Experimental study on self-leveling behavior of binary-mixed particles in cylindrical bed using gas-injection method

Self-leveling of debris beds is one of the key phenomena to be clarified for the safety assessment of material relocation and decay heat removal phases in postulated core disruptive accidents of sodium-cooled fast reactors. This is because it is likely to affect the heat removal capability of debris beds. In recent years, we have conducted several series of experiments extensively using solid particles to simulate the self-leveling phenomenon. One of the experimental series was performed in a cylindrical tank using a gas injection method, which simulates sodium boiling caused by decay heat in the debris bed. We also developed an empirical model to predict variations of the bed mound height for homogeneous particles with the same size and density. In actual accident conditions, however, the core debris can be a mixture of fuel and structure materials with non-uniform sizes, and hence further investigations are necessary to clarify the characteristics of self-leveling behavior for inhomogeneous particles or mixed particles. In the present study, we represented inhomogeneous mixtures by binary-mixed particles, which contain two different particles with the same size and different density or with the same density and different size. Variations in bed mound height during the self-leveling process over time were measured for various particle mixtures and gas injection velocities. The empirical model for homogeneous debris beds was modified to reasonably predict the behavior of binary-mixed particle beds. These results are valuable to step further into the study of the self-leveling behaviors of mixed particles, which have particle-size distribution.