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[en] Highlights: • Effect of gas phase on debris bed formation behavior investigated experimentally using gas-injection. • Increasing gas velocity leads to enhanced pool convection and weakened role of particle inertia. • Effect of gas phase on regime boundary verified. • Influence of gas flow on bed geometric properties confirmed. - Abstract: Studies on debris bed formation behavior are important for the improved evaluation of Core Disruptive Accident (CDA) of Sodium-cooled Fast Reactors (SFR). To clarify the flow-regime characteristics underlying this behavior, in recent years a series of simulated experiments was performed at the Sun Yat-sen University by discharging various solid particles into Two-Dimensional (2D) water pools. Based on the experimental observation, it is found that, due to the different interaction mechanisms between solid particles and water pool, four kinds of regimes, termed respectively as the particle-suspension regime, the pool-convection dominant regime, the transitional regime and the particle-inertia dominant regime, are identifiable. In this work, aimed at providing some evidence for understanding the effect of coolant boiling on the regime transition, a number of new experiments are performed by percolating nitrogen gas uniformly through the water pool during the particle sedimentation. It is recognized that, possibly caused by the enhanced pool convection as well as the weakened role of particle inertia, increasing the gas velocity are confirmable to have an evident impact on the regime transition. On the other hand, even for the cases without regime transition, the gas flow injected is also verifiable to have a great influence on the particle-bed properties (e.g. specific geometric angles), regardless what regime it is. Knowledge and evidence from our work might be utilized for future development of a general model directly applicable for reactor safety analyses as well as the verifications of SFR severe accident analysis codes in China.