The four different geometries of a reactor vessel auxiliary cooling system (RVACS) are designed to investigate how the heat removal capability changes by the geometries. Each geometry has different heat transfer characteristics regarding airflow and the presence of an air separator and an insulation material. The heat removal performance is evaluated with the reactor vessel (RV) temperature, the RV wall emissivity, and the airflow gap. For the same RVACS volume, 600 °C RV temperature, and 6-cm gap, the heat removal capability varies from 240.8 kW to 308.5 kW, depending on the geometry. The wall emissivity is less effective for Geometry 2, which has a large cavity volume and a small heat transfer area compared to the other geometries. The highest heat removal performance was obtained using Geometry 3 because cold air flows in from the bottom of the RVACS, improving both radiative and convective heat transfer. Reducing the gap size by 3 cm results in only 80.0 kW of heat removal capability for Geometry 1, and the heat removal dramatically decreases to 0.2 kW at an RV temperature of 800 °C. A sufficient RVACS gap size of at least 6 cm with a 0.6-m diameter intake pipe is required to provide adequate natural circulation and eventually enhance the heat removal capability.