Fission products and other compounds released during severe nuclear power plant accidents will form aerosol particles, which include water-soluble compounds such as cesium hydroxide (CsOH), cesium carbonate, and cesium iodide. These hygroscopic particles will grow in a humid environment, and thus their settling rate is increased significantly at high relative humidities. This paper evaluates the hygroscopicity of CsOH and other water-soluble compounds released under severe accident conditions. The effect was incorporated into the kinetic particle growth model based on coupled mass and heat transport to evaluate the growth rates of single particles at different atmospheric conditions. Finally, the kinetic growth model for hygroscopic particles was included in the NAUA aerosol code to predict the general behavior of aerosols released into the containment atmosphere. A sensitivity analysis of this model was carried out to guide further work on important parameters and to decrease computing time. It is concluded that hygroscopic properties of radioactive cesium can, in favorable conditions, suppress the release of radioactive materials (source term) by orders of magnitude.