Study of the ruthenium fission-product behaviour in the containment of a nuclear reactor, in case of a severe accident occurring in oxidizing conditions

During an hypothetical severe accident on a Pressurized Water Reactor (PWR), Fission Products (FP) are released from fuel and reach the reactor containment building. The fuel oxidation enhanced by air ingress can impact some FP releases. Among the FP, ruthenium is of particular interest due to its ability to form oxide compounds, some of which are very volatile. In addition, Ru is a very hazardous compound because it is chemically toxic but also in reason of its radiotoxicity (T1/2106Ru = 369 d, and T1/2103Ru = 39.3 d). Several experiments carried out by Canadian (AECL) or Hungarian (AEKI) teams, have shown an increase of ruthenium release rate from fuel in oxidizing conditions. It is thus important to study the ruthenium behaviour, in the reactor containment, under severe accident conditions (temperature between 400C and 1400C, presence of a high steam fraction in the atmosphere, presence of an aqueous phase [sump], high dose rate [about 10kGy/h], air and sump water radiolysis ...), to estimate the potential release of volatile Ru species into the environment. Various ruthenium species can reach the reactor containment, but the experimental study performed by the French Institut de Radioprotection et de Surete Nucleaire, is focused on the main gaseous form, which is the ruthenium tetroxide (RuO4). It is highly reactive, and it is the most probable gaseous chemical form under the thermal-hydraulic conditions mentioned above. The ruthenium dioxide (RuO2) may also be considered in case of a possible revolatilisation under super-oxidizing atmosphere. First of all, experiments dedicated to generate in situ the unstable form RuO4 as solid crystals as pure as possible, with a reproducible and quantitative method, have been developed. Then, several adsorption experiments of ruthenium tetroxide, onto surfaces representative of those present in PWR containments (stainless steel and epoxy paints), have been carried out. XPS, XRD, and SEM/EDS analysis were performed to study the interaction between RuO4 and these surfaces, in order to identify the nature of the Ru deposit layer. Moreover, ruthenium deposit revolatilisation tests have been started and the first results seem to confirm this revolatilisation phenomenon. (author)