The sodium–concrete reaction (SCR) is an important phenomenon during severe accidents in sodium-cooled fast reactors (SFRs), as it generates large volumes of hydrogen and aerosols in the containment vessel along with structural concrete ablation. In this study, the chemical reaction beneath the internal heater (800 °C) was investigated in SCR experiments with internal heating. The experiments simulate the effects of obstacles and heating on the SCR. Especially, we focused on the concrete ablation phenomenon because the hydrogen generation is sourced from the moisture in the concrete. The effects of internal heating on the self-termination mechanism are also discussed. The internal heater on the concrete hindered the transport of sodium (Na) into the concrete. Therefore, the reaction between Na and the concrete began at the periphery of the internal heater, where the concrete ablation depth was larger than under the internal heater. The high Na pool temperature (800 °C) largely increased the Na aerosol-release rate, which was explained by Na evaporation and formed a porous reaction-product layer. The Si mass balance and image mapping by an electron-probe micro-analyzer yielded consistent porosities in the reaction-product layer (0.54–0.59). The porous reaction products suppressed the amount of Na transported into the reaction front. Regardless of the internal heater placement, the Na concentration around the reaction front was limited to around 30 wt %. The Na concentration condition was dominantly responsible for the self-termination of the internally heated SCR.