Ab Initio Molecular Dynamics Calculation of Diffusion Coefficients of Molten Materials

In order to understand the evolution of the microstructure of the molten materials in the lower chamber of the reactor pressure vessel (RPV) of the nuclear reactor at the later stage of a severe accident, it is necessary to investigate the physical properties of the molten material. The atomic diffusion behavior of molten materials in high temperature liquid is simulated by ab initio molecular dynamics based on the first principles, with the actual materials in the melting process in the melting pool, including fuel pellet UO₂, U-Zr-O materials after melting cladding tubes and U-Fe-O materials after melting stainless steel components as the research objects. The results show that the atomic diffusion coefficients of U, Zr, Fe, O in the high temperature liquid phase are negatively related to the atomic mass, and are less affected by the components at the same temperature, maintaining a relatively stable proportional relationship. The difference of different atomic diffusion coefficients will theoretically lead to the formation of layered structure in the melting pool. Therefore, the diffusion coefficients of various atoms in the above three materials at high temperature can be compared, and the direct quantitative relationship can be determined, which lays a foundation for the further study of the microstructure evolution of molten materials at large scale.