Computational Simulations Of Element Interactions And Dynamical Properties Of Uranium And Its Alloy Melts

The special property of the uranium is that it releases tremendous amounts of energy by fission reaction,which makes it become one of the most important nuclear fuels in the world.As an important strategic resource,uranium plays a fundamental supporting role in the development of nuclear weapons and nuclear power.However,the shortcomings of pure uranium,such as poor mechanical properties and corrosion resistance,greatly limit its widespread applications.Alloying is an effective way to improve the performance of pure uranium.Hence,the preparation of uranium-based alloys with excellent properties has become a hot topic in the field of nuclear engineering.It is known that the casting process of uranium and its alloys needs to undergo a transformation from a molten state to a solid state.The molten state is the parent state of the solid state.The local structures and dynamic properties of the melts of uranium and its alloys dramatically affect the structures and properties of their final solid phases.Therefore,studying the local structure and dynamic properties of the melts of uranium and its alloys is helpful to understand the mechanics and corrosion performance of uranium-based alloys,and has guiding significance for the preparation of uranium-based alloys with excellent properties.This paper mainly focuses on the elemental interaction and dynamic properties of molten uranium,uranium dioxide,uranium-niobium alloy and uranium-titanium alloy.The research contents and results are as follows:(1)The dynamical properties and solidification simulation of uranium meltIn the first,through ab initio molecular dynamics,the densities of uranium melt at different temperatures were calculated,and the corresponding equations were established,thereby achieving the high-throughput modeling of uranium melt.Furthermore,based on the Einstein relationship,Stokes-Einstein and Arenius equation,the mathematical equations of diffusion coefficients and viscosities of uranium melt were establishd,which realizes the rapid calculations of diffusion coefficients and viscosities in the whole uranium liquid region.In the second,the evolution of atomic local structures during solidification of uranium were investigated in terms of the pair distribution function,structure factor,bond angle analysis,coordination number,HA and Voronoi index.The results show that with decreasing the temperature,the local order in the uranium melt is enhanced.The analyses of HA and Voronoi indices indicate that the uranium melt is mainly composed of icosahedral and bcc-type local clusters.At high temperature,the icosahedral clusters are the most abundant and,however,as the temperature decreases,the content of the bcc-type local clusters still increases.Finally,at 600 K the content of bcc-type local clusters is higher than that of icosahedral clusters.Note that,although the content of icosahedral clusters in uranium melt is the highest at high temperature,the bcc-type local clusters also occupy a large proportion and,during the cooling process,the dominant role of bcc-type local clusters is still increasing.(2)The dynamical properties and solidification simulation of UO2 meltThe dynamical properties of UO2 melt,containing densities,diffusion coefficients and viscosities were calculated by ab initio molecular dynamics and,their corresponding mathematical equations were further established,respectively.Afterwards,the solidification of UO2 melt was studied.The results indicate that the UO2 melt is mainly composed of rhombohedral and hexagonal clusters.The local structures in UO2 melt mainly comes from the contributions of the O-U and O-O pairs,while the contribution of the U-U pairs can be negligible.Interestingly,at above 3100 K,the contribution of both O-U and O-O pairs to the local structures in UO2 melt is almost the same.Whereas,at 2700 K and below,the HA index of 1301 mainly comes from the contribution of the O-U pairs,and the HA index of 1201 is mainly derived from the contribution of the O-O pairs.(3)The element interactions and dynamical properties of U-Nb and U-Ti alloy meltsBy means of first-principles molecular-dynamics,cluster expansion(CE),Monte Carlo(MC)and NEB method,a systematic study of U-Nb alloy from melt to its solid phase was performed.In the first,we used the first-principles molecular dynamics method to calculate the densities,diffusion coefficients and viscosities of U-Nb alloy melt,and constructed their corresponding mathematical equations to realize the rapid calculations of these three properties in the whole U-Nb liquid region.In the second,we investigated the evolution of atomic structures during solidification of U116Nb12 alloy melt from 2600 K to 600 K.It is found that icosahedral and bcc-type clusters play a dominant role in the solidification process of U116Nb12 alloy melt,and Nb atoms are beneficial to the occurrence of evolution of local structure order.In the third,using the combined techniques including first-principles,cluster expansion and Monte Carlo as well as NEB methods,we studied the effect of Nb atom distribution on the mechanical property and the hydrogen diffusion behavior in U-Nb alloy.It is found that the icosahedral and bcc-type clusters play a dominant role in U-Nb alloy melt,and the increase of Nb content can promote the formation of icosahedral clusters.The analyses of chemical short-range order(CSRO)parameters and diffusion coefficients of U-Nb alloy melt indicate that the repulsive interactions between U and Nb atoms can be regarded as the resistance of atom diffusion.In the melt and y phase of U-Nb alloy,the CSRO parameters U-U and Nb-Nb pairs are negative,while the U-Nb CSRO parameter is positive.This feature indicates that there exists similarity of the element interactions between the melt and ? phase of U-Nb alloy.The findings of repulsive interactions between U and Nb atoms can be used to explain well the fact that there is no intermetal lic compound in y-U-Nb alloys.Furthermore,the Nb aggregation inherited from the melt to ? phase may be beneficial to improve the mechanical property of the ?-U-Nb al loy and protect the ?-U-Nb alloy from hydrogen damage.Finally,based on the first-principles molecular dynamics method,the mathematical equations of densities,diffusion coefficients and viscosities of the U-Ti alloy melt were established to achieve the high-throughput calculations of these properties in the entire U-Ti liquid region.Afterwards,the evolution of atomic local structures during solidification of U101Ti27 alloy melt from 2600 K to 300 K were investigated.The results show that the U-Ti alloy melt is mainly composed of icosahedral and bcc-type clusters,and the icosahedral clusters plays a dominant role.During the solidification process of U101Ti27 alloy,Ti atoms are beneficial to the occurrence of evolution of the local structure order.