Chemical Forms of Important Fission Products in Primary Circuit of HTR-PM under Conditions of Normal Operation and Overpressure and Water Ingress Accidents: A Study with a Chemical Thermodynamics Approach

Abstract

The chemical forms of important fission products (FPs) in the primary circuit are essential to the source term analysis of hightemperature gas-cooled reactors because the volatility, transfer, and diffusion of these radionuclides are significantly influenced by their chemical forms. Through chemical reactions with gaseous impurities in the primary circuit, these FPs exist in diverse chemical forms, which vary under different operational conditions. In this paper, the chemical forms of cesium (Cs), strontium (Sr), silver (Ag), iodine (I), and tritium in the primary circuit of the Chinese pebble-bed modular high-temperature gas-cooled reactor (HTR-PM) under normal conditions and accident conditions (overpressure and water ingress accident) are studied with chemical thermodynamics. The results under normal conditions show that Cs exists mainly in the form of Cs 2 CO 3 at 250 ∘ C and gaseous form at 750 ∘ C, and for I and Ag, Ag 3 I 3 and Ag convert to gaseous CsI and AgO, respectively, with increasing temperature, while SrCO 3 is the only main kind of compound for Sr. It is also observed that new compounds are generated under accidents: I exists in HI form when a water ingress accident occurs. Regarding tritium, the chemical forms of FPs change little, but compounds need higher temperature to convert. Furthermore, hazard of some FPs in different chemical forms is also discussed comprehensively in this paper. This study is significant for understanding the chemical reaction mechanisms of FPs in an HTR-PM, and furthermore it may provide a new point of view to analyze the interaction between FPs and structural materials in reactor as well as their hazards.

Study on the actual particle size, activity concentration, and migration process adsorption behavior of radioactive substances in liquid effluents from nuclear power plants

“…It should be pointed out that the research on the particle size of radioactive substances in the industry mainly focuses on the primary circuit (Geraldo et al, 2019;Li et al, 2019;Tessaro et al, 2020;Guo et al, 2022), while there is very little research on the particle size of radioactive substances during the migration process outside the primary circuit system. Studying and analyzing the changes in particle size morphology of radioactive materials throughout the entire life cycle of nuclear power plants is a prerequisite for constructing a full life cycle source term evaluation and radiation protection.…”