First Principle Mechanical Properties Study Of U₃Si_1.75N_0.25

In the wake of the Fukushima nuclear accident in 2011,the international nuclear industry has placed enormous demands on the safety of reactors in accident conditions and the design concept of accident-tolerant fuel installations has emerged.The rational use and development of nuclear energy has been actively advocated against the backdrop of a changing existential environment and the urgent need for China to reverse its reliance on single-fund consumption and an improved environmental situation.However,in recent years,due to the corresponding boom in statistical theory,number crunching and electronic technology,analogue practical techniques have been increasingly applied in the study of physics,chemistry and materials science.Alloyed fuels（e.g.U-Si branched alloy fuels）are one of the many families of accident-resistant nuclear fuels that are being studied by nuclear fuel-related practitioners today.Uranium dioxide（UO₂）is the most widely used nuclear fuel today,thanks to its ability to withstand high temperatures,similar structure in all directions,structural stability under high temperature radiation,corrosion resistance in underwater structures and compatibility with a wide range of materials.However,UO₂ has a major drawback.However,one of the major drawbacks of UO₂ is its lack of thermal conductivity,which can lead to a rapid rise in temperature in the core of the fuel under abnormal conditions,resulting in the escape of large amounts of fission gas,which in turn can cause the Zr alloy to react with water to produce hydrogen gas,leading to a major accident such as a violent explosion.In the wake of the Fukushima disaster,one of the priority needs for the development of new fuel systems is the modification of UO₂ fuel or the search for new materials with similar properties to UO₂ to ensure better thermal conductivity or other superior properties of the fuel.A new type of U-Si alloy material is needed.The exploration and innovation of U-Si alloy fuels has become a central research objective in the development of accident-resistant fuels,and is one of the more difficult problems to solve in China’s nuclear-related research.The mechanical and thermodynamic properties of N doping have not been fully investigated by researchers in the past.For these reasons,this thesis focuses on the role of N elements in improving the mechanical and thermodynamic properties of U₃Si₂,such as improving mechanical properties and improving oxidation resistance.Specifically,the study will focus on.（1）The feasibility of doping with N elements was verified by evaluating the vacancy and solvation energies of U₃Si₂ based on the first principle calculation,demonstrating the thermodynamic and kinetic stability of U₃Si_1.75N_0.25 and comparing the electronic structure changes before and after doping.The results show that U vacancies are easily formed,but that the formation of Si vacancies and their replacement with N are also thermodynamically feasible.The changes in the electronic structure of the material before and after doping are analysed as well as the charge transfer.（2）On the basis of the first part of the work,the mechanical properties of U₃Si_1.75N_0.25 were further calculated to analyse and compare the changes in brittleness and anisotropy of the material before and after doping.The results show that the values of bulk modulus K,shear modulus G and Young’s modulus E decrease slightly to79.851,46.518 and 119.014 GPa,respectively,but the degree of anisotropy of the toughness and the three-dimensional Young’s modulus are improved,and the Poisson’s ratio and B/G are 0.279 and 1.932,respectively,indicating the significance of the material in terms of improvement in toughness.（3）Concentrated analysis of the calculated data shows that the U₃Si_2-xN_x compound is composed of Si-site defects substituted by N elements and exhibits the properties of a metallic material,and that the differential and local charge density maps obtained for U₃Si_2-xN_x show the formation of metallic,ionic and covalent bonds between U-U,U-Si,U-N and Si-N,respectively.