[en] In this paper an ettempt is made to model the release of I-131 from UO₂ fuel with a burn-up greater than 5 MWd/kg U. As a basis for the model the experimental results from the S176 series of experiments performed at Studsvik have been used. The simple theory in which the release mechanism is sweeping up the iodine by the moving grain boundary and subsequent release through the grain boundary porosity seems to work quite well when grain growth occurs and there is grain boundary porosity up to the fuel center. Both the theoretical considerations and the experimental results show that a knowledge of the irradiation history is important and that there is no unique time-temperature-release relationship for the release of I-131. (author)