Fortschritte bei der Beherrschung und Begrenzung der Folgen auslegungsüberschreitender Ereignisse: Bericht vom KTG Fachtag der Fachgruppen Reaktorsicherheit und Thermo- und Fluiddynamik : Karlsruhe, 25./26. September 2003

The workshop was opened with welcome greetings by Dr. Fritz, Forschungszentrum Karlsruhe, and Prof. Hartkopf, EnBW. Subsequently Dr. Fabian explained goals and content of the workshop. The status was presented in 20 papers in a compact and complete manner. Having recognised that even in the case of design extension events measures can be taken in German nuclear plants for control and limitation of consequences, the relevant potential was assessed and studied in the past 20 years. The plant behaviour was analysed and evaluated and relevant severe accident phenomena were studied extensively. After having understood the sequences and after having developed and validated numerical tools, technical and administrative precautions were implemented in the nuclear plants in form of preventive and mitigative emergency measures. They add a fourth safety level to the well established and accepted multi-barrier safety concept and result in a further and condsiderable reduction of the remaining risk, where - depending on the plant type - the core damage frequency is lowered by about half an order of magnitude. Viewed globally, the measures that are implemented in the German nuclear power plants correspond to the international state of the art. Existing measures can be optimised after plant specific severe accident analyses, especially when using PSA level 2. Evaluation of information and presentation techniques, using simulation programmes and specific procedures, are under development for support of the plant operation in safety management. For new plants, extended licensing requests for limited releases in severe accidents must be followed and proved analytically. Early large releases must be excluded. Retention of core melt within the containment, at least temporarily, is requested to further reduce releases. Depending on plant design, different but always elaborate concepts are followed. The knowledge gained by extended R&D work in the severe accident area demonstrates significant progress in phenomenological understanding and analytical description. Most of the relevant phenomena are basically understood. In many cases, plant specific details dominate the accident sequence. Undoubtedly, a number of details must still be investigated which is being performed in international cooperation. Concerning this topic it was proposed to discuss the identified open problems with utilities, vendors, research and expert organisations, in order to reach agreement on relevance and priorities. For a risk oriented evaluation of open topics it is recommended to elaborate plant specific aspects on the basis of PSA level 2 studies. It was remarked from utilities point of view that, because of limited plant life time due to political boundary conditions, support for severe accident research is understandably rather limited.