Theoretical study for the reaction of fission products Cs and I elements with steam in the HTR-PM primary loop

Abstract

Mastery with regard to the evolution of the chemical form of fission products in the primary circuit is essential for the development and improvement of models, which can predict the release of fission products into the primary loop under normal or accidental operating conditions and analyze the consequences of severe accidents. Here, the chemical forms of fission products in the primary loop of the HTR-PM are predicted by the thermodynamic database method. Furthermore, the microscopic mechanisms of the reactions between radionuclides Cs and I with steam are systematically investigated using density functional theory. The predominant reaction products of Cs + H₂O and I + H₂O include CsO, CsOH, IO, IOH, and HI. The bond evolution properties are analyzed using the electron localization function, quantum theory of atoms in molecules, bond order density, and mayer bond order. The reaction rate constants are determined utilizing the variational transition state theory. Within the temperature range of the HTR-PM primary circuit (250–750 °C), the reaction rate constant of OIH₂ is effectively zero, and the reaction rate constant of HI is higher than that of IOH. The results of the above microscopic calculations are consistent with the macroscopic thermodynamic calculations, and these data serve as valuable ESI for the experiments.