High Temperature Reactor-Pebblebed Modules (HTR-PM) is a typical high-temperature gas cooled reactor (HTGR) [1]. In the HTR-PM, helium is used as the coolant to the primary circuit and the fission products released from fuel elements would be carried into circulation by helium [2]. When analyzing the source terms in HTR-PM, it is important and necessary to know the amount of nuclides adsorbed on the component materials of primary circuit [3] and furthermore, the detail mechanism of adsorption is also essential, which could not be obtained from traditional phenomenological analysis and conservative estimation. In order to solve this challenge, we established a framework with ab-initio methods. In this paper, the detail theory of ab-initio theory and the actual usage in the calculation of the adsorption energy, Fermi level, density of state and charge density difference are given firstly. And then, we show the calculated results of adsorption behaviors of radioactive fission products (Cs, Sr, Ag, I) on 2•1/4Cr1Mo and SiC, which are important structural materials for steam generator and coated particle of fuel elements for HTR-PM, respectively. It is found that Ag and I atoms prefer to be adsorbed at the square hollow site of the face-centered cubic iron cell with binding energy of about 1eV and 3eV respectively. By contrast, Cs and Sr atoms are not adsorbed on the surface of 2•1/4Cr1Mo. For the study of adsorption on SiC, it shows that all the four nuclides can be adsorbed on the surface of SiC with the binding energy of about 1∼3 eV. Finally, the adsorption rates of these nuclides are estimated by using the first-principle calculation results of adsorption energy. The adsorption rate can be used to determine the amount of adsorbed radioactive nuclides for nuclear safety evaluation of HTR-PM. These results can illustrate the micro pictures of the interaction of fission products and material, which is a new and useful way to analyze the source term in physical level.

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