The Phebus Fission Product Programme

Nuclear power plants are designed with engineering systems and associated operational procedures which provide an in-depth defence against accidents. These features in principle, avoid severe accidents. Nevertheless, TMP-2 and Chernobyl have dramatically demonstrated that such accidents may happen and need to be investigated in order to reduce their consequences to an acceptable level. As a consequence, most of the national authorities responsible for the safety of nuclear installations have undertaken a revision of their positions with respect to 'beyond design basic accidents' It is now common practice to assess the risks to the population associated with severe accident conditions in commercial NPPs. As the estimations of LWR severe accident hazards are deduced from computer code simulations, it was realised that the knowledge of the main phenomena taking place during the evolution of severe accidents had to be improved and transferred into computer codes able to cover a wide range of conditions. Despite such an extensive effort, there are still areas of uncertainties which need to be further investigated. This is due to the complexity of the phenomena under investigation. Many phenomena of F P transport are a result of mechanisms in which the thermohydraulics, radioactive physics and chemistry aspects are intimately mixed