事故工况下核电厂安全壳内放射性气溶胶电荷分布研究

Study on Radioactive Aerosol Charge Distribution in Containment of Nuclear Power Plant during Severe Accident

核电厂发生严重事故后,在安全壳内形成大量的放射性裂变产物气溶胶。由于核电厂气溶胶放射性这一特殊性,放射性核素的衰变过程及衰变粒子与周围介质的相互作用过程会使得气溶胶粒子带电。同种电荷及不同电荷之间的相互作用,可能会影响气溶胶粒子的输运过程。然而,目前的核电厂源项评估过程中忽略了电荷对气溶胶输运过程的影响。考虑到放射性气溶胶所带电荷量及电荷分布是后续实验研究电荷对气溶胶输运影响的基础,本文研究了放射性气溶胶的放电机理,编写电荷分布及电荷量求解程序,并对计算过程进行了实验验证,最终得到了典型核电厂严重事故工况下安全壳内气溶胶所带的电荷量及电荷分布。结果表明:在核电厂事故条件下安全壳内的气溶胶整体带负电荷;对于典型粒径的气溶胶(0.1,5)μm,对应的电荷区间为(0,-25);电荷量随粒径的增大而增加;气溶胶粒子电荷呈正态分布。

A large amount of radioactive fission products will be released into the containment after severe accidents in a nuclear power plant.Except for noble gases and a small amount of iodine,these fission products are mainly in the form of aerosols.The fission products aerosols can be removed by natural processes,for instance,gravity settlement,agglomeration,Brownian diffusion,electrophoresis,etc.Due to the particularity of aerosol radioactivity in nuclear power plants,the decay process of radionuclides and the interaction process between decay particles and surrounding air will make aerosol particles charged.The interactions between the same sign charges and different sign charges may affect the transport process of aerosol particles,in particular,affecting the aerosol particle agglomeration process.Charges of same sign make aerosol particles repulsive,while charges of different signs make aerosol particles attractive.Therefore,the charge on the aerosol particles will affect the removal process of the fission product aerosol.However,in the current source term evaluation process for the nuclear power plant,the effect of electric charge on the aerosol transport process has been ignored.Considering that the charge amount and charge distribution of radioactive aerosols are the basis for subsequent experiments to study the effect of charge on aerosol transport,the aerosol charge amount and charge distribution were studied.In this paper,the charging mechanism was researched and introduced,mainly including decay process,ionization process,recombination process and attachment process.The models for charging process including the above four processes were developed,and then the charging balance and ions equilibrium equations were obtained.The method for solving the equilibrium equations was introduced.A program for solving the charge distribution and charge amount was developed by FORTRAN language and the calculation results were verified by the experiment results.Referring the parameters in NUREG/CR-6189 report as the input parameters,the charge amount and charge distribution of the aerosol in the containment under the severe accident condition of a typical nuclear power plant were finally obtained which would provide boundary conditions for experimental studies.According to the calculation results,it can be concluded that the aerosol in the containment is negatively charged as a whole under the accident condition of the nuclear power plant.For the aerosol with a typical particle size of(0.1,5)μm,the corresponding charge range is(0,-25).The amount of charge is related to the particle size.The larger the particle size,the greater the charge.The amount of aerosol particle charge exhibits a normal distribution.