Insight From Recent Experimental and Empirical-Model Studies on Flow-Regime Characteristics in Debris Bed Formation Behavior

Studies on debris bed formation behavior are important for 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 flow-regime characteristics underlying this behavior, both experimental investigations and empirical-model development are being performed at the Sun Yat-sen University in China. 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 (0.000125– 0.008 m), particle density (glass, aluminum, alumina, zirconia, steel, copper, and lead), particle shape (spherical and nonspherical), and water depth (0–0.8 m) along with the particle release pipe diameter (0.01–0.04 m) 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-inertia dominant regime, were identifiable. As for the empirical-model development, aside from a base model which is restricted to predictions of spherical particles, in this paper considerations on how to cover more realistic conditions (esp. debris of nonspherical shapes) are also discussed. It is shown that by coupling the base model with an extension scheme, respectable agreement between experiments and model predictions for regime transition can be achieved for both spherical and nonspherical particles given our current range of conditions.