The steady-state release of active noble gas and iodine from defective fuel elements is described either in terms of a kinetic or a diffusion model. Both models assume a diffusional release in the fuel. Transport of fission products in the fuel-to-sheath gap is represented either by a first-order rate process or diffusion process, and is characterized with an escape-rate constant or diffusion coefficient, respectively. The kinetic model predicts a release dependence on the decay constant of λ−1/2 to λ −3/2. The diffusion model predicts a dependence of λ−1. Observed release data from inpile loop experiments, for a wide range of defect states, confirm the predictions of the models. A fitting of the model to the measured data yields estimates of the empirical diffusion coefficient in the fuel matrix, and the escape-rate constant or diffusion coefficient in the fuel-to-sheath gap. Evaluation of the fitted parameters enables the various rate-controlling processes to be deduced as a function of the defect size.