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The research project was related to the Advanced Fuel Cycle Initiative and was in direct alignment with advancing knowledge in the area of Nuclear Fuel Development related to the use of TRISO fuels for high-temperature reactors. The importance of properly coating nuclear fuel pellets received a renewed interest for the safe production of nuclear power to help meet the energy requirements of the United States. High-temperature gas-cooled nuclear reactors use fuel in the form of coated uranium particles, and it is the coating process that was of importance to this project. The coating process requires four coating layers to retain radioactive fission products from escaping into the environment. The first layer consists of porous carbon and serves as a buffer layer to attenuate the fission and accommodate the fuel kernel swelling. The second (inner) layer is of pyrocarbon and provides protection from fission products and supports the third layer, which is silicon carbide. The final (outer) layer is also pyrocarbon and provides a bonding surface and protective barrier for the entire pellet. The coating procedures for the silicon carbide and the outer pyrocarbon layers require knowledge of the detailed kinetics of the reaction processes in the gas phase and at the surfaces where the particles interact with the reactor walls. The intent of this project was to acquire detailed information on the reaction kinetics for the chemical vapor deposition (CVD) of carbon and silicon carbine on uranium fuel pellets, including the location of transition state structures, evaluation of the associated activation energies, and the use of these activation energies in the prediction of reaction rate constants. After the detailed reaction kinetics were determined, the reactions were implemented and tested in a computational fluid dynamics model, MFIX. The intention was to find a reduced mechanism set to reduce the computational time for a simulation, while still providing accurate results. Furthermore, fast chemistry techniques would be coupled to MFIX to effectively treat the complex chemistry thus improve the computational efforts. Based on the reaction kinetics modeling, it was determined that the detailed set of chemical reactions for the thermal decomposition of a methyltrichlorosilane (MTS)/H2 mixture consisted of 45 species and 114 gas-phase reactions. Further work identified a mechanism consisting of approximately 60 surface reactions for the surface chemistry of SiC chemical vapor deposition. A reduced mechanism for the MTS gas-phase pyrolysis was constructed using the scanning method based on optimization concepts, which consisted of only 28 species and 29 reactions. The benefits of this project are that we have determined gas-phase species produced during and after the various decomposition reactions of MTS. The success of the computational approaches can now be used to predict the complex chemistry associated with the CVD process in producing nuclear fuel. It is expected that the knowledge we acquired can be easily transferred and that it will contribute to further experimental investigations. Furthermore, the computational techniques can now be used for reactor design and optimization for the next generation of nuclear reactors.
High fidelity nuclear reactor thermal hydraulic simulations are a hot research topic in the development of nuclear engineering technology. The three-dimensional Computational Fluid Dynamics (CFD) and Computational Multi-phase Fluid Dynamics (CMFD) methods have attracted significant attention in predicting single-phase and multi-phase flows under steady-state or transient scenarios in the field of nuclear reactor engineering. Compared with three-dimensional thermal hydraulic methods, the traditional one-dimensional system analysis method contains inherent defects in the required accuracy and spatial resolution for a number of important nuclear reactor thermal-hydraulic phenomena. At present the CFD method has been widely adopted in the nuclear industry, across both light water reactors and liquid metal cooled fast reactors, providing an effective solution for complex issues of thermal hydraulic analysis. However, the CFD method employs empirical models for turbulence simulation, heat transfer, multi-phase interaction and chemical reactions. Such models must be validated before they can be used with confidence in nuclear reactor applications. In addition, user practice guidelines play a critical role in achieving reliable results from CFD simulations.
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.
Advances of Computational Fluid Dynamics in Nuclear Reactor Design and Safety Assessment presents the latest computational fluid dynamic technologies. It includes an evaluation of safety systems for reactors using CFD and their design, the modeling of Severe Accident Phenomena Using CFD, Model Development for Two-phase Flows, and Applications for Sodium and Molten Salt Reactor Designs. Editors Joshi and Nayak have an invaluable wealth of experience that enables them to comment on the development of CFD models, the technologies currently in practice, and the future of CFD in nuclear reactors. Readers will find a thematic discussion on each aspect of CFD applications for the design and safety assessment of Gen II to Gen IV reactor concepts that will help them develop cost reduction strategies for nuclear power plants. Presents a thematic and comprehensive discussion on each aspect of CFD applications for the design and safety assessment of nuclear reactors Provides an historical review of the development of CFD models, discusses state-of-the-art concepts, and takes an applied and analytic look toward the future Includes CFD tools and simulations to advise and guide the reader through enhancing cost effectiveness, safety and performance optimization
The first portion of this work is a comprehensive analysis of the chemical environment in a High Temperature Gas-Cooled Reactor TRISO fuel particle. Fission product inventory versus burnup is calculated. Based on those results a thermodynamic analysis is performed to determine fission product vapor pressures, oxygen partial pressure, and carbon monoxide and carbon dioxide gas pressures within the fuel particle. Using the insight gained from the chemical analysis, a chemical failure model is incorporated into the MIT fuel performance code, TIMCOAT. Palladium penetration of the SiC layer is added to the fracture mechanics failure model. Rare-earth fission product and palladium corrosion of the SiC layer are additionally modeled. The amoeba effect is added as a new failure mode. The palladium penetration model has the most significant result on the overall fuel performance model and increases the number of predicted particle failures. The thinning of the SiC layer due to fission product corrosion has a slight effect on the overall fuel performance model. Finally, the amoeba effect model does not lead to any particle failures, but adds to the completeness of the overall model.
(cont.) It is concluded that accurate modeling of TRISO particles depends on having very high accuracy data describing material properties and a very good understanding of the uncertainties in those measurements.
Computational Fluid Dynamics (CFD) evaluation of homogeneous and heterogeneous fuel models was performed as part of the Phase I calculations of the International Atomic Energy Agency (IAEA) Coordinate Research Program (CRP) on High Temperature Reactor (HTR) Uncertainties in Modeling (UAM). This study was focused on the nominal localized stand-alone fuel thermal response, as defined in Ex. I-3 and I-4 of the HTR UAM. The aim of the stand-alone thermal unit-cell simulation is to isolate the effect of material and boundary input uncertainties on a very simplified problem, before propagation of these uncertainties are performed in subsequent coupled neutronics/thermal fluids phases on the benchmark. In many of the previous studies for high temperature gas cooled reactors, the volume-averaged homogeneous mixture model of a single fuel compact has been applied. In the homogeneous model, the Tristructural Isotropic (TRISO) fuel particles in the fuel compact were not modeled directly and an effective thermal conductivity was employed for the thermo-physical properties of the fuel compact. On the contrary, in the heterogeneous model, the uranium carbide (UCO), inner and outer pyrolytic carbon (IPyC/OPyC) and silicon carbide (SiC) layers of the TRISO fuel particles are explicitly modeled. The fuel compact is modeled as a heterogeneous mixture of TRISO fuel kernels embedded in H-451 matrix graphite. In this study, a steady-state and transient CFD simulations were performed with both homogeneous and heterogeneous models to compare the thermal characteristics. The nominal values of the input parameters are used for this CFD analysis. In a future study, the effects of input uncertainties in the material properties and boundary parameters will be investigated and reported.