Dimensional analysis and scenario discussion for the spreading of a corium melt (EPR)

Future generations of pressurized water reactors (e.g. the European Pressurized Water Reactor EPR) will have to realize safety concepts, which go significantly beyond the present standard. With respect to a severe core melt down accident, precautions need to be taken to ensure coolability of the corium melt and the integrity of the containment under all circumstances. In detail a reference concept relies on (a) accumulation of the melt below the pressure vessel, (b) dry spreading and stabilization of the melt on a horizontal spreading area of sufficient extent, (c) passively initiated flooding of the spreaded melt from above using water, (d) removing the decay heat by means of natural circulation within the pool and to the outside by immersed coolers connected to external heat exchangers. The problem of melt spreading is the focus of several investigations at the Forschungszentrum Karlsruhe (FzK). In the framework of the large-scale KATS experiments the spreading of both metallic and oxidic components of a Thermite melt in ceramic-coated channels is investigated. In parallel, physical models are being developed to describe the flow, heat transport and solidification processes during the spreading. Hereby additional small-scale experiments using low-melting waxes and alloys are performed to identify the basic physical phenomena. The present paper focuses on the development of models for the spreading flow. A dimensional analysis in conjunction with a scenario discussion is given. (orig./GL)