Ab Initio Study Of Defect Behavior In U Compounds

Nuclear energy is a safe,sustainable and large-scale clean energy.Uranium dioxide（UO₂）is the most widely used fuel in nuclear power generation due to its high melting point and excellent radiation tolerance under radiation environment.The radiation response of UO₂ has been extensively studied both experimentally and theoretically.However,the results of classical molecular dynamics（MD）simulations rely on the accuracy of empirical interatomic potential,and the strong correlation interaction between U 5f and U 5f electrons is not considered.In addition,U₃Si₂ has been proposed to be an excellent accident tolerant fuel due to its low uranium enrichment and high thermal conductivity.Until now,there still lacks an atomic-level understanding of the formation of point defects in U₃Si₂.It is thus of importance to study the stability of point defects in U₃Si₂ and provide a fundamental insight into the tolerance of U₃Si₂under radiation environment.In recent years,the ab initio molecular dynamics（AIMD）method has made it possible to simulate radiation damage of materials with the inclusion of motion of electrons.The threshold displacement energy,the type of created defects and the mechanism of defect generation can be determined with ab initio accuracy.Additionally,the density functional theory plus Hubbard U correction（DFT+U）has been widely employed to study the structural,electronic and energetical properties of strongly correlated electronic systems.In this study,the AIMD method is used to simulate the low-energy recoil events of UO₂,and the Hubbard U correction is also employed to deal with the U 5f electrons.Moreover,the generalized gradient approximation（GGA）method based on density functional theory plus Hubbard U correction and spin-orbit coupling（GGA+U+SOC）is employed to study the stability of point defects in U₃Si₂.The effects of Hubbard U correction and spin-orbit coupling on the defect formation energy in U₃Si₂ have been explored.The presented results will advance the understanding of the microscopic mechanism of displacement events in UO₂,and provide a fundamental insight into stability of point defects in U₃Si₂.The main contents of this study are as follows:1.The low-energy recoil events of UO₂ has been studied by AIMD+U method.The threshold displacement energies（E_ds）of oxygen and uranium atoms are shown to be dependent on the direction.As for uranium atoms,the maximum and minimum kinetic energies are determined to be 38 eV for[100]direction and 51 eV for[110]direction,which are comparable with the other MD results and smaller than the values obtained by sudden approximation（SA）method.The oxygen atom prefers to be displaced along the[100]direction,while it is more difficult to form stable defects along the[111]direction.The mean E_d values of oxygen and uranium atoms are determined to be 25.56and 46.96eV,respectively.It is also noted that the mean E_d for oxygen atom is slightly larger than the experimental and other theoretical results,while the mean value for uranium atoms is generally comparable with the experimental results.2.The type of the created defects and the mechanism of defect generation have been further explored.The defects created by oxygen atoms generally contain vacancy and interstitial defects,i.e.,Frenkel pair defect.As for uranium recoil events,the defects are generally Frenkel pair defect,and a few antisite defects are also created in UO₂,which are not observed in the classical MD and SA simulation.Moreover,we find that the oxygen and uranium interstitials prefer to occupy the octahedral site.3.The formation energies of the vacancy and antisite defects in U₃Si₂ are studied by GGA,GGA+U（1.5eV）and GGA+U+SOC methods.The results show that the formation energies of vacancy defects are lower than those of antisite defects.Especially,the results obtained by GGA method show that the formation energies of silicon vacancy are lower than those of uranium vacancy,whereas the results of GGA+U and GGA+U+SOC calculations demonstrate that the uranium vacancy is more easier to form than silicon vacancy.We also find that the results obtained by the GGA+U+SOC method are generally higher than those of GGA and GGA+U methods.These results suggest that the Hubbard U correction and spin-orbit coupling have obvious effects on the stability of point defects in U₃Si_2.4.The GGA,GGA+U and GGA+U+SOC methods show diffecent effects on the change of volume and bond length of defective U₃Si₂.The results obtained by GGA method show that the bond length of defective U₃Si₂ is shorter than that of ideal U₃Si₂.By contrast,the GGA+U results show that the point defects have slight effects on the<U-U>bond.The GGA+U+SOC results show that the point defects have profound effects on the volume change of U₃Si₂,and the vacancy defects cause a significant decrease in the maximum value of the<U-Si>bond length,indicating that the spin-orbit coupling affects the<U-Si>interaction remarkably.