| Energy is the fundamental to develop human social civilization.While with the rapid development of industrial technology and the progress of modern human civilization, the earth’s resource is consumed rapidly. In order to realize the sustainable development, human beings start to develop the nuclear power, which has been used widely at present. However, nuclear safety has been a controversial issue. Fermi mentioned that "The development of nuclear technology mainly depends on the behaviors of nuclear materials in the radiation environment of nuclear reactor". Nuclear reactor structural material is a important part of the nuclear materials. The metal of beryllium and zirconium and related compounds are the most important structural materials of nuclear^reactors. Beryllium has some good qualities, such as high atomic density, high elastic modulus, high thermal conductivity and the small neutron capture cross-section. Besides, its specific strength is higher than titanium alloy and aluminum alloy. Zirconium carbide, a high temperature refractory material, possesses a high melting pointing, high strength with good conductivity.However, due to the temperature limitation of the nuclear reactor, beryllium, zirconium with their alloys have to expose in the strong irradiation environment which will undoubtedly be radiated by particles, such as neutrons, electrons, and irons, occur the alpha decay and fission reaction, and transfer the energy to the atoms in the crystal. When the atoms in the crystal have enough energy that can jump out the normal lattice position to become the interstitial atoms, they will form the crystal defects. These defects will play a stimulative role for micro structure evolution of nuclear material, which affects the property and lifetime of the material. Therefore, to control and predict the micro structure evolution of materials in irradiation environment, it is key to investigate the point defect formation process, stability and diffusion properties of hydrogen and helium. Aimed at above reason, in our thesis, we have systematically studied the defects formation process in beryllium and zirconium carbide, together with diffusion behaviors of interstitial impurities, helium and hydrogen using first-principles calculations based on density functional theory. The rest of the paper is organized as follows.(1)The ground-state electronic, mechanical, thermodynamic, and point defects properties of zirconium carbide are studied. The results show that zirconium carbide is mechanical stable at the 0 pressure. Zr-C bond displays ionic/covalent characteristics. The covalent bonding nature exhibit in the hybrid properties of C-2p and Zr-4d4p states near the Fermi level, and the ionic character mainly exhibits that the 1.71 electrons from the Zr-4d4p state are transformed to the C-2p state. Through calculating the formation energies of all kinds of point defects in zirconium carbide, it is found the C-related defects are easiest produced among all kinds of points defects in irradiation environment.(2) The point defect structures, solubility, and diffusion behaviors of impurity helium in zirconium carbide are investigated. We also predict its temperature-dependent diffusion coefficient. It is found that helium atoms are hard to dissolve in zirconium carbide. Helium atom tends to occupy the tetrahedral interstitial site in defect-free lattice. While in pre-existing vacancy range, the zirconium vacancy is favorable to be occupied. The interstitial assisted diffusion mechanism is the mainly way for helium diffusion in zirconium carbide. Besides, we also given its diffusion coefficient, which is nearly 5 orders of magnitude lower than that of helium in silicon carbide, indicating that the ability of resistance helium irradiation for zirconium carbide is stronger that for silicon carbide.(3) For comparison, we calculated the formation energy of hydrogen in zirconium carbide. A single hydrogen atom tends to occupy the carbide vacancy since it can obtain 0.702 electrons from the nearest zirconium atoms to form the stable 1s2 state, which is difference with helium’s results. Two hydrogen atom will form a H-H pair in the<110> direction when both of them are located at the same tetrahedral site. The minimum energy pathway of hydrogen migration in zirconium carbide is searched using the CI-NEB method. According to the diffusion manner, we built the corresponding physical diffusion model and calculated the temperature-dependent diffusion coefficient. Here, we also considered the isotopes effects which shows a large difference in the phonon dispersion curve. The frequencies related to interstitial atoms decrease with increasing mass from hydrogen to its isotopes deuterium and tritium. The main reason caused the difference between diffusion coefficients is from zero-point energy and atom mass.(4) The formation energies for interstitial helium atom at various high-symmetry positions, interaction energies with the surrounding beryllium atoms are calculated. The most stable site for helium is BT site followed by BO site, which is explained from the lattice deformation energies and charge density. Through investigated the diffusion manner for helium in beryllium, it is found that helium atoms diffusion two-dimensionally at low temperatures, however, they tend to diffuse three-dimensional as the temperature increases. Besides, the microscopic parameters of the pre-factor and activation energy of diffusion coefficients for helium diffusion along or vertical the basal planes are estimated using the transition state theory. |
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