Ab Initio Molecular Dynamics Calculation of Diffusion Coefficients of Molten Materials
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摘要: 为获得核反应堆严重事故后期反应堆压力容器（RPV）下腔室内熔融物微观组织的演化规律，需要对熔融物的材料物理性质进行研究。以熔融池中发生熔化过程的实际材料，包括燃料芯块UO2、包壳管熔融后的U-Zr-O材料以及不锈钢构件熔融后的U-Fe-O材料为研究对象，采用基于第一性原理的从头算分子动力学模拟了熔融物材料高温液态下的原子扩散行为。研究结果表明，在高温液相中的U、Zr、Fe、O的原子扩散系数与原子质量呈负相关，且在相同温度下受组分的影响较小，仍保持相对稳定的比例关系。不同原子扩散系数的差异理论上会导致熔融池形成分层结构，因此，可对比上述3种材料在高温液态下各种原子的扩散系数，确定直接的量化关系，为在大尺度下进一步研究熔融物微观组织的演化奠定基础。Abstract: 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.
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Key words:
- Molten materials /
- Uranium dioxide /
- Diffusion coefficient /
- Ab initio molecular dynamics
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图 1 熔融物材料UO2、UZrO4、UFeO4的原子超胞结构
Figure 1. Atomic Supercells of Molten Materials, UO2, UZrO4 and UFeO4
图 2 UO2高温熔化过程的原子结构和RDF
g(r)—RDF
Figure 2. Atomic Structure and RDF of UO2 in High Temperature Melting Process
图 3 UZrO4高温熔化过程的原子结构和RDF
Figure 3. Atomic Structure and RDF of UZrO4 in High Temperature Melting Process
图 4 UFeO4高温熔化过程的原子结构和RDF
Figure 4. Atomic Structure and RDF of UFeO4in High Temperature Melting Process
图 5 3种熔融物液态下原子的MSD
Figure 5. MSD of Atoms in the Liquid State of Three Molten Materials
表 1 3种超胞的晶格常数
Table 1. Lattice Constants of Three Supercells
超胞类型 a/Å b/Å c/Å UO2 10.750 10.750 10.750 UZrO4 14.963 7.481 18.011 UFeO4 9.756 10.214 11.239 1 Å=10−10 m；a、b、c—晶格x、y、z三个方向的晶格常数 
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表 2 U、Zr、Fe、O原子在高温液相下的扩散系数
Table 2. The Diffusion Coefficient of U, Zr, Fe and O Atoms in High-temperature Liquid Phase
原子类型 U Zr Fe O 扩散系数/（ ${10^{ - 8} }{\text{ } }{ {\text{m} }^{\text{2} } } \cdot { {\text{s} }^{ {{ - 1} } } }$） 0.84 0.63 1.15 1.55
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