Study of the iodine kinetics in thermal conditions of a RCS in nuclear severe accident

•High temperature iodine reactivity in gas phase.•Kinetics limitations were evidenced in severe accident gas atmosphere conditions.•Iodine kinetic scheme involving steam/di-hydrogen/di-oxygen. During the PHEBUS-FP integral severe accidents simulation tests, gaseous iodine was detected in earlier stages of the simulated accident, coming from the experimental circuit modelling a reactor coolant system. One possible explanation is the existence of some kinetic limitations which promote the persistence of gaseous iodine at low temperature. This paper sums up some analytical and modelling works performed to check this assumption. Results show that the chemical speciation of iodine cannot be calculated by assuming chemical equilibrium, kinetics have to be considered, in particular for oxidising atmosphere with an excess of steam. A kinetic model for gaseous iodine is proposed and qualified by comparison with experimental works. Such modelling should be considered to calculate the transport of iodine through the reactor coolant system for a severe accident because it can significantly impact iodine source term evaluations.