Ruthenium behavior in the reactor cooling system in case of a PWR severe accident: Study of oxidative conditions with stainless steel tube

In the research field on severe accidents in nuclear power plant, a specific scenario correspond to accident with oxidative conditions for which damaged fuel can be highly oxidised with significant releases of ruthenium oxides. Ruthenium chemistry is complex, and the current knowledge has to be deepened to better assess ruthenium source term with potential radioactive releases to the environment as volatile ruthenium tetroxide. In this work, experimental studies are focused on ruthenium behaviour along a stainless-steel thermal gradient tube, with maximum temperature of 1200 °C, simulating the reactor cooling system in oxidizing conditions with mainly steam/air gas mixtures. Results showed that few % of ruthenium oxides (< 10% with SS tube) can reach low temperature, representative of containment temperature, even with low oxygen content in the carrier gas. The ruthenium revaporization process from the Ru deposits along the tube on mid-term was studied. Influence of carrier gas composition (steam %), flow rate and NO _x feed are discussed.