Study On The Loss Mechanism Of Radioaerosol During The Transportation In The Containment

During the operation of the nuclear power plant,there may exist cracks or crevasses,which may lead to the loss of coolant.In order to ensure the safe operation of the nuclear reactor and in the concept of leak before break（LBB）,it is of great importance to monitor the leak of the coolant and quantify the flow rate as soon as possible to prevent the severe accident.Among a variety of leakage monitoring methods,the method of monitoring the activity concentration of specific radioactive aerosols in the containment is concerned because of its quantitative inverse estimation of the leakage rate of the coolant.In the application of this method,mastering the transport and loss mechanism of radioactive aerosol is the key to accurately monitor and estimate the leakage rate of coolant.In this paper,the transport and loss mechanism of aerosol particles are studied by means of theoretical analysis,numerical simulation and experimental research.The correctness of aerosol transport and loss model and research method are verified.The transport,deposition and coagulation laws of aerosol particles are analyzed,and the coupling effect of the transportation,deposition and coagulation of aerosol particles is mastered.The corresponding relationship between the monitoring value and the leakage rate is obtained through the simulation of the transport and loss process of the F-18 aerosol in the containment.The main contents and conclusions of this study are as follows:（1）The motion models of air and aerosol particles are established.The method of direct numerical simulation coupled with Lagrange particle tracking is used to study the transport and loss of aerosol particles in the flow field.The calculation results are compared with the theoretical and empirical formulas,and the correctness of the model and simulation are proved.（2）The method mentioned above is used to study the transport and deposition of aerosol particles in the air.The deposition velocity of the micron-sized aerosol particles in the still air is measured.The effects of gravity,thermophoresis,and turbulence on deposition are analyzed and the deposition laws are obtained.It shows that in the sill air,thermophoresis has a significant influence on the deposition of small particles（smaller than 2μm）and can increase the deposition velocity to the cold wall.But for large particles,gravity will dominate the deposition.In the turbulent flow field,the deposition of small particles will be influenced mainly by turbulence,while the large particle is mainly influenced by gravity.If the initial aerosol particles are polydisperse,the particle size distribution of the deposited particles is different with the initial particle size distribution.The median diameter of the deposited particles is much larger than that of the undeposited particles.The deposition of particles toward the sidewalls in turbulent flow is mainly affected by turbulence and thermophoresis,and thermophoresis can increase the deposition number of particles to the cold wall in the near wall region.（3）Aiming at the limitation of the coagulation theory and numerical simulation method,a new method based on Monte Carlo algorithm and Finite Active Samples Assumption（MC-FASA）is proposed and verified to directly simulate the collision and coagulation of particles.This method is of high efficiency to describe the particles motion and collision by Monte Carlo method,and avoids numerous judgments of collisions for all particles.Directly study on the particle trajectory,motion and collision can reveal the details of coagulation.It shows that,the effect of coagulation on the change of particle number concentration and the particle size depend on the initial particle number concentration.When the initial number concentration is 10¹³/m³,the phenomenon of particles coagulation is obvious.The coagulation coefficient of monodisperse aerosol particle decreases with the increase of particle size.The mean coagulation coefficient of the polydisperse aerosol particles is three times larger than that of the monodisperse aerosol particle with the same count mean diameter.（4）Based on the above research results,the CFD method is used to study the transport process of F-18 in the third generation PWR nuclear power plant containment.Results show that after the leakage of the coolant,the total radioactivity of the monitoring nuclide in the containment is increased first and then gradually become stable.Taking F-18 as an example,the balance time is about four times of the half-life period.Selecting a higher monitoring location is more effective to detect the leakage as early as possible.The change of monitoring points at the same height does not significantly affect the monitoring results.The work in this thesis can provide a basis for the selection of the sampling and monitoring location,for the assessment of the relationship between the monitoring results and the leakage rate.