Ablagerungsverhalten von Kernschmelz- und Brandaerosolen in einem DWR-Sicherheitsbehälter

A main focus of reactor safety research is the mitigation of radiological consequences during a severe accident of a nuclear power plant. In the case of such an event a significant amount of radioactivity is released into the containment in form of aerosols. Knowledge about their size distribution, concentration and chemical composition during a severe accident scenario is crucial regarding source term estimations for environment and also accident management measures, e.g. the design of venting systems. Even if much knowledge is available concerning aerosol behaviour under the complex boundary conditions of a severe accident, only little is known about the impact of fire aerosols on the characteristics of the nuclear aerosols in the containment atmosphere. The current work is an experimental investigation aiming to examine the potential impact of fire products on core melt species and their depletion behaviour. For this purpose an experimental facility is set up being able to continuously generate, mix and measure chemically representative core melt species and representative cable fire aerosols. As chemically representative core melt species the materials SnO2, CsI, Ag and Cs2MoO4 are used. As representative cable fire products pyrolysis products and soot from under-ventilated as well as well-ventilated combustion of a fire resistant non-corrosive containment cable (FRNC-BX) and soot of a well-ventilated PVC cable fire are generated. Then single components and multi-component aerosols were mixed with fire aerosols in order to study changes in size distribution, concentration, morphology and chemical composition. Results show that the presence of fire aerosols induces physical and chemical interaction. Compared to representative core melt aerosols, cable fire aerosols are a large source of small particles having an aerodynamic number mean diameter of about 0,3 µm, whereas the used core melt aerosol particles are twice as large. Physical interaction leads to a broader size distribution which is shifted to smaller mean particle sizes. The depletion behavior is enhanced for aerosol species with small initial diameters by preventing accumulation of particles in a size range between 1 µm and 3 µm AMMD. This is mainly observed for CsI and Cs2MoO4. For large silver particle, which mainly sediment during depletion, a reduced deposition due to cable fire aerosols is observed. Significant results concern the chemical impact of cable fire aerosols on the composition of CsI particles during the experimental feed-in and depletion phase. Results confirmed that a decomposition of CsI particles under the presence of cable fire products takes place. The decreasing iodine to cesium ratios are leading to the conclusion that volatile, probably organic iodine was formed. Volatile iodine species production is known to be dominated by radiolytic reactions, but an additional production path due to fire aerosol interaction may also have a non-negligible impact on the airborne iodine activity and iodine source term estimation