## 4 Conclusions  
Investigations on DBF behavior are important for the assessment of the PAMR phase in CDAs of SFRs and the evaluation of debris bed coolability, which is of great importance to the mitigation of accident consequences. Knowledge from experimental and modeling work can be applied to the evaluation and verification of SFR safety analysis codes. Recently, a large number of experiments and modeling studies on DBF behavior have been carried out considering the overall accumulated-bed characteristics and flow regime characteristics. In the studies on DBF behavior focusing on the overall accumulated-bed characteristics, the key factors indicating the accumulated-bed characteristics (such as bed and mound heights, dimple area, and volume) were investigated in light of experiments in a cylindrical water pool to gain insight into the basic parametric effects on DBF behavior. It was confirmed that due to a more significant impact of particle jets on the accumulated particle beds, more concave and flatter particle beds (i.e., particle beds with a lower mound height, smaller repose angle, larger dimple area, and volume) could be attained when smaller-sized particles or a nozzle with larger diameter or higher placement were employed. However, in these experiments, only convex and concave beds were observed as a result of the relatively small parametric ranges. Considering the importance of bed height in evaluating the bed characteristics, two predictive models were empirically proposed for mound height and repose angle, and one was subsequently modified, becoming more applicable in predictingthe bed height under more realistic accidental situations (i.e., the situations where debris are composed of mixed particles). On the other hand, aimed at enabling experimental visualization and obtaining more comprehensive knowledge on the flow regime characteristics of DBF, additional parameters with wider ranges and more potential and realistic accidental situations (e.g., complex particle compositions in debris and coolant boiling resulting from the decay heat of debris) were considered in the experiments by using a narrow 2D viewing tank. From the experiments, it was confirmed that the opposed effects of pool convection and particle inertia result in different inter-particle and particle-fluid interactions and play a prominent role in flow regime transitions during the DBF process. Generally, four typical flow regimes, including particle-suspension, pool-convection dominant, transitional, and particle-inertia dominant regimes, were identifiable. Due to their different characteristics, after particle accumulation, four formed bed shapes, including flat, concave, trapezoid, and convex shapes, could be found, respectively. Larger particle inertia (e.g., resulting from particles with bigger sizes and densities) and lower intensity of pool convection (e.g., resulting from a lower liquid height or lower particle discharging rate) can promote the flow regime transition to a larger-numbered one, leading to the formation of a sharper bed summit geometry. Moreover, recognizing that information concerning the bed height was not adequate in analyzing the coolability of the debris bed because different debris bed shapes may possess the same bed height, another predictive model for flow regime transitions and final bed shape was developed in accordance with the parametric effects on flow regime characteristics. Later, thanks to the extensive data and understanding accumulated from the experiments using non-spherical and mixed-sized particles, the applicable range of this model was further enlarged to include more realistic situations by introducing two corresponding extension schemes that characterize the relevant shape and mixed-sized influences on flow regime transition.