Abstract:  The nuclear reactor accident in Fukushima, caused by a strong earthquake followed by a tsunami, is one of the most severe accidents in the history of nuclear power and its consequences are not yet completely known. This presentation attempts a preliminary technical interpretation of the available information about the development of the accident within the Japanese nuclear power plants. The relevant ‘Generation II’ Boiling Water Reactor and its safety concept in case of beyond design failure of important emergency systems is explained. The emphasis of the presentation is laid on the thermo-hydraulic aspects of a severe accident, in particular the decay-heat removal and consequences of a common failure of all cooling systems. Various known scenarios of the then following degradation of the overheating nuclear core are described and compared with the observations in Fukushima, e.g. steam venting, direct contact condensation and pool boiling, oxidation of the core cladding material, hydrogen generation and mixing, hydrogen explosions, core fragmentation, and possible core melt. An overview of ongoing experimental and theoretical research at the University of Stuttgart on the understanding and modelling of some of these phenomena is given. Recently developed ‘Generation III+’ reactors with advanced safety systems, as well as ‘inherently safe’ reactors, for which a core melt is not possible, will also be discussed.   Host: Prof. Eckart Meiburg