Abstract: Liquid lead-bismuth eutectic (LBE, 44.5%Pb-55.5%Bi), owing to its superior neutronic, physical, and chemical properties, serves as both a spallation neutron target material and a coolant for accelerator-driven systems (ADS) and reactor systems. However, conventional source term calculation methods for light water reactors and sodium-cooled fast reactors fail to account for the unique chemical interactions between LBE and nuclides, rendering them inapplicable to lead-bismuth fast reactors. To address this challenge, evaporation experiments were conducted to investigate the migration behavior of volatile nuclides from LBE coolant to the cover gas and containment, and to establish a dedicated source term calculation method for lead-bismuth fast reactors. The study focused on the evaporation behavior of polonium (Po) and its Henry constants under different atmospheres (pure Ar, Ar+5%H₂, Ar+2%H₂O) at 300-1 000 ℃, as well as the aerosol particle size distributions of LBE-3%CsI and LBE-1%Te alloys at 300-600 ℃. Results indicate that Po Henry constants exhibit strong consistency at high temperatures (>600 ℃). However, at lower temperatures (300-600 ℃), Po evaporation is significantly influenced by redox reactions and surface enrichment, leading to overestimated Henry constants. The chemical speciation of volatile Po remains unresolved, necessitating further systematic studies to elucidate the mechanisms driving enhanced Po evaporation. Aerosol particle sizes for LBE alloys range from hundreds of nanometers to micrometers. The LBE-3%CsI alloy aerosols exhibit an average particle size of about 4 μm, substantially larger than the 0.3 μm average size of LBE-1%Te aerosols. Due to the higher purge flow rates and increased evaporation at higher temperatures, the probability of particle collisions increases, resulting in the aggregation of smaller aerosol particles into larger ones. Therefore, as the temperature increases, there is a trend of increasing particle size for both LBE-3%CsI and LBE-1%Te alloy aerosols. By integrating experimentally derived Henry constants and aerosol size distributions, a source term calculation method was developed for volatile nuclide migration across the cover gas, containment, and environmental release pathways. This method enables quantitative evaluation of volatile nuclide release under both normal operation and accident conditions in lead-bismuth fast reactors, providing critical insights for shielding design, waste management, and environmental impact assessment.