Analysis of the results of the Phébus experimental program identified phenomena related to fission product behaviour that are insufficiently described by available empirical models. The combining experimental and theoretical strategy implemented by IRSN to develop models that are able to predict any pressurised water reactor (PWR) severe accident transient is illustrated here for one of these phenomena, the presence of gaseous iodine at the break of the reactor coolant system (RCS). Kinetics modelling of the {I, O, H} chemical system in the RCS under severe accident conditions computed using quantum chemistry and validated against separate-effect experiments are used to re-interpret with a notable improvement the FPT1 experimental data contrary to the FPT3 one. After being implemented in the ASTEC (Accident Source Term Evaluation Code) software, these new models will contribute to a reduction in uncertainties in the prediction of the iodine source term.