Simulating the Behavior of Fission Product Aerosols in the Containment

During a severe accident at an NPP involving loss of the primary coolant circuit and reactor core tightness, the majority of volatile and nonvolatile fission products are released into the containment atmosphere in the form of aerosols, the particles of which are multicomponent in composition and polydisperse in size. The particle sizes may vary from a few nanometers to several tens of micrometers. The size and mass of aerosol particles determine their lifetime in a suspended state in the atmosphere of the reactor premises. The radiation activity of particles depends on the content of radionuclides in them and is only indirectly connected with their mass and size. Therefore, in simulating the behavior of radionuclides, it is very important to take into account not only the change in the disperse characteristics of aerosol particles, e.g., their mass and size, but also the content of low- and even very low-mass but highly radioactive admixtures in the particles. The article presents a fission product aerosols' behavior model, which takes into account a change in the concentration, size, and composition of aerosols, including a change in the particle material density. The model is implemented in the fraction method approximation for a discrete mass of particles. For taking into account the effect that the composition of components has on the density of mixed aerosol material, the ideal solution approximation is considered. Test calculations for the model problem with two sources of aerosol particles having different composition and diameter in the modeled room volume were carried out. The first source consisted of CsOH particles, and the second one of UO₂ particles. It is shown that it is important when simulating the behavior of multicomponent fission product aerosols in premises to take into account the change in the composition of aerosols as a result of coagulation and, as a consequence, the change in aerosol particle material density, which depends both on the composition and density of individual components.