## 5 Discussion of Future Prospects  
Though much valuable knowledge and understanding of DBF behavior have been accumulated through these previous experiments and modeling studies, it is expected that actual severe SFR accident conditions would be more complex than the conditions that have been taken into account. Therefore, in order to gain a more elaborate insight into the flow regime mechanisms and the characteristics of DBF behavior and to obtain more knowledge for SFR safety analysis, future investigations concerning DBF behavior phenomenology are required. Possible future experimental and modeling studies may involve but are not restricted to the following points:  
1. Further analyses and validations of DBF characteristics in cases of multicomponent (i.e., more than three components) mixed-sized particles. As noted in Sections 2 and 3, previous investigations under mixed-sized spherical particle conditions were carried out by only using bicomponent or a few tricomponent mixed-sized particle mixtures. To attain more reliable insights into DBF behavior under more realistic particulate situations, more experimental studies, as well as corresponding modeling verifications, with three (or even more) component mixed-size solid particles can be conducted. 2. Further experimental investigations on the characteristics of particle separation and stratification under mixed-size particle conditions. As mentioned in Section 3, from experiments focusing on flow regime characteristics, it was implied that the stratifications and separations of different particle components in the mixtures, which may affect the coolability of the debris beds, can possibly appear under some specific accident situations (e.g., significant sodium boiling) as a result of the differences in component inertia. Therefore, to study the characteristics of particle separations and stratifications, experiments using mixed-size particles can be carried out under the gas-injection method, which can effectively simulate the violent sodium boiling conditions. 3. Further experimental and modeling studies under mixed-density conditions. For the experimental studies, although fundamental mixed-density influences were preliminarily investigated through several experiments focusing on accumulated-bed characteristics, it is necessary to point out that due to the rather limited experimental parametric conditions and non-visual experimental processes, some potential valuable experimental evidence may have been missed. Therefore, mixed-density experiments should be continuously performed under more realistic parametric situations (e.g., smaller particle size and particle mixtures composed of components with densities comparable to the densities of debris in actual reactor accidents (such as MOX fuel and SS)) and in a visual quasi-2D water tank to study in detail the flow regime mechanism under mixed-density particle conditions. Again, since the particle inertia for mixed-density particle components is different, it is reasonable to imagine that particle separation and stratification phenomena may also be found and investigated in bubbling cases with mixed-density particles. Further, for the modeling studies, referring to the modeling developments regarding the molten-pool sloshing and debris bed self-leveling behaviors [81, 82], some effective (or equivalent) density to estimate the overall density of the particle mixture can be tested by employing the base model. Further, the establishment of an extension scheme that can appropriately take the mixed-density effect into account can also be attempted. 4. Further modeling studies for bubbling conditions. To continuously extend the model's predictability, an extension scheme can be developed for bubbling conditions.