Influence of multidimensionality and interfacial friction on the coolability of fragmented corium

After the shutdown of a nuclear power plant continued decay heat is released from the reactor core. To avoid environmental pollution, even during a very unlikely severe accident with failure of all cooling devices, the nuclear material has to be retained in the reactor containment. Due to the decay heat and a missing heat sink the core may dry out, and subsequently heat up and melt. This core melt, called "corium", is a mixture of nuclear fuel, cladding and structure material. It will flow downwards, and, after some temporary configurations, pour down into the lower plenum of the reactor pressure vessel. Here the corium jet gets in contact with residual coolant water, and break up into fine fragments, that settle down as particulate debris bed. If the reactor pit is flooded, a similar configuration may also arise after vessel failure in the containment. To achieve a stable cooled state enclosing the contaminated material, the decay heat of the corium has to be removed. Due to the magnitude of the internal power, and the non-availability of active cooling components, this heat can only be removed by evaporation of cooling liquid. The produced vapour escapes from the bed through the upper surface. To establish a steady cooled state, the evaporated water has to be replaced by a coolant inflow driven by gravity. Thus, a two phase flow of liquid water and steam establishes inside the particulate debris and determines the coolability. The central aim of this work is to present a model for the calculation of the amount of heat that can be removed by this mechanism. As will be shown, this depends mainly on the friction laws and the geometric configuration of the particle bed. Especially the friction laws, with main emphasis on the interfacial drag between the steam and the water, are regarded in detail. For reactor typical configurations it will be shown, that the coolability is significantly increased in realistic multidimensional geometries, compared to commonly used 1D considerations. This increased coolability potential is due to preferred flow paths of the water.