Investigations on flow-regime characteristics during debris bed formation behavior in sodium-cooled fast reactor by releasing high-temperature particles

It is crucial to understand the Debris Bed Formation (DBF) behavior for the improved assessment of Core Disruptive Accident (CDA) of Sodium-cooled Fast Reactors (SFR). To elucidate the DBF flow-regime characteristics, a series of visualization simulated experiments were conducted at Sun Yat-Sen University by releasing solid particles into water pools. Because of different mechanisms of the inter-particle and fluid-particle interactions, four different flow regimes were identified along with four different final bed shapes found. Motivated to comprehensively understand the effect of coolant boiling on flow-regime transitions during the falling of hightemperature debris, new experiments are performed in present work by releasing high-temperature particles under varying parametric conditions. It is found that the boiling generated from falling high-temperature particles can effectively centralize the particle accumulation and diminish the particle-flow-induced pool convection, leading to the potential regime transitions. Based on the detailed experimental analyses and data attained, an extension function for evaluating the boiling effect caused by falling high-temperature particles is successfully developed to incorporate into our base model, whose applicability is examined to be restricted to the predictions under non-boiling condition. Knowledge from present work is useful for future development and verifications of SFR safety analysis models and codes in China.