MODELING THE RELEASE BEHAVIOR OF CESIUM DURING SEVERE FUEL DEGRADATION

An analytical model has been developed to describe the diffusional rel ease of fission product cesium from Zircaloy-clad fuel under high temp erature reactor accident conditions. The model is based on the analysi s of recent annealing experiments performed at the Chalk River Laborat ories in steam. The present treatment accounts for the influence of th e atmosphere (i.e., changing oxygen potential) on the state of fuel ox idation and the release kinetics. The effects of fuel dissolution on t he volatile release behaviour (under reducing conditions) is considere d 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 a nd hydrogen experiments at the Vertical Irradiation (VI) Facility in t he Oak Ridge National Laboratory and at the HEVA/VERCORS Facility in t he Commissariat a l'Energie Atomique.