Based on a number of in- and out-of-reactor experiments at the Chalk River Nuclear Laboratories, a physically based model has been developed to predict the activity release of radioactive noble gases from defected UO2 fuel elements during steady-state reactor conditions. This model has been interfaced with the ELESIM fuel-performance code, and verified against all-effects experiments in the National Research Experimental reactor with defected elements containing various sizes and types of sheath failure, and operating at linear powers ranging from 22 to 67 kW/m up to a maximum burnup of 278 MW.h/kg U. The model accounts for various interrelated phenomena that can affect the prediction of fuel temperature and fission product release. The transport of fission products in the fuel matrix is described by a diffusion mechanism. The kinetics of fuel oxidation are treated as a rate-determining reaction at the fuel/steam interface. Such oxidation can lead to a degradation of the fuel thermal conductivity, and a direct enhancement of the rare gas diffusivity in the fuel matrix. Migration of fission products along the fuel-to-sheath gap to the defect site is also modeled by a diffusion process.