http://hdl.handle.net/1721.1/7681 Wed, 22 May 2013 09:56:02 GMT 2013-05-22T09:56:02Z http://hdl.handle.net/1721.1/77677 Phoenix : an interactive hierarchical topological floorplanning placer Chow, Chee-Seng Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science; and, (B.S.)--Massachusetts Institute of Technology, Dept. of Physics; and, (B.S.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1985.; Bibliography: leaf 141. Tue, 01 Jan 1985 00:00:00 GMT http://hdl.handle.net/1721.1/77677 1985-01-01T00:00:00Z http://hdl.handle.net/1721.1/77534 Accelerated clustering through locality-sensitive hashing Kishore, Shaunak We obtain improved running times for two algorithms for clustering data: the expectation-maximization (EM) algorithm and Lloyd's algorithm. The EM algorithm is a heuristic for finding a mixture of k normal distributions in Rd that maximizes the probability of drawing n given data points. Lloyd's algorithm is a special case of this algorithm in which the covariance matrix of each normally-distributed component is required to be the identity. We consider versions of these algorithms where the number of mixture components is inferred by assuming a Dirichlet process as a generative model. The separation probability of this process, [alpha], is typically a small constant. We speed up each iteration of the EM algorithm from O(nd2k) to O(ndk log 3(k/a))+nd 2 ) time and each iteration of Lloyd's algorithm from O(ndk) to O(nd(k/a). 39) time. Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 18). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/77534 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/76934 Experimental simulation of crevice corrosion of a functionally graded composite system of F91 and Fe-12Cr-2Si exposed to high-temperature lead-bismuth eutectic coolant Ferry, Sara Elizabeth In a system in which metal corrosion is of concern to its long-term structural integrity, crevice corrosion can be a significant cause of damage. Small crevices in a metal exposed to a working fluid (such as a reactor's coolant) may be prone to the development of a localized, aggressive reducing environment. If the metal relies on a passivating layer of oxides for corrosion protection, it may be vulnerable to corrosion attack within the crevice due to a drastically reduced oxygen potential and low pH. Furthermore, in a liquid metal environment, the reducing conditions combined with typically high solubilities of alloy components in the liquid metal can result in severe, localized crevice corrosion that surpasses that which might occur in the aqueous environment of a LWR. In this study, F91 and Fe-12Cr-2Si, two alloys used in previous experiments were exposed to lead-bismuth eutectic maintained at 715*C with a cover gas of pure hydrogen for thirty hours. The conditions were kept extremely reducing, via the initial removal of oxygen and the subsequent maintenance of an environment of pure hydrogen gas, in order to simulate conditions inside a crevice. Following the experiment, the materials were analyzed for corrosion damage via optical microscopy, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. F91 was found to have sustained significant corrosion damage, as expected based on previous experiments, in addition to chromium depletion at the sample surface. Fe-12Cr-2Si was also found to have sustained corrosion damage as a result of lead-bismuth attack. No significant oxide formation or alloying element depletion was observed at the Fe-12Cr-2Si surface. The observed damage in Fe-12Cr-2Si was not entirely expected due to its excellent corrosion resistance in less reducing environments. This raises the concern that crevice corrosion could be an important damage mechanism in applications of the Fe-12Cr-2Si/F91 composite if crevices are present, either due to design flaws or due to cracking during service. Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2011.; "June 2011." Cataloged from PDF version of thesis.; Includes bibliographical references (p. 58-60). Sat, 01 Jan 2011 00:00:00 GMT http://hdl.handle.net/1721.1/76934 2011-01-01T00:00:00Z http://hdl.handle.net/1721.1/76931 Computational neutronics analysis of TRIGA reactors during power pulsing Bean, Malcolm (Malcolm K.) Training, Research, Isotopes, General Atomics (TRIGA) reactors have the unique capability of generating high neutron flux environments with the removal of a transient control rod, creating conditions observed in fast fission reactors. Recently, several TRIGA reactors have had issues with the deformation of fuel rods nearest the transient control rod, where the neutron flux is highest. This is a difficult problem to analyze because the damage is not simply due to rods overheating, but rather the pressurization of hydrogen, from the Uranium Hydride fuel, that has diffused into the spacing between fuel and cladding. Previous neutronic analyses utilized point kinetics; a model which assumes changes in reactivity uniformly affect the reactor's flux, resulting in no relative spatial variation over time. Point kinetics is attractive because of its low computing costs, however the pulse's localization, theoretically, should generate a pronounced flux spike and radial neutron wave, which violates an assumption of point kinetics. The aim of the research is not to explicitly describe the cause of fuel rod deformation, but rather generate time dependent, high-resolution 3-dimensional flux maps. The Purdue Advance Reactor Core Simulator (PARCS) was used to simulate a TRIGA pulse with both nodal and point kinetics. Assuming our nodal kinetics models accurately simulate TRIGA pulses, we find that point kinetics methods are ill suited to simulate TRIGA pulses. By maintaining the steady-state flux profile, point kinetics does not capture the fact that the power peak actually occurs in the center assembly, from which the transient control rod is removed. In our simulations, point kinetics underestimated the normalize power in the central assembly by as much as 46.19%. Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2011.; "May 2011." Cataloged from PDF version of thesis.; Includes bibliographical references (p. 28). Sat, 01 Jan 2011 00:00:00 GMT http://hdl.handle.net/1721.1/76931 2011-01-01T00:00:00Z