170 research outputs found
Hilbert Schemes and Seshadri Constants
No full textIn this paper we will propose a new method to investigate Seshadri constants, namely by means of (nested) Hilbert schemes. This will allow us to use the geometry of the latter spaces, for example the computations of the nef cone via Bridgeland stability conditions to gain new insights and bounds on Seshadri constants. Moreover, it turns out that many known Seshadri constants turn up in the wall and chamber decomposition of the movable cone of Hilbert schemes.This preprint is based on one of the chapters of the author\u27s PhD thesis supervised by Frank Gounela
Seshadri Constants and Fujita's Conjecture via Positive Characteristic Methods
No full textIn 1988, Fujita conjectured that there is an effective and uniform way to turn an ample line bundle on a smooth projective variety into a globally generated or very ample line bundle. We study Fujita's conjecture using Seshadri constants, which were first introduced by Demailly in 1992 with the hope that they could be used to prove cases of Fujita's conjecture. While examples of Miranda seemed to indicate that Seshadri constants could not be used to prove Fujita's conjecture, we present a new approach to Fujita's conjecture using Seshadri constants and positive characteristic methods. Our technique recovers some known results toward Fujita's conjecture over the complex numbers, without the use of vanishing theorems, and proves new results for complex varieties with singularities. Instead of vanishing theorems, we use positive characteristic techniques related to the Frobenius-Seshadri constants introduced by Mustata-Schwede and the author. As an application of our results, we give a characterization of projective space using Seshadri constants in positive characteristic, which was proved in characteristic zero by Bauer and Szemberg.PhDMathematicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149842/1/takumim_1.pd
Impact of reactor environment on quenching heat transfer of accident tolerant fuel cladding
No full textThis electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2018Cataloged from student-submitted PDF version of thesis. Page 123 blank.Includes bibliographical references (pages 106-116).Development of accident tolerant fuels (ATF) for light water reactors (LWRs) came into focus for the nuclear engineering community after the accidents at Fukushima-Daiichi. The primary focus of the ATF program is to identify alternative fuel and cladding technologies that may provide enhanced safety, competitiveness, and economics. The new fuel design must also be compatible with present-day LWR design. For near-term applications, coatings on the nominal Zirconium-based cladding material and other metallic materials are being considered to improve the corrosion resistance and reduce the generation of hydrogen at high temperatures. Major ATF coating choices under consideration include chromium as a coating, iron-chromium-aluminum alloys (FeCrAl) as cladding and molybdenum as a coating, which have demonstrated better mechanical and oxidation behavior during the experimental testing.Thermal-fluids characteristics are pivotal for a robust testing of ATF concepts as the proposed candidates may have an entirely different thermal-hydraulic behavior when compared to Zircaloy-4. ATF coatings may display very different boiling characteristics as a result of different microstructures and surface characteristics. In the present work, transient boiling heat transfer during quenching of the candidate ATF claddings on vertical rodlets is studied experimentally. The candidate ATF material (chromium, FeCrAl, and molybdenum) are applied on Zircaloy-4 rodlets. The vertical solid rodlets are heated to temperatures up to 1000 °C and are quenched in a saturated pool of water at atmospheric pressure. The temperature variation during the quenching of rodlets was recorded insitu with synchronized visualization of boiling regimes over the test specimen using a high-speed video camera.The quench performance of the ATF coatings was analyzed based on the examination of various surface parameters such as wettability, roughness, emissivity and capillary wicking. In order to obtain a more realistic picture of the candidate performance during the emergency cooling reflood phase in a nuclear reactor, the coated rodlets are also oxidized in an autoclave before quenching. The performance of the candidate claddings is evaluated after oxidation and the surface characterized. It was observed from the post-test analysis that the surface characteristics and oxidation had a significant impact on the quench performance of ATF coatings, which varied between different coating materials. In order to better understand the thermal margins in a reactor specific environment, an analysis was performed on samples after exposing them to gamma rays. The gamma rays tend to change the surface wettability through a phenomenon called Radiation Induced Surface Activation.A Gammacell 220E irradiator that uses 12 cobalt-60 pencil sources, arranged axially in a sample chamber at MIT, was used to irradiated the samples. The results of water quenching and contact angle studies showed a higher Leidenfrost temperature and wettability in both samples exposed to gamma irradiation. The detailed microscopic analysis attributed the enhanced wettability to oxidation of the surface under gamma irradiation.by Arunkumar Seshadri.S.M.S.M. Massachusetts Institute of Technology, Department of Nuclear Science and Engineerin
Development of Cr cold spray–coated fuel cladding with enhanced accident tolerance
No full textAccident-tolerant fuels (ATFs) are currently of high interest to researchers in the nuclear industry and in governmental and international organizations. One widely studied accident-tolerant fuel concept is multilayer cladding (also known as coated cladding). This concept is based on a traditional Zr-based alloy (Zircaloy-4, M5, E110, ZIRLO etc.) serving as a substrate. Different protective materials are applied to the substrate surface by various techniques, thus enhancing the accident tolerance of the fuel. This study focuses on the results of testing of Zircaloy-4 coated with pure chromium metal using the cold spray (CS) technique. In comparison with other deposition methods, e.g., Physical vapor deposition (PVD), laser coating, or Chemical vapor deposition techniques (CVD), the CS technique is more cost efficient due to lower energy consumption and high deposition rates, making it more suitable for industry-scale production. The Cr-coated samples were tested at different conditions (500°C steam, 1200°C steam, and Pressurized water reactor (PWR) pressurization test) and were precharacterized and postcharacterized by various techniques, such as scanning electron microscopy, Energy-dispersive X-ray spectroscopy (EDX), or nanoindentation; results are discussed. Results of the steady-state fuel performance simulations using the Bison code predicted the concept's feasibility. It is concluded that CS Cr coating has high potential benefits but requires further optimization and out-of-pile and in-pile testing. Keywords: Accident-Tolerant Fuel, Chromium, Cladding, Coating, Cold Spray, Nuclear Fue
Understanding the Impact of Nuclear Environment on the Hydrothermal Corrosion in SIC
No full textSiC/SiC fiber composites are potential candidates for advanced cladding materials to improve the accident tolerance of commercial light-water nuclear reactor fuel. To evaluate the fiber composites’ viability, understanding the kinetics of their corrosion in Light Water Reactor (LWR) conditions is critical. SiC corrosion results in the formation of silica, which can then be rapidly dissolved in the LWR environment. LWR conditions are demanding on materials because they are subjected to irradiation, high pressure, high temperature, and aqueous chemistry. Experiments were performed under prototypical LWR conditions (Pressure, temperature, flow rate) to understand the corrosion and silica dissolution characteristics of high purity chemical vapor deposited (CVD) SiC. Sensitivity studies are performed to develop a comprehensive model for silica dissolution considering the impact from irradiated microstructure (through Si/ proton, Co60 gamma, and neutron pre-irradiation), flow rate, electrical resistivity, surface roughness, surface wettability, and CRUD (metallic oxide impurities) deposition. The corrosion rate in the irradiated microstructures was found to be an order of magnitude higher compared to the unirradiated microstructure under boiling water reactor (BWR) conditions. Electrochemical and spectroscopic studies revealed that the enhanced corrosion in irradiated samples was the result of an increased surface reaction potential that can be associated with the structural defects and the electronically excited states produced by irradiation. Surface roughness effects on hydrothermal corrosion also accelerated the corrosion rate significantly at high mass flow rates relevant to LWR operating conditions. Based on the experimental results, the existing semi-empirical SiC hydrothermal corrosion kinetic models are updated to include the effects of irradiation, resistivity, flow rate, and pH. Further, the experimental results suggest that the CRUD deposition on the CVD SiC would reduce corrosion significantly. Enhanced CRUD formation was observed under gamma irradiation and was correlated to the reduced zeta potential and the contact angle of the surface. Further adhesion properties responsible for CRUD deposition in SiC are investigated to evaluate the likelihood of CRUD deposition in LWR conditions.
The silica dissolution rate of nuclear grade Hi-Nicalon type S fibers and fibers manufactured with Rapid Laser chemical vapor deposition (R-LCVD) with varying surface chemistries were also obtained through experiments performed in static autoclave simulating PWR conditions. The hydrothermal corrosion behavior of stoichiometric R-LCVD fibers was observed to be comparable to the nuclear grade Hi-Nicalon Type S fibers. The results show that the impact of stoichiometry was much higher than the particular manufacturing technique, though the higher surface roughness in R-LCVD fibers significantly affected the corrosion kinetics. Thermal pre-treatment of R-LCVD fibers leads to a drastic reduction in the corrosion of SiC fibers and was correlated to the increased grain size on the fiber surface when exposed to high temperatures. The effect of pre-ion irradiation on the hydrothermal corrosion behavior of SiC fibers was found to exhibit a complex relationship based on the stoichiometric composition of the fibers.
Finally, the Radiation Chemistry Analysis Loop (RADICAL) code that models the complete coolant loop chemistry, radical, and species transport in LWRs, is modified to include SiC/SiC cladding corrosion and silica transport based on experimentally determined silica formation and dissolution rates. Sensitivity analysis is further carried out on several parameters in RADICAL to inform the industry on the extent of spatial inventory of silica deposition in typical BWR and pressurized water reactor (PWR) primary loops. RADICAL modeling suggests that silica deposition in PWR components and CVD SiC thickness loss is not of great concern even when the effect of irradiation damage on SiC corrosion is considered. However, for BWRs, significant silica deposition on components and CVD SiC thickness loss is expected unless the fuel rod is covered entirely in stable CRUD within the first few months of operation. As such, a feasibility study on different protective metallic coatings applied on the SiC/SiC fiber composite was conducted to reduce the thickness loss. Out of different coatings tested, plasma spray coated and vacuum annealed FeCrAL with blended FeCrAl, Cr coating served as a stable protective barrier against SiC dissolution in hydrothermal conditions.Ph.D
Parallelizing Unstructured Sparse Matrix Computations on Large-Scale Multiprocessors
No full textProblems in the class of unstructured sparse matrix computations are characterized by highly irregular dependencies and communication patterns that are not known at compile-time, but can be completely determined at run-time before the computations are actually performed. For this class of problems, current parallelizing compilers are unable to produce efficient code on large-scale distributed memory MIMD multiprocessors, and manual techniques are inflexible and too ad hoc to be generally effective.
In this thesis, we propose a run-time automatic partitioning and scheduling methodology for unstructured sparse matrix computations on large-scale multiprocessors. Our methodology is based on extracting information from the problem instance by preprocessing its symbolic structure, and using this information to achieve high performance in repeated iterations of the computations during which the symbolic structure is unchanged. We present efficient software tools to help users build their parallelization system by following this methodology.
We demonstrate the efficacy of our methodology on sparse Cholesky factorization, which has historically proven to be hard to parallelize. The highlight of our approach is a new two-dimensional block partitioning scheme. We build a run-time parallel system for block sparse Cholesky factorization called Sparse Hybrid Automatic Parallelization Environment (SHAPE), consisting of a parallel partitioner, a parallel scheduler and a parallel communication optimization algorithm. These are modular tools tied together by an explicit representation for block-based unstructured computations. We employ SHAPE to carry out an extensive experimental study of sparse Cholesky factorization on the iPSC/860. The experimental results show that with a judicious choice of partitioning parameters, our block-based partitioning and scheduling method outperforms a well-known column-based method in delivering high performance on a variety of structured and unstructured matrices. The preprocessing itself is shown to be very efficient, its cost being recovered in a small number of iterations of the factorization.
Our methodology and tools may be used to parallelize other unstructured sparse matrix computations for which the same symbolic structure is used in several iterations of the computations. Such computations include sparse triangular solution and sparse
matrix-vector multiplication.Technical report DCS-TR-30
SHAPE: A Parallelization Tool for Sparse Matrix Computations
No full textWe describe the design, implementation and performance of a Sparse Hybrid Automatic Parallelization Environment (SHAPE). SHAPE partitions and schedules sparse matrix computations for Cholesky factorization with the goal of achieving good performance at low cost, while providing flexibility for use as an experimental tool. It employs efficient parallelization algorithms which reduce the communication cost without adversely affecting the load balance by using a hybrid mixture of column and block partitions. Through several parameters, SHAPE aims for portability across a diverse range of sparse matrix structures and message-passing multiprocessors with different communication cost parameters. We present preliminary timing results on the iPSC/860 and compare the performance of SHAPE with that of a commonly used column-based method. The results show that SHAPE significantly reduces computation time, number of messages, and overall communication time for a variety of test matrices.Technical report dcs-tr-29
A Review of Irradiation Damage and Effects in α-Uranium
No full textUnderstanding irradiation damage and effects in α-uranium (α-U) is critical to modeling the behavior of U-based metallic fuels. The aim of this review is to address the renewed interest in U-based metallic fuels by examining the state-of-the-art knowledge associated with the effect of irradiation on the microstructure, dimensional changes, and properties of α-U. We critically review the research progress on irradiation-induced growth and swelling, the enhancement of plastic flow and superplasticity by irradiation, and the effect of irradiation on thermal and electrical properties of α-U. Finally, we outline the research directions that require advancements, specifically the need to carry out fundamental research on several of the less understood mechanisms of irradiation damage and effects in α-U
Sourcing Reform Competency and Effective Collaboration: A Resource Based View
No full textNOL Fellowship; Singapore MOE Academic Research Fund Tier 1The author gratefully acknowledges partial support for this research from the NOL Fellowship under grant C216/MX09B001 and from Office of Research at SMU under grant 08-C207-SMU-012</p
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