Steam-aerosol interaction experiments: Measurement of aerosol and liquid droplet size distributions

Radioactive aerosol particles, along with steam, may get discharged into the Reactor Coolant System (RCS) and containment following severe nuclear reactor accident. The interaction between aerosol and steam lead to the condensation of steam on particles, facilitating effective particle growth in comparison to dry agglomeration. This, in turn, promotes rapid settling or wet washout of the particles. Limited studies of this nature currently exist for Pressurized Heavy Water Reactors (PHWRs). Therefore, experiments have been carried out to understand interaction of steam with aerosol and to quantify droplet diameter and associated parameters within the Nuclear Aerosol Test Facility (NATF). This facility is specifically designed for the aerosol-related safety assessment of Indian Pressurized Heavy Water Reactors (IPHWRs). Four sets of experiments have been carried out with two different chemical species and air flow conditions. Aerosols were generated using Plasma Torch Aerosol Generator (PTAG) and their total number concentration and number size distribution were measured using Condensation Particle Counter (CPC), Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS) in dry conditions. Whereas Cloud Droplet Probe (CDP) was used for measuring droplet properties under wet (steam) conditions. In experiments with steam, all aerosols, varying in initial diameter, reached a size of 300 nm after exposure to steam. Tin oxide aerosols were noted to have greater Liquid Water Content (LWC), Mean Volume Diameter (MVD) and Droplet Number Concentration (DNC) when compared to zinc oxide aerosols. This can be attributed to the difference in the chemical properties, indicating high hygroscopicity of tin oxide compared to zinc oxide. Comparison with NAUA code exhibits code's limited predictive capability concerning condensational behavior, lacking provisions for chemical interactions and subsequent water uptake. Enhancing the accuracy of the safety codes is imperative through the incorporation of experimental inputs.