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Oxidation Resistance Enhancement of Metallic Materials in Nuclear Reactors Environment
Materials Science and Engineering
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The research in my dissertation aims at a better understanding of the corrosion mechanism of alloys applied in nuclear reactors and potential methods to enhance their corrosion resistance based on reliable computational simulations. First-principles based calculations and density functional theory (DFT) simulations are taken to investigate the chemical reaction between nuclear structural materials and corrosive coolants. Zirconium (Zr) based cladding materials are widely used in commercial light water nuclear reactors and it is essential to prevent them from water oxidation to avoid serious safety issues in nuclear power plants such as Fukushima nuclear accident. To provide guidelines to design novel Zr alloys with enhanced water oxidation resistance, we performed a first-principles high-throughput screening (HTS) search that is based on the water dissociation mechanism over Zr basal plane. 53 metal dopants, including transition and non-transition metals, were selected to determine the promising dopants in Zr-X binary alloys with significantly improved resistance of water oxidation. Firstly, the adsorption and dissociation mechanisms of water molecules on the zirconium basal (0001) surface are determined using the density functional theory (DFT) calculations. Then the water dissociation barrier is used as a descriptor for the HTS approach. Next, the neutron cross-section is considered for the realistic applications of Zr-X alloys as cladding materials in nuclear reactions. Finally, the stability is checked for the possibility of processing these binary Zr-X alloys experimentally. Al, Zn, Ge, As, Sn, Sb, Pb, and Bi are singled out as promising dopants that could improve the corrosion resistance of zirconium alloys. In fact, aluminum alloys have already been used as fuel cladding, and Zr alloys such as Zircaloy, ZIRLO, which contain 1% ~ 1.5% Sn, have been used as fuel cladding for PWRs and BWRs for decades. Eutectic LiCl-KCl molten salt is often used in molten salt reactors as the primary coolant due to its high thermal capacity and high solubility of fission products. Thermophysical properties, such as density, heat capacity, and viscosity, are important parameters for engineering applications of molten salts, but may be significantly influenced by metal solute from corrosion of metallic structural materials. The behavior of the LiCl-KCl eutectic composition is well-researched, yet the effects on these properties due to chlorocomplex formation from metals dissolved in the salt are less well known. These properties are often difficult to accurately measure from experimental methods due to issues arising from the dissolved species such as volatility. Here we applied a combination of quantum mechanics molecular dynamics (QM-MD) and deep machine learning force field (DP-FF) molecular dynamics simulations to investigate the structure and thermophysical properties of LiCl-KCl eutectic as well as the influence of dissolved transition metal chlorocomplexes NiCl2 and CrCl3 at low concentrations. We find that the dissolution of Ni and Cr in the LiCl-KCl system forms the local tetrahedral (NiCl4)2- and octahedral (CrCl6)3- chlorocomplexes, respectively, which do not have a significant impact on the overall liquid salt structures. In addition, the thermodynamic properties including diffusion constant and specific heat capacity are not significantly affected by these chlorocomplexes. However, the viscosity is significantly changed in the temperature range of 673 ~ 773 K. This study thus provides essential information for evaluating the effects of dissolved metals on the thermophysical and transport properties of molten salts.