Theoretical Calculation Of Electronic Structure,Mechanical Properties And Service Behavior Of U₃Si₂ Nuclear Fuel Based On The First Principles

After the Fukushima nuclear accident in 2011,U₃Si and U₃Si₂ have received extensive attention as alternative nuclear fuels for UO₂.Research results in recent years show that U₃Si₂has better radiation resistance than U₃Si,and has a higher research value.However,the lack of important physicochemical data and the rarity of systematic studies on U₃Si₂ compounds make it difficult to develop fuels for practical use.In this paper,the electronic structures,polycrystalline elastic properties,ideal tensile/shear strength,and elastic anisotropy of U₃Si₂ were systematically calculated by the first principles based on density functional theory with on-site Coulomb correction（DFT+U）.Meanwhile,the effects of fission atoms Cs,Sr and I on the crystal structure and properties of U₃Si₂ were investigated.The calculation parameters for optimizing the U₃Si₂ unit cell and the value of U in the DFT+U method were determined through the convergence test and the calculation of the structural properties of U₃Si₂.When a cut-off energy of 600 e V,a k-point grid of 7×7×13 and a SIGMA value of 0.2 are chosen as optimization parameters,the energy convergence conditions required for the accuracy of the calculation are met.At a U value of 1.5 e V,the relaxation-optimized U₃Si₂ structure parameters,equilibrium volumes and elastic stiffness constants agree well with the available experimental data and other theoretical results,and the optimized U₃Si₂structure is mechanically and dynamically stable.The calculated results for the charge density and differential charge density of U₃Si₂ in the（110）plane show that there is a weak electron aggregation between the nearest neighbor U atom and Si atom that forming the U-Si chemical bond.It suggests that in addition to the formation of ionic bonds between U and Si atoms,there is also covalent bonding behavior with shared electrons.The relationships between mechanical failure modes and deformation processes of U₃Si₂ on three typical crystallographic directions and four slip systems were established using the first principles computational tensile/shear test（FPCTT/FPCST）method.Among the three crystallographic directions,U₃Si₂has the lowest tensile strength and the smallest deformation along the[110]crystallographic direction,which is the direction most susceptible to fracture,with a corresponding ideal strength of 6.54 GPa and a tensile strain of 0.08.Meanwhile,the results show that the tensile loads acting on the three crystalline directions did not cause brittle fracture of the U₃Si₂crystal.However,U₃Si₂exhibits characteristics of brittle fracture under ideal shear strain.In the[（?）10]crystal direction of the（110）crystal plane,U₃Si₂ possesses the lowest strength of 2.15 GPa and the smallest deformation of 0.05.Therefore,the disintegration fracture and dislocation slip of U₃Si₂ should occur on the（110）[（?）10]slip system.It is evident from the strain-induced charge density changes that the ideal strength of U₃Si₂ under tensile strain is mainly related to the breakdown of chemical bonds.However,the ideal strength values of U₃Si₂ under shear strain may be influenced by strain-induced structural phase transitions.The calculations of the three-dimensional surface structure of the elastic modulus and the elastic anisotropy factors of U₃Si₂amply demonstrate that U₃Si₂ has very weak elastic anisotropy,that its elastic anisotropy behavior is better than that of U₃Si,and that on the（001）crystal plane U₃Si₂ exhibits an almost isotropic elastic property.However,the ideal strength values of U₃Si₂ under shear strain may be influenced by strain-induced structural phase transitions.The calculations of the elastic anisotropy factor and the three-dimensional（3D）surface structure of the elastic modulus of U₃Si₂ amply demonstrate that U₃Si₂ has very weak elastic anisotropy,that its elastic anisotropy behavior is better than that of U₃Si,and that on the（001）crystal plane U₃Si₂ exhibits an almost isotropic elastic property.In the study of the service behaviour of U₃Si₂,the calculations of the point defect energies in U₃Si₂ crystals determine that U_Ⅰvacancies and Si vacancies as the types of vacancies more likely to arise in U₃Si₂ crystals.Also,U_Ⅰvacancies are the most favorable incorporation sites for Cs and Sr atoms,while Si vacancies are the most favorable incorporation sites for I atoms.The elastic constants C_ij of 2×2×2-U₃Si₂ supercell containing fission atoms Cs,Sr and I were calculated respectively using the volume conservation method,and the effects of fission atoms on the polycrystalline elastic properties,mechanical stability,elastic anisotropy and Debye temperature of U₃Si₂ crystals were further estimated in conjunction with the relevant theoretical formulae.The results show that the fission atoms Cs,Sr and I in vacancies lead to a decrease in the uniaxial strain stiffness of U₃Si₂ crystal along the a,b and c axes,as well as a decrease in the Debye temperature.The effect on the elastic modulus of U₃Si₂ crystal is more pronounced when the fission atoms occupy Si vacancies compared to U_I vacancies.The doping of all three fission atoms increases the B/G values of U₃Si₂ crystals.Among other things,the doping of metal Cs and Sr atoms improves the brittleness of U₃Si₂.Furthermore,the calculation results show that the doping of fission atoms Cs,Sr and I increase the elastic anisotropy on different crystallographic planes of U₃Si₂.Among these,the fission atoms on Si vacancies play a major role in the effect of the elastic anisotropy of U₃Si₂.When the fission atoms occupy the Si vacancies,a negative Poisson’s ratio occurs in the direction of the minima.It may be responsible for the expansion of U₃Si₂.