A review of the processes important to the behavior of aerosols during a severe reactor accident involving core melting shows processes leading to particle size change (agglomeration, condensation, and evaporation) and processes leading to removal of particles from the atmosphere (diffusion, sedimentation, thermophoretic, and inertial deposition). The NAUA model and computer code developed at the Karlsruhe Nuclear Research Center treats these processes in a hypothetical core melt accident. The NAUA code is based on first principles, without further restrictions. Its application to such an accident in a pressurized water reactor (Biblis B) shows that the mass of aerosol leaked from a containment building during an accident is strongly dependent on the aerosol source from the core and the existing steam conditions. Condensing steam is effective in reducing leaked aerosol mass. Most of the leakage would occur during the first 12 h of an accident; such leakage is not directly proportional to the aerosol source strength but tails off significantly as the initial aerosol concentration increases.