Experimental investigations for the demonstration of the long-term coolability of core melts

To guarantee the enclosure of radioactivity also in case of core melt accidents in nuclear power plants, the integrity of the containment has to be maintained even in these accident situations. Therefore, the core melt released from the reactor pressure vessel has to be transferred into a stable configuration and has to be cooled down. At the Institut fuer Angewandte Thermo- und Fluiddynamik of Forschungszentrum Karlsruhe a core catcher is developed based on the fragmentation of the melt. The fragmentation is formed by penetration of water into the melt from below which results in a break up of the melt and a porous solidification. The described investigations contribute to the validation of the concept and to the description of the occurring processes. Three kinds of experiments were conducted, their results can be summarized as follows: Small scale experiments are conducted to clarify which hydrodynamic parameters are dominating the bubble and channel formation in the melt. The melt is represented by a liquid of high viscosity, air is injected via a single nozzle, representing the gas phase. The experiments show that the formation of bubble swarms at high volume rates does not lead to a uniform fragmentation of the melt. Consequently, the coupled process of water inlet from below and evaporation is fundamental for the fragmentation of melts. This is investigated by experiments with hot plastic and metal melts with a single water inlet at the bottom. The temperatures in these experiments are some 200 C and herewith far above the evaporation temperature of water. An extensive mixing of melt and coolant with a subsequent porous solidification is observed. (orig.)