Research on Resuspension Mechanism of Aerosols inside Containment during Severe Accidents at Nuclear Power Plant

When a severe accident occurs in a nuclear reactor core, a large amount of radioactive aerosols exist within the containment. These radioactive aerosols deposit on the surfaces of the containment and internal structures due to mechanisms like gravity settling and diffusion swimming, reducing the aerosol concentration inside the containment. During severe accidents, various events can cause gas flow within the containment, broadly categorized into two types. One type includes events like steam explosions, hydrogen combustion, which generate transient high-speed airflow locally; the other type includes continuous airflow generated between different compartments of the containment, natural convection between compartments, and steam condensation near the wall. When gas flow occurs within the containment due to these events, previously deposited aerosols are resuspended into the air due to fluid drag force, becoming a continuous source of radioactive aerosols. Resuspended aerosols can then migrate with the airflow, affecting the distribution of radioactive substances within the containment. Therefore, aerosol resuspension introduces more uncertainty into the distribution and source term calculations of radioactive substances within the containment during severe accidents. Visual experiments and torque balance models were employed to investigate aerosol resuspension characteristics under different deposition conditions (relative humidity, deposition surface orientation, deposition time) and airflow conditions (airflow velocity, transient airflow, and continuous airflow) in this paper. Experimental results indicate that humidity in different deposition environments affects aerosol resuspension. With increasing humidity in deposition environments, the proportion of aerosol resuspension significantly decreases. For aerosols deposited under three humidity environments of RH30%, RH70%, and RH98% at an airflow velocity of 60 m/s, the resuspension proportions are 93.2%, 32.5%, and 13.5%, respectively. Similarly, compared to horizontally deposited surfaces, vertically deposited surfaces exhibit lower deposition amounts, making it difficult to form loose deposition structures and thus reducing resuspension proportions. Aerosol deposition time also affects resuspension proportions; as deposition time increases, aging of deposits occurs, increasing adhesion forces between aerosols and deposition surfaces as well as among aerosols themselves, thereby decreasing aerosol resuspension proportions. At an airflow velocity of 8 m/s, aerosol resuspension proportion for deposits aged for 24 hours is 58.8%, whereas those aged for 72 hours decrease to 36.3%. Under the influence of low-speed continuous airflow, small particle size aerosols underwent resuspension and were carried downstream, whereas larger clusters underwent rolling motion, significantly reducing the median particle size of aerosols in downstream fluids. Under high-speed transient airflow, multiple layers of aerosols were suspended simultaneously, and clusters formed during the deposition process were carried by the airflow, resulting in a second peak in the particle size distribution after resuspension with larger corresponding particle sizes. Due to its greater acceleration, transient airflow generates additional removal forces such as the Basset force, resulting in significantly higher resuspension proportions for transient resuspension at the same velocity compared to continuous erosion resuspension.