The self-leveling of debris beds is a critical phenomenon to be clarified in the safety assessment of material relocation and decay heat-removal phases in postulated core-disruptive accidents in sodium-cooled fast reactors. In recent years, we have conducted several series of experiments using simulant materials to develop an experimental database of self-leveling processes in particle beds and proposed an empirical model to predict the bed height during self-leveling. The obtained experimental knowledge and proposed model applicability have been limited to homogeneous particles, although fragmented core debris could be mainly mixtures of fuel and stainless-steel particles with a size distribution. We conducted self-leveling experiments using mixtures of solid particles with different properties to understand the characteristics of self-leveling phenomena under a wider range of conditions. An improved empirical model for transient bed height was developed to correlate the experimental data under various conditions of particle mixtures. The developed model reproduces the self-leveling development of mixed particle beds and those of the homogeneous particle reasonably.
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