<p>In the event of a hypothetical core melt accident in a sodium-cooled fast reactor (SFR), the fragmented corium settles on the core catcher, forming a porous debris bed that continues to generate decay heat. Efficient removal of this heat is essential to prevent failure of the reactor vessel. Therefore, a thorough understanding of the heat transfer mechanisms within the debris bed is crucial for assessing its coolability. Due to the high thermal conductivity and boiling point of sodium, both conduction and natural convection contribute significantly to heat transport within the bed. The effective thermal conductivity generally characterizes conduction in such porous media, while the onset of natural convection is predicted using the Rayleigh number (<i>Ra</i>). However, there is uncertainty in the critical Rayleigh numbers (<i>Ra</i>_c) reported in the literature, and experimental data in sodium systems remain limited. To address this gap, an experimental study was conducted to investigate heat transfer behavior in sodium-filled beds composed of stainless-steel particles of varying sizes under bottom-heated conditions. Experiments were performed across a range of heat fluxes to examine the heat transfer regimes as a function of <i>Ra</i>. The results indicate that the onset of natural convection in sodium-filled debris beds is primarily governed by <i>Ra</i> and is independent of the Prandtl number (<i>Pr</i>). The critical <i>Ra</i> was found to be about 38, in agreement with theoretical predictions.</p>