Knowledge From Recent Investigation on Flow Regime Characteristics in Debris Bed Formation Behavior Related to SFR Severe Accident Analyses

Studies on debris bed formation behavior are of crucial importance for the improved evaluation of core relocation and debris bed coolability that might be encountered in a Core Disruptive Accident (CDA) of Sodium-cooled Fast Reactors (SFR). Motivated to clarify the characteristics of flow regimes underlying this behavior, both experimental investigations and empirical-model development are being performed at the Sun Yat-sen University. As for the experimental study, several series of simulated experiments are being conducted by discharging various solid particles into water pools. To obtain a comprehensive understanding, a variety of experimental parameters, including particle size (0.256∼8mm), particle density (glass, alumina, zirconia, steel and lead), particle shape (spherical and irregularly-shaped), water depth (0∼60cm), particle release pipe diameter (10∼30mm) as well as the particle release height (110∼130cm) were varied. It is found that due to the different interaction mechanisms between solid particles and water pool, four kinds of flow regimes, termed respectively as the particle-suspension regime, the pool-convection dominant regime, the transitional regime and the particle-inertial dominant regime, were identifiable. As for the empirical-model development, by using dimensional analysis technique, a regime map (base map), which is restricted to predictions of spherical particles up until now, was recently suggested. It is shown that a respectable agreement between experiments and the regime-map predictions could be obtained. This work, which gives a large palette of favorable data and insight for a better understanding and an improved estimation of CDAs in SFRs, is expected to benefit future analyses and verifications of SFR severe accident analysis codes in China.