A kinetic model for iodine, oxygen and hydrogen is proposed in order to simulate transport of iodine along the reactor coolant system (RCS) of a Pressurized Water Reactor (PWR) in case of a severe accident. This kinetic mechanism is composed of 33 reactions. Due to lack of data in the literature for many gaseous iodine reactions, ab initio methods have been employed to determine rate constants as it was already the case in our previous works (Canneaux et al. J. Phys. Chem. A 114 (2010) 9270; Hammaecher et al. J. Phys. Chem. A 115 (2011) 6664). These computational studies have been completed here with calculations for the I/H substitution in HOI and iodine abstraction from hypoiodous acid by H and I (2P3/2) atoms. The rate constants have been estimated using the same methodology over the temperature range (300–2500 K). After being implemented in the ASTEC software (Accident Source Term Evaluation Code), simple computations were performed to assess time needed to reach equilibrium at three different temperatures: 1000, 1200, and 1400 K in a batch reactor at a constant pressure of 2 × 105 Pa. The influence of gas composition has been also examined using four different gas compositions: pure steam, pure hydrogen, and two hydrogen/steam intermediate mixtures containing in weighted percentages either 20/80 or 80/20%w. The PHEBUS-FPT1 experimental test was re-interpreted in activating with the kinetics of the I–O–H reaction system instead of thermodynamic equilibrium assumption. As expected, the kinetics promoted the persistence of gaseous iodine fraction at low temperature (428 K) as observed in the PHEBUS tests and thermodynamic equilibrium assumption appeared to be too simplistic to get accurate iodine speciation at the break and thus reliable source term1 estimations.