In a severe nuclear accident in a pressurized water reactor, a part of the fissio products (FPs) is released from the core and transported vis the primary circuit to the confinement vessel, and in some cases to the environment, in variable proportions according to the scénario. During such transport, an important fraction of the FPs is deposited on the different compartments visited and constitutes a potential reservoir for a differed remobilization. The evaluation of the source term needs a fine modelling of the physico-chemical phenomena related with the FPs in the transpost (gas-phase reactivity, interface, nucleation…) The goal of the present PhD project is to improve our capacity to account for the surface effects, in particular non congruent condensation, in the calculation tool developed at IRSN used for severe accident, ASTEC/SOPHAEROS. The condensed phases MoO3 and MoO2 and their reaction with the species present in the vessel, will be characterized. The PhD candidate will : (i) investigate, with ab initio calculations, the reactions adsorbate-substrate, and obtain their Gibbs energies of formation of adsorbed state (ii) implement these values in the code chemical transport ASTEC/SOPHAEROS to model a series of experiments where Ru and Mo are involved Expected results and skills developped : This PhD project will allow the candidate to master theoretical tools to model physico-chemical processes for interest in nuclear processes, in close contact with experimental groups.