As part of a major nuclear accident, the products resulting from the degradation of the control rods are likely to influence the transport of iodine in the Reactor Coolant System (RCS) of a pressurized water reactor as evidenced by the results of the Phebus-FP program. Three experimental Phebus-FP tests (FPT0, FPT1, and FPT2) were performed with Silver-Indium-Cadmium (AIC) control rods wheras in FPT3 test, the control rod is boron carbide (B4C). For FPT3, a much larger fraction of gaseous iodine was observed at the RCS break. It is suspercted that the CsI (caesium iodide) formation has been restricted due to Cs_xB_yO_z (especially caesium metaborate CsBO2) formation. The PhD results allow us to consolidate thermochemical data on cesium borates, which are poorly known in the literature, and to get first kinetic data for reactions leading to the formation of CsBO₂ (CsI + H₃BO₃ ⇄ CsBO₂ + HI + H₂O et CsOH + H₃BO₃ ⇄ CsBO₂ + 2 H₂O). In order to estimate these thermokinetic parameters, theoretical chemistry tools were used, with the help of statistical thermodynamics and appropriate kinetic theories; a special care was dedicated to the validation of the applied methodologies. All data have been implemented in the severe accident simulation software ASTEC (Accident Source Term Evaluation Code) and allow us reconciling the simulation results with experimental data concerning gaseous iodine at the break for FTP3 test.