Investigation of fragmentation phenomena and debris bed formation during core meltdown accident in SFR using simulated experiments

The event of a severe core melt down accident, resulting in the relocation of the active core is analyzed as a part of the nuclear reactor safety research in order to ensure safe removal of decay heat. Molten Fuel Coolant Interaction (MFCI) and debris bed configuration on the core catcher plate assumes importance in assessing the post accident heat removal capability. The key factors affecting the coolability of the debris bed are the bed porosity, morphology of the fragmented particles, degree of spreading/heaping of the debris on the core catcher and the fraction of lump formed. A well defined debris bed is helpful in fixing a prototypical source term for the PAHR studies. Towards this, a series of experiments on fragmentation kinetics and subsequent debris bed formation is conducted with molten Wood's metal (an alloy of Bi 50%, Pb 25%, Sn 12.5% and Cd 12.5% with melting point of 346 K) in water simulant system. The experiments are carried out using 2 kg, 5 kg and 20 kg melt inventories. The particle size distribution obtained for the fragmented debris is fit using an Upper Limit Log Normal (ULLN) distribution. The dependence of particle size distribution on initial melt temperature and interaction height is quantified by correlating them to the key parameters i.e. shape factor and location factor in the ULLN expression. Morphology of the debris particles is investigated to understand the fragmentation mechanisms involved. Three major mechanisms of fragmentation are identified namely melt entrainment mechanism, boundary layer stripping and hydrodynamic breakup due to capillary forces. Finally, an approach to quantify the stratification and spreading behaviour of debris on the collector tray is presented by analysing the particle size and mass percentage in different radial zones of the collector tray.