Reaction Kinetics of a Fission-Product Mixture in a Steam-Hydrogen Carrier Gas in the Phebus Primary Circuit

Radioiodine entering the containment from the postaccident primary circuit in vapor or gaseous form, as observed in the Phebus FPT0 and FPT1 tests, has a direct impact on the source term evaluation. State-of-the-art fission-product transport codes based on the assumption of thermochemical equilibrium failed to predict this phenomenon. In this work the standard approach of assuming the instantaneous establishment of thermochemical equilibrium is questioned and it will be argued that kinetic limitations may have existed under the severe-accident boundary conditions of the FPT0 and FPT1 tests. To this end a simple monodimensional transport model was developed in an attempt at introducing kinetic aspects within the primary circuit. A number of homogeneous gas-phase reactions between selected fission products and structural materials, complemented by condensation reactions, underlies the kinetic model. In the absence of experimental data, the kinetic constants were estimated using the transition-state theory or semi-empirical methods. The kinetic model was then applied to the analysis of Phebus FPT0 and FPT1 yielding a satisfactory agreement between experimental data and model predictions.