Experimental determination and numerical prediction of dryout heat flux in heat generating debris beds

During severe accidents in nuclear reactors, the core can melt and get relocated in the reactor cavity/core catcher. If there is water present in the cavity, the melt forms a heat generating debris bed. Ensuring further long term coolability of such heat generating debris beds is very much important in the context of safety with respect to coolability as well as failure of cooling, yielding re¬melting and subsequent attack of structures. To achieve long term coolability of the configuration, all evaporated water has to be replaced by water inflow due to natural forces. At the same time the produced steam must escape the porous structure driven by buoyancy forces. If the heat generation is too high and the coolant is unable to take out the heat, the bed reaches dryout condition where the temperature of the bed increases sharply. The heat flux corresponding to this is called as dryout heat flux (DHF). It is important to determine the diyout heat flux of the bed in order to assess the coolability of the beds.The heat transfer behaviour in debris bed is very complex and is influenced by many factors like the mean size of the particles, porosity, operating conditions such as water entry from the top or the bottom of the debris bed, water temperature, Magnitude of non-condensable gases generated during MCCI, Spatial distribution of the bed porosity etc. In the present work, experimental results an determination of dryout heat flux in a realistic debris bed with irregularly shaped particles mixed with stainless steel balls have been reported. Influence of pressure and bottom injection on dryout heat flux has also been presented. It is observed that dryout heat flux increases with pressure. It also increases substantially with a small inflow. (author)