Chemical states and reactions of typical nuclides in the primary circuit under normal conditions of HTR-PM

The chemical states of nuclides significantly affect their behaviors in multicomponent and multiphase systems, including adsorption, desorption, deposition, diffusion, migration, and chemical reactions. Based on the design and operating parameters of the world's first pebble-bed modular high-temperature gas-cooled reactor demonstration power plant (HTR-PM), the chemical states and potential reactions of 14 typical nuclides in the primary circuit under normal operating conditions of HTR-PM were investigated under a thermodynamic framework and a newly developed two-dimensional projected phase diagram. A correlation matrix was used to quantitatively analyze the factors influencing the chemical states, including the temperature, pressure, and amount of impurity elements (C, H, O, and N). The results showed that the temperature had the greatest effect on the chemical states of the nuclides, while the pressure and N content had negligible effects. A mirror-symmetry relationship was discovered between the effects of C and O on the chemical states of typical nuclides. The phase boundary caused by this symmetry was largely influenced by the chemical states and contents of the compounds of minor elements. This study systematically provides the chemical states and reactions for typical nuclides in the primary circuit of HTR-PM, which is of great significance for research into the behaviors of fission products in advanced nuclear energy systems. The developed technologies and methods are also widely applicable to multicomponent and multi-phase chemical systems.