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In a hypothetical severe accident in a Pressurised Water Reactor (PWR), Fission Products (FPs) can be released from the overheated nuclear fuel and partially transported by gases, composed of a mixture of superheated steam and hydrogen, to the reactor containment. Subsequent air ingress into a damaged reactor core may lead to enhanced fuel oxidation, affecting some FP release, especially that of ruthenium. Ruthenium is of particular interest because of its high radiotoxicity and its ability to form very volatile oxides. In the reactor containment, such volatile forms are very hazardous as they are much less efficiently trapped than particulate forms by emergency filtered venting.
In the four and a half years of SARNET, collaborative research dedicated to the "ruthenium story" has been performed by several partners. This paper presents the main achievements over the whole project period.
Starting from experimental observations showing that fuel could be extensively oxidised by air to, and that a significant fraction of ruthenium inventory can be released, rather satisfactory models have been developed. In addition, the effect of the air interaction with Zircaloy cladding, as well as with UO(2) itself, has been studied.
Experiments on the complex transformations of ruthenium oxides upon cooling through the reactor circuit have been performed. An unexpectedly large effect of temperature on the decomposition rate of gaseous ruthenium compounds has been found, as well as effects of the nature of circuit internal surfaces and other FP deposits. So it has been highlighted that various forms of ruthenium can reach the containment, but the most probable gaseous species under these conditions is ruthenium tetroxide. Preliminary analysis of ruthenium transport supports these conclusions.
Experiments and analysis have also been launched on the radio-chemical reactions undergone by these ruthenium oxides in the reactor containment.

Additional information

Authors: AUVINEN A, Technical Research Centre of Finland (VTT), Espoo (FI);KÄRKELÄ T, Technical Research Centre of Finland (VTT), Espoo (FI);GIORDANO P, Institut de Radioprotection et de Sûreté Nucléaire (IRSN), St-Paul-Lez-Durance (FR);BRILLANT G, Institut de Radioprotection et de Sûreté Nucléaire (IRSN), St-Paul-Lez-Durance (FR);COLOMBANI J, Institut de Radioprotection et de Sûreté Nucléaire (IRSN), St-Paul-Lez-Durance (FR);MUN C, Institut de Radioprotection et de Sûreté Nucléaire (IRSN), St-Paul-Lez-Durance (FR);DAVIDOVICH N, Ente per le Nuove Tecnologie, S. Maria di Galeria (IT);DICKSON R, Atomic Energy of Canada Ltd. (AECL), Ontario (CA);PONTILLON Y, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);HASTE T, Paul Scherrer Institut (PSI), Villigen (CH);LAMY J S, Electricité de France (EdF), Villeurbanne (FR);OHAI D, Institute for Nuclear Research (INR), Pitesti (RO);STEINBRÜCK M, Forschungzentrum Karlsruhe (FZK) GmbH, Karlsruhe (DE);VÉR N, Hungarian Academy of Sciences KFKI Atomic Energy Research Institute (AEKI), Budapest (HU)
Bibliographic Reference: An article published in: Progress in Nuclear Energy, Volume 52, Issue 1, January 2010, Pages 109-119
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