镧系裂变产物（La、Ce、Pr和Nd）在α-U表面偏析行为的第一原理研究 #br#

镧系裂变产物引起的燃料肿胀及包壳脆化是UZr金属燃料服役中的主要问题。其快速扩散通常借助于裂变气体释放通道，实验可观察到镧系裂变产物会在UZr金属燃料氙气泡处偏析。为理解裂变产物扩散机理及表面偏析现象，本文采用第一原理方法，对低温α-U中（100）、（112）、（001）、（021）、（110）、（010）表面的原子结构及形成能进行了研究，并讨论了常见镧系裂变产物La、Ce、Pr和Nd在不同表面的偏析行为。计算结果表明，（110）表面的形成能最低，为1.75 J/m²，（112）、（021）和（001）表面的形成能次之，为1.81~1.83 J/m²，（010）和（100）表面的形成能最高，分别为1.96 J/m²和2.04 J/m²。4种裂变产物在6种表面都表现出明显的偏析效应，对于同一种表面，其偏析驱动力排序为：La>Ce>Pr>Nd。对于同一种镧系裂变产物，偏析能随层间距的增加而减小。此外，采用Mc-Lean方程从热力学上评估了4种裂变产物在α-U表面的占据率，结果表明在服役温度范围内，4种镧系裂变产物均表现出明显的表面偏析。

Segregation of Ln Fission Product (La,Ce, Pr and Nd) on Surface of α-U from First-principle

The embrittlement of the cladding materials and the fuel swelling caused by Ln fission products are the main problems in the service of UZr metal fuel. The quick diffusion is usually by means of fission gas release channels, and it is observed in experiments that the Ln fission products will segregate to the fission gas bubbles and bubble connection zone in UZr metal fuel. In order to understand the diffusion mechanism of fission products and the phenomenon of surface segregation, in this work, first-principle calculations were carried out to investigate the atomic structure and formation energy of (100), (112), (001), (021), (110) and (010) free surfaces in low-temperature α-U, and the segregation behavior of fission products La, Ce, Pr and Nd on different free surfaces. The results show that the formation energy of (110) free surface is the lowest, 1.75 J/m², followed by (112), (021) and (001) free surfaces, 1.81-1.83 J/m², and the formation energy of (010) and (100) free surfaces are 1.96 and 2.04 J/m², respectively. Ln fission products all tend to segregate to the free surfaces, and the segregation strength of the four Ln fission products is in the order of La>Ce>Pr>Nd. The segregation energy decreases with the increase of the interlayer spacing for six free surfaces. Furthermore, using the Mc-Lean equation, the occupation of the fission products on the surfaces of α-U was estimated thermodynamically, and the results suggest that within the service temperature range, the four Ln fission products all segregate to the surfaces.