosensitive organs. The simulation design was validated with in-phantom measurements using gold foil activation and the dual ion chamber technique. By using a two-field irradiation, a relatively uniform dose distribution could be delivered to the rat lung. The mean lung dose rate was 18.7 cGy/min for neutron beam only irradiation and 37.5 cGy/min with neutrons plus BPA and a blood boron concentration of 18 gg/g.; (cont.) The delimiter designed for rat lung irradiation...