Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004. / Includes bibliographical references (p. 290-300). / The performance of coated fuel particles is essential for the development and deployment of High Temperature Gas Reactor (HTGR) systems for future power generation. Fuel performance modeling is indispensable for understanding the physical behavior of fuel particles and achieving their high reliability during operations and accidents through a guided design process. This thesis develops an integrated fuel performance model of coated particle fuel to comprehensively study its mechanical behavior and define an optimum fuel design strategy with the aid of the model. Key contributions of the thesis include a pyrocarbon layer crack induced particle failure model with a fracture mechanics approach, mechanical analysis of particles with better representation of irradiation induced creep, a proposed fuel optimization procedure, the capability to simulate arbitrary irradiation histories, and the incorporation of Monte Carlo sampling to account for the statistical variation of particle properties. / (cont.) Stress calculations in this model were benchmarked with the FUEL code and finite element calculations of Idaho National Engineering and Environmental Laboratory (INEEL) and with model predictions for High Temperature Test Reactor (HTTR) first- loading fuel. Fuel failure predictions were made for New Production-Modular HTGR irradiated fuel capsules, which show good agreements with experiments. Based on the simulations, it is suggested that in most cases the pyrocarbon crack induced fuel failure mechanism plays a much more important role in fuel performance than the more widely accepted pressure vessel fuel failure mechanism. After the establishment of the model, parametric study was conducted to find out the effects of various input variables on fuel performance, and fuel design and optimization procedure was proposed accordingly. Simulations with optimized fuel configurations demonstrate that superior fuel performance can be achieved with model analysis. The model also prepares interfaces for further improvements on various modules upon arrival of new information. / by Jin Wang. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003. / Includes bibliographical references (p. 239-244). / A nuclear power plant (NPP) has an intricate operational domain involving systems, structures and components (SSCs) that vary in scale and complexity. Many of the large scale SSCs contribute to the lost availability in the operation of the NPPs, when their malfunctions cannot be detected in a timely manner. The lost availability can result in millions of dollars economic loss. Currently, one of the main reasons of the incapacity for avoiding such critical system failures is the lack of an appropriate health monitoring system (HMS). A comprehensive HMS can help prevent the system failures, by analyzing the large amount of information for determining the performance status of the SSCs and for providing decision support in the NPP operations. The immediate goal of this work is to design the methodology for the cognition system of an automated multi-faceted HMS to be implemented at NPPs. The tasks of this system are providing efficient and reliable fault diagnosis, failure prediction, and decision support in NPP operations. The ultimate goal of this work is to enhance the NPP operations by increased availability, and consequently, further improved reliability and safety. This work presents the design of the cognition system of the HMS as a unique hybrid intelligent system. In this hybrid structure, we use the Bayesian network (BN) and neural network (NN) techniques in conjunction, for the first time, in order to provide complementary probabilistic performance status estimates and fault diagnosis concerning the monitored SSCs. This strategy makes the real-time implementation of this diagnostic model feasible in large scale, complex problem domains. / (cont.) It permits an important step in the applied artificial intelligence field. We use the influence diagram (ID) technique for extending the Bayesian network structure to provide decision support to the plant personnel. The advisory model using the ID technique derives recommendations concerning the optimal alternative actions to be executed during incidents. Finally, we adopt an analytical model to estimate the mean time to occurrence of the initiating faults and failures of the monitored systems. This model considers the multi-state dynamic behavior of the SSCs. The ultimate configuration of these models for various tasks of the HMS is named the HIS-N, hybrid intelligent system for NPP operations. The HIS-N has been developed, tested and verified on a test bed that mimics the bearing system of a horizontal charging pump. The results from the implementation of the HIS-N on the target system show consistent behavior in fault diagnosis, decision support and failure prediction tasks. The features of our HMS methodology gives us new capabilities to enhance the NPP operations, in order to reach its ultimate goal. The new monitoring method with the HIS-N developed in this work is not restricted to NPP operational applications. We recommend that an identical approach be adopted for similar purposes also in other large scale, complex process domains. / by Bilge Yildiz. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001. / Includes bibliographical references (p. 357-366). / The feasibility of a lead-bismuth (Pb-Bi) cooled fast reactor that eliminates the need for steam generators and coolant pumps was explored. The working steam is generated by direct contact vaporization of water and liquid metal in the chimney above the core and then is sent to the turbine. The presence of a lighter fluid in the chimney drives the natural circulation of the Pb- Bi within the reactor pool. Three key technical issues were addressed: 1) the maximum thermal power removable by direct contact heat transfer without violating the fuel, clad and vessel temperature limits, 2) the consequences of Pb-Bi aerosol transport on the design and operation of the turbine and 3) the release of radioactive polonium (a product of coolant activation) to the steam. Modeling of the multi-phase phenomena occurring in the chimney confirmed the effectiveness of the direct contact heat transfer mode within a well-defined design envelope for the reactor power, chimney height and steam superheat. A 1260MWth power is found possible for 10m chimney height and 25°C superheat. The temperature of the low-nickel steel clad is maintained below 600°C, which results in limited corrosion if tight control of the coolant oxygen concentration is adopted. / Generation, transport and deposition of Pb-Bi aerosols were also modeled. It was found that the design of a chevron steam separator reduces the heavy liquid metal in the steam lines by about three orders of magnitude. Nevertheless, the residual Pb-Bi is predicted to cause embrittlement of the turbine blades. Four solutions to this problem were assessed: blade coating, employment of alternative materials, electrostatic precipitation and oxidation of the Pb-Bi droplets. An experimental campaign was conducted to investigate the polonium release from a hot Pb- Bi bath to a gas-streamn. Th thermodynamics of the polonium hydride formation reaction (free- energy vs. temperature). as welQ as the vapor pressure of the lead-polonide were measured and then utilized to model the polonium transport in the reactor. It was found that the polonium concentration in the steam and on the surface of the power cycle components is significantly above the acceptable limits, which makes the very concept of a direct contact reactor open to question. / by Jacopo Buongiorno. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005. / Includes bibliographical references (leaves 141-148). / This thesis is organized into two parts which focus on the studies of the dynamic structure factor and static inter-particle structure factor respectively. In the first part, we have measured and analyzed the dynamic structure factors of aligned 40 wt% calfthymus Na-DNA molecules with the inelastic X-ray scattering (IXS). In the second part, we have developed a new efficient method to calculate the inter-particle structure factor in a simple fluid interacting with a two-Yukawa term potential and apply it to study the kinetic phase diagram and analyze the small angle neutron scattering (SANS) intensity distribution of colloidal systems. By analyzing the dynamic structure factor measured with IXS, the phonon dispersion relations of 40 wt% calf-thymus Na-DNA with different counterion atmosphere are constructed. It is found that the addition of extra counterions will increase phonon damping at small scattering wave vector, Q. At the intermediate Q range (12.5 nm- < Q < 22.5 nm-l), it may even overdamp the phonon so that the phonon feature can not be extracted from the IXS spectra. The measured sound speed is 3100m/s, which is much higher than the sound speed, - 1800m/s, obtained by Brillouin light scattering. This difference shows that the atoms of DNA molecules are closely coupled to the surrounding water molecules. / (cont.) Therefore, the different dynamic response of water molecules in different Q range affects the overall dynamic response of the hydrated DNA molecules. By analyzing the IXS spectra, the intermediate scattering function is extracted and shows a clear two step relaxation with the fast relaxation time ranging from 0.1 to 4 ps and the slow relaxation time ranging from 2 to 800 ps. In order to understand the phase behavior and the interactive potential of a colloidal system, we have developed a new and efficient method to calculate the inter-particle structure factor of a simple fluid interacting with a two-Yukawa term potential. We have applied this method to study the kinetic phase diagram of a system interacting with a short-range attraction and a long-range repulsion. A new glass phase, cluster glass, is determined through the theoretical analysis by the mode coupling theory (MCT). The SANS intensity distribution of cytochrome C protein molecules in solutions is measured and analyzed with our method. A sharp rising intensity at very low Q value has been consistently observed, which is named zero-Q peak. The existence of the zero-Q peak implies that a weak long-range attraction between protein molecules in solutions exists and has a even longer range than the electrostatic repulsion. / by Yun Liu. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / CDROM contains thesis in .pdf format; appendix B (MD simulation code: SPC/E Potential) and appendix C (Programs to fit QENS data by RCM) in .doc format; and a readme file in .txt format. / Includes bibliographical references (leaves 101-108). / The slow dynamics of supercooled water is studied by modelling the spectrum of test particle fluctuations: intermediate scattering function (ISF). The theoretical models are compared with experimental measurements by quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulation results. The dynamics of supercooled water can be decoupled into a product of translational and rotational dynamics. While the translational dynamics is modelled well by a product of a ISF representing short time in-cage vibrations and long time cage relaxation processes, the rotational dynamics is an aspect we shall study in this thesis. We introduce a model for the first, second and third order rotational correlation functions, which are required for the computation of the rotational intermediate scattering function corresponding to the incoherent QENS spectra from supercooled water. The model is tested against MD data generated from an extended-simple-point-charge (SPC/E) model of water and is found to be satisfactory. The analysis can be used as a practical method for extracting rotational relaxation parameters from QENS spectra measured at large Q from supercooled bulk water or interfacial water in porous materials. / (cont.) By confining water in nano-pores of silica glass, we can bypass the crystallization and study the pressure effect on the dynamical behavior in deeply supercooled state using incoherent QENS technique. we investigated the dynamics of water confined in nanoporous alumino-silicate matrices, from ambient temperature to the deeply supercooled state. We collected data on three instruments, with widely different resolutions, the Fermi chopper (FCS), the disk chopper (DCS) and the backscattering (HFBS) spectrometers at the NIST Center for Neutron Research (NIST NCNR). The confining systems were lab synthesized nanoporous glasses MCM-41-S and MCM-48-S. Inside the pores of these matrices the freezing process of water is retarded so that freezing occurs at a temperature about 50 K less than in bulk water. Thus, with the combined use of different instruments and nano-pore confinement, we were able to study the system over a wide temperature range, 160-325 K, with different energy resolutions. The data from all the three spectrometers were analyzed using a single consistent expression based on the relaxing cage model (RCM) for the translational and rotational dynamics. / (cont.) A remarkable slowing down of the translational and rotational relaxation times has been observed. The behavior of shear viscosity j7 or equivalently the structural relaxation time T of a supercooled liquid as it approaches the glass transition temperature is called 'fragile' if it exhibits non-Arrhenius character, such as that described by the Vogel-Fulcher-Tammann (VFT) law; otherwise, with q and obeying Arrhenius law, it is called 'strong'. The fragile behavior is typical to ionic and van der Waals systems. In contrast, a liquid being strong reflects that its structural makeup, to a large extent by strong (commonly covalent) bonds forming a network structure. Bulk water is considered as a fragile liquid at room temperature but for supercooled water, a 'fragile-to-strong' (F-S) transition at around 228 K has been proposed to occur at around 228 K, based on a thermodynamic argument. The F-S transition in a molecular liquid like water may be interpreted as a variant of kinetic glass transition predicted by the ideal Mode-Coupling Theory (MCT), where the real structural arrest transition is avoided by an activated hoping mechanism below the transition. Experiments at DCS and HFBS for water in MCM-41-S with 14 A pore diameter were also done under selective pressures, from ambient to 2400 bar. / (cont.) We observe a clear evidence of a cusp-like F-S dynamic transition at pressures lower than 1600 bar. Here we show that the transition temperature decreases steadily with an increasing pressure, until it intersects the homogeneous nucleation temperature line of bulk water at a pressure of 1600 bar. Above this pressure, it is no longer possible to discern the characteristic feature of the F-S transition. The discussion part of this thesis concludes that the high-temperature liquid corresponds to the high-density liquid (HDL) of which the hydrogen bond network is not fully developed to conform a locally tetrahedral coordination, while the low-temperature liquid corresponds to the low-density liquid (LDL) of which the more open, locally 4-coordinated, ice-like hydrogen bond network is fully developed. Identification of the end point of F-S transition with a possible second critical point is also discussed. / by Li Liu. / Ph.D.

Includes bibliographical references (p. 164-165). / Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005. / (cont.) A storage system was designed to contain the lithium-6 filter safely when it is not in use. A mixed field dosimetry method was used to measure the photon, thermal neutron and fast neutron dose. The measured advantage depth is 9.3 ± 0.1cm without filter and 9.9 ± 0.1cm with 8mm lithium-6 filter. The result is consistent with the result of Monte Carlo calculation. / The design of a lithium-6 filter to be used in Boron Neutron Capture Therapy was developed. The lithium-6 filter increases the average energy of the epithermal neutrons in the epithermal neutron beam. This filter allows the beam to be used for effective BNCT treatment at greater depth in tissue. Based on Monte Carlo calculations, 8mm thick lithium-6 filter was found to be the optimum filter thickness for the MIT fission converter based epithermal neutron beam (FCB). The highly reactive lithium metal filter is sealed with aluminum covers against the humidity and surrounding air. A well shielded and convenient frame was also designed to hold the lithium-6 filter. The frame is separated into two parts. The fixed part of the frame will be mounted into the patient collimator of the FCB and provides a slot for the lithium-6 filter. The filter itself will be connected to the movable part of the frame and slid in and out of the beam through a pair of roller bearing tracks like a vertical drawer. Both parts of the frame are built with borated polyethylene (RICORAD) and steel to insure good shielding. Many safety issues have been considered in the design including tritium production, nuclear heating, pressure from released gases and radiation leakage on the side of the collimator. / by Wei Gao. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1996. / Includes bibliographical references (p. 101). / by Xinhui Chen. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000. / Includes bibliographical references (p. 344-360). / This is a study of the microstructural influences on thermo-mechanical behavior of selected metals and ceramics using computer simulation, with original contributions in both theoretical and applied aspects. There are three major thrusts. First, by constructing a many-body empirical potential for ZrCx and then carrying out MD simulations to calculate its lattice thermal conductivity, I obtain the first quantitative evidence ever that the vibrational contribution is only a small part of the total thermal conductivity of refractory carbides at realistic carbon vacancy concentrations. This is a long-standing problem which even the most recent review article on the subject give what I now believe is the wrong estimate. Second, ideal strengths are calculated for Ar,Cu,SiC crystals using both lattice and molecular dynamics methods. A set of homogeneous instability criteria are derived. Tension tests are performed on amorphous and nanocrystalline SiC at room temperature, based on which a grain size cutoff of ~20 nm is extrapolated for the Hall-Petch effect. Nano-indentation is performed on single-crystal and nanocrystalline Cu, and bursts of dislocation loops is observed at a local stress level consistent with recent experiments on Cu thin films. Third, an invariant loop summation similar to the J-integral is derived for the driving force on defect motion, but with the loop size now down to nanometers, and the summation now expressed in terms of interatomic forces instead of stress, a field concept which is hard to use in atomistic calculations and becomes ill-defined when defect separations approach the nanometer scale. It is shown first that the change in a system's total Helmholtz free energy due to a defect's move can be approximated by a local quantity involving only scores of atoms immediately surrounding the defect. Then, perturbation expansion is used to evaluate this local invariant for defect translation using only the current configuration. This driving force measure is then tested on a) self-interstitial diffusion near free surface in [alpha]-iron, b) crack-tip extension near a void in Si, c) screw dislocation translation in Si, with convincing results down to literally r = 1 nm, at a fraction of the cost of a full relaxation or free energy calculation for the whole system. This means that defect mobility can now be characterized by a universal and invariant standard, computable from a tiny atomistic calculation without relying on elasticity formulas or image summations. The standard is then used to determine the true Peierls-Nabarro stress in Si-like materials. / by Ju Li. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1999. / Includes bibliographical references (leaves 223-224). / by Feng Li. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1999. / Includes bibliographical references (leaves 173-180). / by Antonis A. Armoundas. / Ph.D.