## 4 Conclusions  
This research aims to develop a numerical tool for investigating the effects of different dynamic transformations of the heterogeneous bed configurations on the debris bed's criticality/cooling safety assessment under hypothetical SAs in SFRs. To realize this goal, the coupled CFD-DEM-MCN methodology was newly proposed in this research, which can relieve the bed's safety assessment from the constraint of the static particle conditions, considering the multiphysics interplay between the thermal-hydraulics and neutronics. In addition, the coupled CFD-DEM-MCN solver used in an automatically cyclical manner was developed successfully using the originally created interface (workflow control and data exchange), integrating two other existing codes (the STAR-CCM+ code for the CFD-DEM solver, and the MVP3 code for the MCN solver). The effects of different transformation processes of the heterogeneous bed configuration induced by the particle redistribution, including (1) the effect of particle-centralized fissile material (PC effect, by comparing the Mixed model with the Homogeneous model) and (2) the effect of the bed-centralized fissile material at the bottom of the debris bed (BC effect, by comparing the Stratified model with Mixed model), were quantitatively confirmed in this research. The analytical results showed that PC and BC effects had negative and positive dominant effects on securing the criticality safety margin (\(1-k_{eff}\)) for the criticality assessment after particle redistribution had ceased. For the cooling assessment, when attaining the stable cooling state, the results indicated that both PC and BC effects had dominant negative effects ensuring the safety margin of avoiding particle re-melting accidents (\(T_{p,melting}-T_{p,Max}\)), showing a more \begin{table}
\begin{tabular}{c c c} \hline \hline  & **PC Effect [\%]** & **BC Effect [\%]** \\ \hline \(\text{Nu}_{p,Avg.}\) & \(-0.43\) & \(-1.90\) \\ \(T_{p,melting}-T_{p,Max.}\) & \(-2.51\) & \(-0.57\) \\ \hline \hline \end{tabular}
\end{table} 
Table 10: The PC and BC effects on the cooling assessment of the heat-generating particles after stable cooling (at 100 s). evident impact from the PC effect than the BC effect. Therefore, this can prove that the newly developed tool, the coupled CFD-DEM-MCN solver, is capable of investigating the effects of different dynamic heterogeneous bed configurations on the criticality and cooling safety assessment of debris beds, considering multiphysics interaction. As the preliminary examination of the newly developed numerical tool, the current study focused on the capability of clarifying the uncertainty from the factor of the dynamic heterogeneous configuration on the debris bed's safety assessment by assuming the hypothetically intermediate configurations of the debris bed in the process of the debris sedimentation as the initial state in this research. Hence, the whole debris bed's sedimentation process, including the fragmented particles' falling in the lower plenum of SFRs after FCI, will be integrated into future bed safety assessments by conducting further model validations with more referable materials for the SAs analysis. In addition, along with considering the impacts of different molten fuel compositions and decay heat transients corresponding to various conceivable core burnups in future studies, the effects from other additional heterogeneities of the debris bed, such as the particle size, shape, and the ratio of the fuel to structure material, will also be further investigated with the applicable range, in order to cover potential scenarios existing in the stochastic SAs' development processes. Therefore, the newly developed coupled CFD-DEM-MCN solver can still be expected to be an essential cornerstone in the area of SA safety analysis. Conceptualization, K.O., T.T. and N.E. ; methodology, C.-Y.L., N.E. and M.P. ; software, C.-Y.L. and M.P. ; formal analysis, C.-Y.L. ; investigation, C.-Y.L. and K.W. ; resources, K.O. and Y.O. ; data curation, C.-Y.L.