A comprehensive fission gas release model is developed by considering the behavior of multiple bubble sizes on the fuel grain boundary in terms of relevant physical parameters. This model takes into account bubble migration and coalescence; critical bubble size, which depends on the thermal gradient on the grain boundary; and the lenticular shape of the bubbles. Booth’s classical diffusion theory is directly adopted in the modeling of intragranular fission gas behavior. To consider the bubble drift due to the thermal gradient, those bubbles that exceed the critical bubble size are assumed to be left on the grain boundary and to migrate along the thermal gradient until they encounter free voidages. Use of this model in the KAFEPA code, which predicts the absolute magnitude and the trend of the gas release depending on power history, gives better agreement with the experimental data than the predictions of the model in the ELESIM code, which considers only a single bubble size at the grain boundary.