Dynamic Hygroscopic Growth Mechanism of Aerosol under Severe Nuclear Reactor Accident

Abstract

After a severe accident in a nuclear power plant, the nuclear fuel and fission products inside the core will leak into the nuclear reactor containment vessel in the form of radioactive solid particles, and disperse in the containment space to form aerosols. These aerosols are important carriers of radioactive materials, and are also the key source items of radioactive fission products released into the environment under serious accidents. Besides, serious accidents in nuclear reactors are often accompanied by the loss of coolant accidents in primary pipe sections, resulting in a high-temperature and humid environment with relative humidity close to saturation in the containment. Under such accident conditions, the soluble aerosol in the containment vessel will undergo hygroscopic growth. The particle size will increase significantly due to the absorption of water vapor, and the particle density will also change significantly. Then the kinetic properties of aerosols will change, thus affecting the natural removal, migration and evolution of aerosols in the original containment, which is of great significance for the study of radioactive removal in the containment after serious accidents in nuclear power plants. In order to describe the process parameters of aerosol hygroscopic growth, such as hygroscopic equilibrium time, and characterize the time scale of hygroscopic growth process, an aerosol dynamic hygroscopic model based on the non-equilibrium process of aerosol hygroscopic growth was constructed, considering the relationship between vapor mass conservation and energy conservation during hygroscopic process, as well as the influence of surface vapor pressure on aerosol hygroscopic growth behavior. The aerosol dynamic hygroscopic model takes into account the mass conservation between the vapor phase and the energy change caused by vapor condensation. The surface vapor pressure of the particles was determined by the Kelvin effect and the solute effect. Based on the aerosol dynamic hygroscopic model, the parameters of the hygroscopic process such as particle size change rate and hygroscopic equilibrium time were analyzed. The results show that the solute effect is more significant than the Kelvin effect in the initial stage of hygroscopic growth, but the solute effect decreases rapidly with the process of hygroscopic growth proceeding. The results also show that the increase of particle size due to hygroscopic growth is particularly significant for the small particle size under the condition of high humidity. When the aerosol particle size gradually increases, the effect of hygroscopic growth gradually decreases to negligible. During the hygroscopic growth process, the particle size change rate follows a logarithmic variation pattern with time, while the equilibrium time and particle size change approximately linearly.

Keywords

• severe nuclear reactor accident
• aerosol
• hygroscopic growth