The Electronic,Mechanical And Thermal Properties Of U₃Si₂-Al Compounds:A Systematic Density Functional Theory Research

Since the Fukushima nuclear accident in 2011,the nuclear industry has put forward a major demand for reactor safety under accident conditions and proposed a new concept of developing an accident fault-tolerant fuel system.One of the important branches is developing alloy fuels(such as U-Si series alloy fuels).As a member of U-Si alloy fuel,U₃Si₂ is regarded as an ideal candidate material for new accident-tolerant nuclear fuel due to its ultra-high thermal conductivity and uranium density as high as 11.3 g(U)/cm³.However,the main shortcoming of U₃Si₂ is its poor mechanical properties and oxidation resistance,and alloying elements need to be added into for optimizing original physical and chemical properties,thereby forming a new type of uranium-silicon alloy fuel.Currently the researches and designs of uranium-silicon alloy fuel have become a hotspot within accident fault-tolerant fuel systems,as well as becomes one of the critical themes in the development of our country's nuclear industry.Previous studies show that the addition of Al element can significantly improve the oxidation resistance of U₃Si₂,but the research on its mechanical and thermodynamic properties is lacking.Therefore,this work focuses on exploring the role of Al element in improving the mechanical and thermodynamic properties of U₃Si₂.Up to now,the results and conclusions achieved are as follows:1.The ground-state structures of the U₃Si₂ formed by Al alloying at three internal defect sites are obtained by DFT+U method;and their stabilities are calculated separately.The results show that all the structures of these three compounds are thermodynamically stable.2.The calculation results of the electronic structures show that the U₃Si₂-Al(?)compound formed by the substitution of Si positions exhibits metallic characteristics.The results of the differential charge density and the local charge density show that U-U and U-Al form metal bonds;while U-Si forms an ionic bond,and Si-Al forms a covalent bond.3.The calculation results of elastic properties show that the addition of Al element significantly improves the toughness of U₃Si₂.At the same time,the results of the three-dimensional Young's modulus show that,compared with U₃Si₂,the doped structure formed by the substitution of Al atoms at the Si position still maintains higher isotropy,while the doped structure formed by the substitution at the UI position is showing significant anisotropy.4.The results for the anisotropy of phonon thermal conductivity show that the alloyed ternary compounds keep maintaining high isotropy;while the results of the electronic thermal conductivity show that as the temperature increases,the electronic thermal conductivitives for three compounds increase linearly.