During a severe accident occurring to a pressurized water reactor (PWR), fission products (FPs) are released from the nuclear fuel and may reach the reactor containment building. Among the FPs, ruthenium (Ru) is of particular interest due to its ability to form volatile oxide compounds in highly oxidizing conditions combined with its high radiotoxicity (103Ru and 106Ru isotopes) at middle term after the accident. Uncertainties concerning evaluation of Ru releases are still significant and some R&D efforts are led to get a better understanding of Ru chemistry in such conditions. The thermodynamic database on Ru species used to estimate these releases shows some discrepancies for most Ru oxides and for other species, as oxyhydroxides, data are very scarce and not reliable. Theoritical chemistry tools were used to calculate the thermodynamic properties. Several quantum chemical methods have been investigated (DFT, CASSCF, MRCI, CASPT2, QDNEVPT2, CCSD(T)) in order to predict the geometrical parameters, the energetics including electronic correlation, and spin-orbit coupling. The most suitable method for Ru compounds corresponds to TPSSh-5%HF for geometry optimization, followed by CCSD(T) with complete basis set (CBS) extrapolations for the calculation of the total electronic energies. The influence of spin-orbit coupling has been estimated for the open-shell systems by performing a spin-orbit coupled state-interaction calculation. The energetics are combined with statistical physics to obtain the thermodynamics values. This methodology is applied to explore Ru oxyhydroxide species because they may play an important role on the transport of Ru in the primary circuit due to high steam content. The revised thermodynamic database is then used to compute equilibrium calculations for Ru in order to predict which species are most stable in representative severe accident conditions. Kinetic calculations are also performed to obtain pathways of formation for Ru trioxide and tetraoxyde gaseous compounds, which are the most stable Ru volatile species in steam/air atmospheres, including NxOy, HxOy and Ox reactants. All these values are implemented in nuclear power plant simulation code ASTEC and apply to OCDE/STEM START experiment.