A review of existing data on these phenomena shows relatively good understanding. In terms of oxygen affinity, the fuel is oxidized before ruthenium, from UO2 to UO2+x. Its oxidation is a rate-controlling surface exchange reaction with the atmosphere, so that the stoichiometric deviation and oxygen partial pressure increase. High temperatures combined with the presence of oxygen in the atmosphere lead to fuel expansion and formation of cracks. In these conditions, intra- and inter-granular diffusions of ruthenium in the fuel matrix are so enhanced that it is possible to consider an instantaneous volatilisation of ruthenium oxides at the fuel surface.
Based on these considerations, a completely new model has been implemented in the EDF local version of the MAAP4.07 severe accident code (Modular Accident Analysis Program), owned by EPRI (Electric Power Research Institute). The fuel oxidation modelling takes into account many kinds of atmospheres (steam and/or air and/or hydrogen), the stoichiometric evolution and the oxygen partial pressure of the fuel matrix. The release of ruthenium oxides is calculated considering their particular reaction constants. The model was assessed by the simulation of different CEA-VERCORS experiments in air, steam and mixed atmospheres. These experiments are specifically designed to study FP release from fuel under different atmospheres and temperatures. This paper deals with the main results obtained with MAAP4.07 when simulating these tests.