Oxygen Diffusion And Xenon Segregation At Grain Boundaries In UO₂: A Study Of The Strain Effect

As one of the main fuels widely in commercial PWR,uranium dioxide（UO₂）is subjected to high temperature,strong radiation and other harsh environment factors,as well as the combined effects of thermal stress,mechanical loading stresses and irradiation induced plastic deformation during operation.Various point defects are introduced during irradiation,and the migration and aggregation of point defects eventually may lead to the macroscopic performance degradation or failure of materials.Although the diffusion kinetics of point defects in UO₂ is well investigated both experimentally and theoretically,the kinetics under strain conditions is not well understood.In addition,since the grain boundary（GB）is one of the effective defect absorption traps,the study of the formation energy of different configurations of Xe atom at different GBs can help understand the aggregation and release behavior of fission gas at GBs.The main conclusion of this article are as follows:First,the diffusion behavior of oxygen vacancy and interstitial in UO₂ subjected to uniaxial tensile strain up to 3%in the directions of<100>,<110>,and<111>is investigated by combining molecular dynamics（MD）and temperature-accelerated dynamics methods（TAD）.In addition,the relationship between formation energy,surface energy,and cleavage energy at different rotational axes（<100>,<110>and<111>）of symmetric tilt grain boundaries（STGBs）with various misorientation angle was studied.Finally,minimum incorporation energy and segregation energy of Xe atoms at the octahedral interstitial site（Xe-OIS）and substitution site（Xe-V_U）in STGBs was calculated,considering the effects of interatomic potential,temperature,and tensile strain on the results calculated.The main conclusion of this article are as follows:The responses of oxygen vacancy and interstitial to tensile strain are significantly different,especially along the<100>crystal direction,with both diffusivities changing in opposite trends with strain.Strain did not change the main migration mechanism of the them.The effect of strain on the diffusion coefficient D can be attributed to the combined result of the independent variation of the migration energy E_m and the pre-factor D₀.For oxygen vacancy,the effect of strain on D is more profound above 700 K;while for interstitial,the effect of strain on D is more significant below 1150 K.In addition,stretching along the<100>direction results in a obvious decrease in E_m of oxygen vacancy and interstitial.Stretching in<110>and<111>directions exert negligible effect.The formation energy（E_f）of STGB of different misorientation angles is found to obey the 1D BRK function relation,regardless of the choice of potential function,with the E_f based on Basak potential being more sensitive to temperature and lower at 800K.For<100>STGB with small misorientation angles,the E_f increases with the increase of angle.For<111>STGB,the formation energy increases first and then tends to be stable with angle.Under 3%strain,the formation energy shows a general downward trend.Regarding the surface energy（E_S）,the E_S of<100>STGB is almost unchanged with the angle,while the E_S of<110>STGB decreases first and then increases.As for the cleavage energy(E_cl),only the<110>STGB has a clear correlation with the angle,which decreases first and then increases with the angle.The low-?GB energies and their dependence on the misorientation angle obtained in this study are consistent with the literature,which are of great reference significance for understanding the structural stability and mechanical properties of GBs.The strain effect on the incorporation energy(E_incorp)of Xe-OIS and Xe-V_U in grain boundaries and grain interiors shows different degrees of decrease.For low?GBs,the effect of strain on the E_incorp of Xe-OIS and Xe-V_U is minimal.Moreover,when considering the behavior of segregation,the calculation results demonstrate that the segregation energy of Xe-OIS in low?GBs is lower and the influence of strain is insignificant.This is due to the fact that the atom arrangement in low?GBs is more regular and the structure is more stable.Thus,the effect of tensile strain on the grain boundary structure is minimal.Additionally,regardless of the presence of strain,the incorporation energy of Xe-V_U in<111>STGB decreases with the increase of the angle;its segregation energy decreases with the angle in all small-angle（<20°）GBs.