###### Abstract
In the past few years, by assuming that the bed was initially conical (or convex), extensive investigations on the relocation phenomena of debris beds (termed as self-leveling) triggered by sodium boiling were performed [47; 48; 49; 50; 14; 15; 43; 44; 47]. Based on these investigations, the self-leveling mechanisms (e.g., bubble-particle interactions within particle beds) and characteristics (e.g., parametric influence on the leveling rate) were clarified. However, it should be noted that the complex multiphase flows, including solid debris, liquid coolant, and sodium vapor, involved in the DBF process can result in varying initial shapes of the debris bed, which in turn can influence the progression of severe accidents in SFRs. Hence, it Is of great importance to understand the characteristics and mechanisms of DBF behavior in severe accidents of SFRs. Figure 1: Formation of the debris beds [24]. Here, it should also be noted that in the context of the severe accidents of LWRs, the debris bed formation behavior was widely investigated with the aim of evaluating the debris bed coolability and subsequent Molten Core-Concrete Interaction (MCCI) [41; 51; 52; 53; 54; 55; 56], and many valuable findings were attained from these studies. However, due to the different thermal and hydraulic interactions made between the coolant and discharged molten core materials, it was found that the breakup and fragmentation of the discharged molten jet would be facilitated in SFRs because of the higher heat transfer between the two phases resulting from the direct liquid-liquid contact [27; 57; 58; 59; 60]. Therefore, in contrast to the LWR condition, wherein deep penetration of the discharged molten core jet likely occurred due to the vapor film formed between coolant and molten materials, the core materials are more easily fragmented into solid debris before reaching the reactor vessel or core catcher structures [27; 57; 58; 59; 60]. In addition, the debris size range in SFRs (e.g., 0.1 mm to several millimeters) is generally wider than that of LWRs (e.g., over several millimeters) [26; 39; 60; 61], leading to varying debris bed formation characteristics. Therefore, to elucidate the characteristics and mechanisms of DBF behavior in the case of an SFR, in the past decade, extensive experimental and modeling studies were performed separately at Kyushu University, the Japan Atomic Energy Agency (JAEA) [23; 28; 29; 62; 63; 64], Indira Gandhi Centre for Atomic Research (IGCAR) [65], and Sun Yat-Sen University (SYSU) [24; 66; 67; 68; 69; 70; 71; 72]. Figure 2 summarizes these investigations, which can be divided according to their different focal points: (1) investigations focusing on the overall accumulated-bed characteristics; and (2) investigations focusing on the flow regime characteristics. The research methodology used water and solid particles with different properties to simulate the liquid sodium and debris in CDAs. Solid particles were discharged into the water pool from the top by gravitation through a nozzle and accumulated at the bottom to form the particle bed. In the investigations focusing on the overall accumulated-bed characteristics [24; 29; 30; 63; 64; 65; 66], single-sized particles and mixed-sized solid particles in a range of 1.1\(\sim\)6.0 mm were employed, and some key parameters (e.g., particle type, nozzle height, and nozzle diameter) were quantitatively studied based on inter-particle interactions. It was found that concave or conical accumulated beds may be formed during DBF, dependent on the experimental conditions. Based on the experimental data, empirical models were developed to predict the key parameters that represented the accumulated characteristics (e.g., repose angle, mound, and bed height).