Abstract: During severe accident progression in boiling water reactors (BWRs), reactor pressure vessel failure leads to the release of corium into the deep water pool of the containment pedestal, inevitably triggering fuel-coolant interactions (FCI) that form debris beds—a process threatening containment integrity. The characteristics of the resulting debris bed, including its porosity, morphology, and particle size distribution, critically determine the coolability of the molten material and the potential for steam explosions, thereby directly influencing the effectiveness of severe accident mitigation strategies. To reproduce prototypic debris bed formation phenomena under economically and safely feasible conditions, metallic simulants including tin (Sn), tin-bismuth alloy (Sn-Bi), zinc (Zn), and tin-iron alloy (Fe-Sn) were selected as representatives of actual corium compositions (Zr/Fe mixtures). The fragmentation processes of melt jets, droplet breakup, and debris sedimentation/agglomeration were comprehensively recorded using high-speed videography. The simulant materials were selected based on a simplified scaling analysis considering both hydrodynamic similarity (Froude number) and thermal similarity (modified Stefan number and Abe’s heat transfer regime maps), ensuring that the experimental observations are relevant to prototypical reactor conditions. Precise measurements were obtained for debris bed characteristics including geometric configuration, porosity distribution, as well as morphological parameters and particle size distribution of fragments. The experimental results reveal significant material-dependent behaviors: 1) Steam explosion is observed exclusively in the Sn experiments, attributed to its high thermal conductivity combined with high superheat, which maintains the melt in a fully liquid state with excellent fluidity, enabling violent film boiling collapse and instantaneous heat transfer; 2) The Fe-Sn alloy experiments produce uniquely dense debris beds with extensive agglomeration and low porosity, in stark contrast to the high-porosity (＞90%) honeycomb structures formed by low-melting-point simulants (Sn, Zn, Sn-Bi); 3) Despite having comparable superheat levels, Fe-Sn exhibitsno steam explosion, instead undergoing mild film boiling that generated “externally solidified, internally liquid” particles that subsequently bonded upon settling. Analysis of the Fe-Sn results indicates that its intermediate superheat and stable film boiling regime lead to progressive surface solidification while maintaining a liquid core, with particle collision at the pool bottom rupturing the solid shells and allowing internal melt to weld adjacent particles together, forming dense agglomerates. This mechanism provides critical insights for prototypic reactor conditions, suggesting that the Zr/Fe components in actual corium may form similar dense, low-porosity layers that impede coolant penetration and threaten long-term coolability. The work provides crucial experimental data for enhancing the multicomponent corium debris bed database, offering significant scientific value for understanding the mechanisms by which melt material composition influences debris bed formation, fragmentation behavior, and steam explosion potential. These findings contribute to the development of more accurate severe accident models and inform the optimization of mitigation strategies for both BWRs and pressurized water reactors (PWRs) during the ex-vessel phase of severe accidents.