Evaluation of debris bed self-leveling behavior: A simple empirical approach and its validations

During a hypothetical core-disruptive accident (CDA) in a sodium-cooled fast reactor (SFR), degraded core materials can form roughly conically-shaped debris beds over the core-support structure and/or in the lower inlet plenum of the reactor vessel from rapid quenching and fragmentation of core material pool. However, coolant boiling may lead ultimately to leveling of the debris bed that is crucial to the relocation of molten core and heat-removal capability of debris bed. To clarify the mechanisms underlying this self-leveling behavior, several series of experiments were elaborately designed and conducted in recent years under the collaboration between Japan Atomic Energy Agency (JAEA) and Kyushu University (Japan). Based on the experimental observations and quantitative data obtained (mainly the time variation of bed inclination angle), a simple empirical approach to predict the self-leveling development depending on particle size, particle density and gas velocity was proposed. To confirm the rationality and wide applicability of this approach, over the past few years extensive efforts have been made by performing modeling investigations against a large number of experimental data covering various conditions, including difference in bubbling mode, bed geometry and range of experimental parameters. The present contribution synthesizes these efforts and gives detailed comparative analyses of the performed validations, thus, providing some insight for a better understanding of CDAs and improved verifications of computer models developed in advanced fast reactor safety analysis codes.