###### Abstract  
A Sodium-cooled Fast Reactor (SFR) is one of the optimized candidates in Generation IV nuclear reactor systems, but safety is an essential issue for SFR development and application. Most knowledge was accumulated through SFR safety investigations, especially for Core Disruptive Accidents (CDAs). During the CDA of SFRs, the molten materials in the core region are likely to discharge into subcooled sodium and form a debris bed on the lower region of the reactor vessel. Noticing that elaboration on the characteristics and mechanisms of Debris Bed Formation (DBF) behavior should be essential for the subsequent analysis of debris bed coolability and accident progression through various experimental and modeling studies, much knowledge was obtained during the past decades. Motivated to promote future investigations on CDAs of SFRs, the previous experiments and modeling studies on DBF behavior are systematically reviewed and discussed in this paper. The experimental results showed that the flow-regime and accumulated-bed characteristics during DBF were influenced by varying parameters and realistic conditions. Through the modeling studies, several empirical models were proposed for predicting the flow regime and accumulated-bed characteristics in DBF. In addition, to promote further development of research, the future prospects concerning DBF behavior are also described. nuclear safety; sodium-cooled fast reactor; severe accident; core disruptive accident; debris bed formation behavior; experiments; modeling studies; review +
Footnote †: journal: Medical  
removal (PAHR) phase [18; 19; 20; 21; 22]. During the PAMR phase, in a subcooled sodium pool, the molten fuel, structural components, and other materials may be discharged from the reactor core, rapidly quenched, and fragmented into solid debris due to interactions with the sodium coolant. After falling, the debris will accumulate in the lower plenum of the reactor vessel (or on the core-support structures) to form a debris bed. [23; 24]. Figure 1 illustrates the debris bed formation. It is generally believed that the full fragmentation and solidification of the molten materials can be achieved in the case of a CDA of an SFR [25] forming debris beds consisting of solid particles or fragments of different sizes and porosity in the lower plenum [19; 25; 26; 27]. Considering that the debris bed configurations are of great essence to neutronic re-criticality and self-sustainable long-term coolability, adequate comprehensive evaluations for debris relocation in the CDA of an SFR are necessary to realize the In-Vessel Retention (IVR) for fuel debris [28; 29; 30]. On the other hand, focusing on the IVR of SFRs, relevant safety equipment (e.g., core catchers) is also recommended for employment below the reactor vessel to enlarge the safety margin [26; 31]. It is expected that the stable removal of decay heat from the debris beds is achievable using core-catcher structures thanks to the natural convection of the sodium coolant. Noting that the characteristics of the debris beds can significantly impact their ability to remove decay heat, it is crucial to understand the characteristics and mechanisms of Debris Bed Formation (DBF), which are helpful in improving the design of SFR core catchers (such as their structures, retention capacities, and sizes) to further guarantees the safety IVR during the CDAs. During the past decades, studies focusing on heat transfer within debris beds were performed [32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. In these investigations, in general, the initial debris bed shape was supposed to be homothetically piled [37], cylindrical [38], heap-like [40; 41], and Gaussian-shaped [39] or conical [42; 43; 16; 44]. Nevertheless, it was found that the geometries of debris beds (such as their heights) also significantly affect their heat removal capacity [45; 46].