Recent findings on ruthenium chemistry in a severe accident

The chemistry of radiotoxic ruthenium in a severe nuclear power accident has actively been investigatedespecially during the last decades. The Ru studies have covered the release from a fuel, the transport in the primary circuitand the behaviour in the containment building. The gathered experimental data have been utilized to understand the keyparameters governing the Ru chemistry in a severe accident (SA) and to check the ability of the existing models of SAanalysis codes to explain the experimental results. To further increase the knowledge on Ru behaviour, the collaboration oninternational level has been intensive. Lately, the widest and most active networks have been EU SARNET and EU SARNET2.The valuable effort of these networks on sharing information of e.g. national programs and on interpreting the experimentalresults is continued in EU NUGENIA program. More detailed studies on separate phenomena have been conducted e.g. aspart of OECD/NEA STEM/START and ISTP/VERDON programs. Furthermore, Phébus FP tests have produced valuable dataon integral phenomena.The large-scale integral and semi-integral experiments have confirmed that Ru release depends strongly on carrier gas. Ru issignificantly released from an irradiated fuel sample under oxidizing conditions, in particular when air is involved. Inaddition, the oxidation of UO2 fuel seems to lead to a higher Ru release than in case of MOX fuel. Ruthenium can betransported to the containment atmosphere both in gaseous and particulate forms. The small-scale separate-effectexperiments gave a detailed view on Ru transport. A high fraction of ruthenium was detected as particles at the outlet of themodel primary circuit in an air atmosphere. However, the observed gaseous Ru fraction is higher than what could beexpected based on thermodynamic equilibrium calculations. Further studies on the effect of flow residence time in atemperature gradient for the equilibrium of Ru oxides have been conducted.The effect of other fission products in the gas phase, as well as FP deposits on the surface of primary circuit, on the Rutransport has been investigated. For example, caesium containing deposits seemed to trap gaseous ruthenium effectively.Similarly in case of control rod residues, silver particles in the gas phase of the circuit acted as a sink for gaseous Ru. In anair ingress accident, the effect of air radiolysis products on the Ru chemistry becomes important. As the main air radiolysisproducts can be considered as oxidizing agents, their ability to oxidize the lower oxides of Ru to higher oxidation state has been examined.Most of Ru in the containment building ends up as deposits on the containment surfaces and in the sump. Experiments on theradiolytical revaporisation of ruthenium deposits on the epoxy paint surface indicated the release of gaseous ruthenium andit was enhanced under humid atmosphere and elevated temperature. It appeared that the products of air radiolysis caused by γ-radiation promoted the formation of gaseous ruthenium from Ru oxide deposits on paint in a higher amount than could beexpected by pure ozone action. Concerning the irradiation tests of perruthenate aqueous solutions, they indicated the formation of gaseous Ru by γ-radiolysis products in solution