First - Principles Study On Structural Defects And Impurity Diffusion Behavior In Typical Nuclear Materials

Due to the important role of actinide dioxides in the field of nuclear indus-try, they have been attracting vast attentions both in the field of science and engineering. Among the actinide dioxides, PuO2and ThO2, as the traditional and new resources respectively, are attracting more and more attentions. PuO2is an important part of mixture fuel for fast reactors. Meanwhile, it also plays an important role in the storage and recycle utilization of plutonium. As for the physical characteristics, it plays an referenced role in studying the whole ac-tinide dioxides, since it lies in the intermediate position of the actinide dioxides, where the oxides before it are Mott insulator types while the ones behind it show charge-transfer insulator types. ThO2, acting as a potential alternative fuel of UO2, has many advantages with respect to traditional fuel. ThO2generates little transuranic radioactive waste after combustion. Also, it is miscible with PuO2, thus ThO2can partially replace PuO2to form (Pu, Th) O2mixed fuel, greatly reducing the resource requirements of plutonium. Besides, ThO2fuel has high melting point, high corrosion resistance, and high thermal conductivity, etc.In addition to the nuclear fuel, nuclear reactor material is another impor-tant part of the nuclear materials. Titanium and its alloy are one of the most important nuclear reactor materials. Titanium and its alloys, which have high specific strength, strong corrosion resistance, and high heat resistance, have been widely used in the nuclear industry, aerospace, chemical and other fields. A lot of equipments, pipelines, and related components of nuclear reactor are made of titanium and titanium alloys. Titanium alloys are also used as the protective cover of nuclear reactor to reduce nuclear radiation. For nuclear materials, the structure stability is very crucial for their applications. Due to the specific appli-cation environment of nuclear material, radiation and fission, as well as neutron, ion or electron radiation, will inevitably make the atomic scattering collision of the crystal structure to form crystal defects. These defects will promote the evo-lution of microstructure of the nuclear material, which may cause the degradation of thermal and mechanical properties. For the control and prediction of material in the irradiated microstructure evolution, it is essential to investigate the defect formation mechanism and diffusion behaviors.In this paper, we have studied the formation mechanism of point defects in PuO2, ThO2, Titanium and TixHf1-x binary soluble alloy, as well as the inter-stitial impurities, H and He, diffusion behavior using density functional theory (DFT).(1) We consider the point defect structures at different charged states to find the most favorite ones in materials. And then we use the point defect model to analyze the defect concentration variation trend along with stoichiometric.(2) The interstitial H and He atoms need to release and absorb energies to form defects in titanium respectively, which makes the diffusion manners of them are different. According to the different diffusion patterns, we set up cor-responding diffusion models and obtain the diffusion coefficients as a function of temperatures using transition state theory.(3) Through the calculation of phonon dispersion curves, we have discussed the interaction between impurity atoms and the metal atoms, and compared the various phonon vibration behaviors of interstitial atoms in different transition states. We also investigate the isotope effect (H/D/T) on the phonon energy.(4) By using the special quasi-random structure method, we construct the ordered structures of TixHf1-x; solid solution alloy at x=0.75and x=0.5ratio, respectively. Due to the different component of various interstitial sites in these alloys, the formation and diffusion behaviors of impurity atoms in them are more diversified. By comparing the defect formation energies, we determined the most stable interstitial sites in TixHf1-x alloy. We find that both the formation energy and diffusion barrier in0.75ratio TixHf1-x alloy are much higher than that in pure titanium. The0.75ratio TixHf1-x alloy has superior ability to fix helium atoms in the interstitial sites with respect to the pure titanium metal.