Abstract An analytical model has been developed to describe the diffusional release of fission product cesium from Zircaloy-clad fuel under high temperature reactor accident conditions. The model is based on the analysis of recent annealing experiments performed at the Chalk River Laboratories in steam. The present treatment accounts for the influence of the atmosphere (i.e., changing oxygen potential) on the state of fuel oxidation and the release kinetics. The effects of fuel dissolution on the volatile release behaviour (under reducing conditions) is considered in terms of earlier crucible experiments and a simple model based on bubble coalescence and transport in metal pools. This model has also been used to interpret the cesium release kinetics observed in steam and hydrogen experiments at the Vertical Irradiation NO Facility in the Oak Ridge National Laboratory and at the HEVA/VERCORS Facility in the Commissariat à l'Energie Atomique.