Lead–bismuth-cooled fast reactors (LFRs) are among the most promising fourth-generation nuclear reactor designs. However, during prolonged operation, fuel rods are subjected to high temperatures and corrosive environments, leading to the formation of micro-cracks in the cladding. These micro-cracks facilitate the release of radioactive fission products into the coolant, increasing radioactivity in the primary circuit. Understanding the release behavior of fission products from damaged fuel cladding is therefore critical for evaluating source terms and designing safety systems in severe accident scenarios. In this study, fuel cladding micro-cracks were modeled as radial micro-channels, and the transient release behavior of fission gases was investigated under varying thermal–hydraulic conditions and crack sizes using experimental methods. Theoretical calculations were conducted based on capillary and orifice flow models, with the results validated against experimental data. The findings revealed that fission gas release from damaged fuel cladding followed an exponential decay pattern, which could be accurately described by a first-order kinetics equation. Crack size was identified as the dominant factor influencing gas release dynamics. The micro-channel model provided more accurate predictions for smaller cracks, while the orifice model was better suited for larger cracks due to the differences in frictional resistance effects. This research hope to provide fundamental theoretical insights and modeling approaches for assessing source terms and enhancing safety designs in lead–bismuth fast reactors, particularly in the context of severe accident scenarios.