Ruthenium tetroxide is an extremely volatile and highly radiotoxic species. During a severe accident with air ingression in the reactor vessel, ruthenium oxides may reach the reactor containment building in significant quantities. Therefore, a better understanding of the RuO4(g) behaviour in the containment atmosphere is of primary importance for the assessment of radiological consequences, in the case of potential releases of this species into the environment. A RuO4(g) decomposition kinetic law was determined. This decomposition highly depends on humidity; steam seems to play a catalytic role, as well as the presence of ruthenium dioxide deposits. The temperature is also a key parameter. The half-life time of RuO4(g) is about 5 h at 90°C in presence of steam. The nature of the substrate, stainless steel or epoxy paint, did not exhibit any chemical affinities with RuO4(g). This absence of reactivity was confirmed by XPS analyses, which indicate the presence of the oxyhydroxide RuO(OH)2 in the Ru deposits surface layer whatever the substrates considered. It has been concluded that RuO4(g) decomposition corresponds to a bulk gas phase decomposition, and not to an adsorption process. The ruthenium revolatilisation phenomenon under irradiation from Ru deposits was also highlighted. This process comes from the oxidation of the Ru deposits by air radiolysis products, producing RuO4(g). An oxidation kinetic law was determined. The temperature and the steam concentration are still two key parameters. Their increases promote significantly the oxidation reaction. In addition, it has been shown that the sump is an important source of volatile ruthenium due to gamma radiolysis effect. The establishment of Ru behavioural laws allowed making a preliminary modelling of the Ru source term. The results of the reactor calculations indicate that the values obtained for 106Ru source term are closed to the reference value considered currently by the IRSN, for 900 MWe PWR safety analysis (3.10-4). This first modelling proves that it is necessary to proceed with the study on the "ruthenium containment" issue, and mainly on the ruthenium behaviour in aqueous phase, under gamma radiolysis.