Investigation On Removal Characteristics Of Aerosol By Spray In The Containment

When a severe accident occurs in a nuclear reactor,a portion of radioactive materials is released into the containment building in the form of aerosols.Once these aerosols are accidentally released outside the containment building,they pose a serious threat to the environment and public safety.The containment spray system is an important facility for cooling and reducing the concentration of radioactive aerosols within the containment during severe accidents.Currently,research on aerosol removal through spray systems is relatively limited both domestically and internationally,and the influence of relevant parameters on aerosol removal through spraying is still not well understood.Therefore,this study adopts a combined approach of theory and experiment to investigate the characteristics of aerosol removal through spraying.The main accomplishments of this study are as follows.Firstly,a distribution model for spray droplets was established based on the Lagrangian particle tracking method and the Discrete Parcel Method(DPM).The model consists of droplet motion model,droplet collision and coalescence model,entrainment airflow model,and droplet phase change model,enabling real-time tracking of parameters such as droplet velocity,position,diameter,and temperature.The model calculations for droplet velocity,diameter,and temperature parameters were validated using experimental data.Furthermore,the model was used in conjunction with experimental data to study droplet coalescence behavior and the distribution of entrainment airflow.The results indicate that droplet coalescence leads to an increase in average droplet diameter.As the spray flow rate increases,the influence of droplet coalescence on the average droplet size also strengthens.However,an increase in the spray flow rate results in an increase in droplet size dispersion and a decrease in droplet coalescence probability.Hence,the average droplet diameter does not increase indefinitely with the increase in spray flow rate,suggesting that there is a certain limit to the impact of spray flow rate on droplet coalescence.The entrainment airflow enhances droplet velocity and the coverage range of the sprayed droplets.The flow rate of the entrainment airflow at different positions increases with the vertical distance from the nozzle.Due to the presence of the spray expansion angle and the weakening of gas-droplet momentum exchange,the velocity of the entrainment airflow decreases with increasing distance from the nozzle.Heat exchange between the droplet and the gas results in a rapid increase in droplet temperature.In a saturated vapor environment,droplet diameter continues to increase until it reaches a stable value.In an overheated vapor environment,droplet diameter initially increases,then continuously decreases due to evaporation until the droplet completely evaporates.The study identifies two stages of droplet phase change behavior: transient phase change and thermal equilibrium phase change.Aerosol transport model under spray conditions was established based on the entrainment airflow model,single droplet aerosol removal model,and aerosol settling and coagulation model.This model was coupled with the droplet distribution model to create a spray aerosol removal model.The model calculations were validated using experimental data.The influence of aerosol settling and coagulation on concentration and particle size distribution was analyzed based on the model and experimental data.The results indicate that long-term settling and coagulation of aerosols have an impact on aerosol concentration and particle size,leading to stratification of aerosol concentration in the vertical direction.However,within the 2000 s spraying time,the stratification and settling and coagulation effects of aerosols are relatively weak,and therefore,they do not significantly affect the process of aerosol removal during spraying.Experimental and theoretical research was conducted on the behavior of aerosol removal in high-temperature and high-pressure environments under spray conditions.The experimental data was compared with the spray aerosol removal model to validate the model’s predictive accuracy for relevant parameters during the aerosol removal process.Based on the model analysis,it was found that droplet coalescence reduces the aerosol removal efficiency,with a more significant impact observed for small droplets.The experimental and model results for thermodynamic parameters revealed that a higher spray flow rate leads to faster cooling inside the containment vessel.Additionally,a higher vapor volume fraction results in a higher heat transfer coefficient on the containment vessel walls,leading to higher gas temperatures.The experimental and model results for aerosols showed that an increase in spray flow rate and droplet velocity leads to an increase in aerosol removal rate.Moreover,a higher vapor volume fraction and higher containment vessel wall temperature result in higher aerosol removal efficiency due to the enhanced droplet-driven removal mechanism caused by higher gas-liquid temperature difference.