High Temperature Reactor-Pebblebed Modules (HTR-PM) is a typical high-temperature gas cooled reactor (HTGR) [1]. Tritium is one of the most important radionuclides in reactors owing to its very harmful β-radiation and long half-life. In the HTR-PM, Silicon carbide (SiC) is the main barrier of triisotropic (TRISO) particles to prevent the diffusion of Tritium into the primary circuit [2]. When Tritium into the primary circuit and circulate to Steam Generator (SG), the Incoloy800H alloy is another important material to prevent the diffusion of Tritium into the secondary circuit [3]. When analyzing the source term of Tritium in HTR-PM primary and secondary circuit, it is important and necessary to know the diffusion behavior of Tritium in SiC and Incoloy800H and furthermore, the detail mechanism of diffusion is also essential, which could not be obtained from traditional phenomenological analysis and conservative estimation. In order to solve this challenge, a framework with ab-initio methods is established. In this paper, the detail theory of ab-initio theory and the actual usage in the calculation of the diffusion path, barrier energy are given firstly. And then, the most physical path and the minimum energy barrier will be determined, which can be considered as the diffusion activation energy. The calculated results of activation energy of Tritium in SiC and Incoloy800H are 0.442eV and 0.757eV respectively. Furthermore, the theoretical results are compared with the experimental data, and it is found that both are in agreement with each other. These results are very helpful for understanding the diffusion behaviors of Tritium in HTR-PM materials and can be used to guide the tritium source term analysis in HTR-PM, which are first studied from a micro perspective.

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