First-principles Calculations For Th/Ti Influence On The Behavior Of Fission Gas Xe/kr In UO₂

Compared with traditional fossil fuels,the nuclear fission reaction in nuclear power plants is a green and clean energy with high energy density,high power,no greenhouse gases such as carbon dioxide,and mature technology.Uranium dioxide（UO₂）is currently the most commonly used fuel for light water reactors（LWRs）.Its properties play a crucial role in the reliability and safety of nuclear reactors.Therefore,the behavior of fission nuclides in UO₂has attracted much attention.In this paper,the first-principles method is used to firstly investigate the stable occupation and diffusion mechanism of fission gas Xe/Kr in UO₂ and then study the stable occupation and diffusion behavior of fission nuclide Th/Ti on Xe/Kr in UO₂.The influence of nuclear fuel will provide certain theoretical guidance for the simulation calculation and experiment of nuclear fuel in the future.Hubbard U usually corrects UO₂ due to the strong correlation effect of the 5f electron of the U atom.In this paper,the U_eff value is determined by the DFT+U and the OMC schemes,and it is guaranteed that the system can converge to the ground state when the defects are subsequently calculated.Fission gas Xe/Kr atoms are the main fission gases of UO₂.In order to ensure the safe and efficient operation of nuclear reactors,it is necessary to deeply understand the behavior of Xe/Kr atoms in UO₂.This paper first investigates the stable occupancy and diffusion mechanisms of Xe/Kr atoms in UO₂.According to the calculation results of the binding energies and dissolution energies of Xe/Kr atoms in UO₂,it is shown that when vacancy defects exist,Xe/Kr atoms are more likely to be combined into vacancy defect clusters with larger spatial dimensions.At U-rich,Xe/Kr easily dissolves into Sch2defect clusters;at O-rich,Xe/Kr dissolves more easily into uranium vacancies and uranium-oxygen double-vacancy defects.The dissolution of Xe/Kr atoms in vacancy defects is mainly affected by the formation energy of vacancy defects.Xe/Kr atoms tend to aggregate into clusters in uranium vacancy defects.By establishing different interstitial migration modes,it is shown that the assisted migration of Xe/Kr atoms to defects through Frenkel will reduce their migration energies.UO₂ nuclear fuel will also produce some fission metals,such as Th/Ti atoms,etc.These metals will inevitably exist in UO₂ nuclear fuel as impurities,which will affect the normal reaction of UO₂ fuel,but sometimes also improve the physical properties of nuclear fuel,chemical,and thermodynamic properties,and reduce the release of fission gas during nuclear fuel irradiation.In this paper,the DFT+U method is used to examine the appropriate U_effvalue of Th/Ti atoms,and then the formation energy,binding energy,and dissolution energy of vacancy defects are used to characterize.The influence of impurities on the stable occupancy of Xe/Kr in UO₂ was investigated.Finally,the influence of impurities on the diffusivity of Xe/Kr in interstitial sites was investigated.The results show that the Th atom has less influence on the dissolution energy of Xe/Kr in UO₂ due to the similar ionic radius and electronegativity to the U atom,and the Th atom can reduce the squeezing between the atoms of the Xe/Kr atom during the migration process.Pressure,thereby significantly reducing the diffusion barrier of Xe/Kr atoms in the interstitial sites.The ionic radius of the Ti atom is much smaller than that of the U atom,and its electronegativity is also larger than that of the U atom.Therefore,the Ti atom mainly reduces the formation energy of vacancy defects,making Xe/Kr atoms relatively easier to dissolve into vacancy defects.And Ti atoms make Xe/Kr atoms not easy to aggregate in the vacancy defect but more inclined to distribute around the vacancy to the surrounding octahedral interstitial sites.Ti atoms also greatly reduce the diffusion barrier of Xe/Kr in interstitial sites.