Experimental and numerical investigation of aerosol hygroscopic behavior in sodium fire accidents

During severe core disruptive accidents in sodium-cooled fast reactors (SFRs), liquid sodium may be ejected into the reactor containment and trigger subsequent sodium fires. Sodium combustion products react with steam and core-released radioactive iodine to form NaOH and NaI aerosol particles. The hygroscopicity of these particles is directly correlated with the containment source term of sodium-cooled fast reactors. To clarify the hygroscopic growth characteristics of single-component NaOH, single-component NaI, and binary NaOH-NaI mixed aerosols, this study established an experimental system consisting of atomization, drying, humidity regulation and a scanning mobility particle sizer (SMPS). Particle size data at different relative humidity (RH) levels were measured and used to calculate aerosol hygroscopic growth factors (GF). The experimental GF values were further compared with the predictions from three typical models: a simplified model based on the van’t Hoff factor, a single-component fitting model derived from experimental data, and the ZSR model applicable to mixed aerosol systems. The results show that NaOH aerosols present a continuous hygroscopic growth trend and begin to absorb moisture at a low relative humidity of 25%. By contrast, NaI aerosols follow a completely different hygroscopic pattern. The measured deliquescence relative humidity (DRH) of NaI is approximately 40 %. Significant differences can be observed in its hygroscopic curves before and after the deliquescence point. For binary NaOH-NaI mixed aerosols, their hygroscopic properties change regularly with the mass fraction of NaI, and a transition range exists. When the NaI mass fraction is below the critical value, the hygroscopic growth curve remains smooth. As the NaI proportion rises beyond this critical threshold, a distinct transition point emerges on the hygroscopic curve. This work does not determine an accurate critical mass ratio, but verifies that this value falls within the range of 0.5-0.714(It is identified as the critical mass ratio range of the binary mixture). The research outcomes can facilitate the improvement of numerical models for aerosol growth, sedimentation and source term assessment. It also lays a solid foundation for in-depth investigation on the deposition position and settling velocity of hygroscopically aged aerosols inside the containment of sodium-cooled reactors after accidents.