Low volatile fission-product release and fuel volatilization during severe reactor accident conditions

An analytical model has been developed to describe the release behavior of low-volatile fission products from uranium dioxide fuel under severe reactor accident conditions. The effect of the oxygen potential on the chemical form and volatility of fission products is determined by Gibbs-energy minimization. The release kinetics are calculated according to the rate-controlling step of diffusional transport in the fuel matrix or fission product vaporization from the fuel surface. The effect of fuel volatilization (i.e., matrix stripping) on the release behavior is also considered. The model has been validated against several out-of-pile annealing experiments performed at high temperature in various oxidizing and reducing conditions. (orig.)