[en] The steady state release of active noble gas and iodine from defective fuel elements is described in terms of either a kinetic or a diffusion model. Both models assume a diffusional release in the fuel. However, transport of fission products in the fuel-to-sheath gap is represented either by a first-order rate process or by a diffusion process, and is characterized with an escape-rate constant or a diffusion coefficient, respectively. The kinetic model predicts a release dependence on the decay constant of λ/sup -1/2/ to λ/sup -3/2/, whereas the diffusion model predicts a release dependence of λ/sup -1/. Observed release data from in-pile loop experiments, for a wide range of defect states, confirm these predicted dependencies, A fit of the model to the measured data yields estimates of the empirical diffusion coefficient in the fuel matrix, and the escape-rate constant or the 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