A KINETIC-MODEL FOR FISSION-PRODUCT RELEASE AND FUEL OXIDATION BEHAVIOR FOR ZIRCALOY-CLAD FUEL-ELEMENTS UNDER REACTOR ACCIDENT CONDITIONS

An analytical model has been developed to describe the release behavio ur of fission product cesium from uranium dioxide fuel during severe r eactor accident conditions. The present methodology is based on the re sults of out-of-pile annealing experiments with irradiated fuel (bare and Zircaloy-clad fuel specimens), subjected to a steam atmosphere at high temperature (1200 to 1700 degrees C). In the present framework, t he fuel oxidation kinetics is detailed by a surface exchange reaction at the fuel/steam interface. The dependence of the fission product rel ease kinetics on the state of fuel oxidation is treated. This analysis also considers the inhibiting influence of hydrogen, produced as a re sult of the Zircaloy-steam reaction, on the oxygen potential of the st eam environment.