The innovative design of the AP1000 power plant has various layers of passive safety systems aiming to enhance reactor safety during normal and transient conditions. The passive containment cooling system (PCCS) is a safety-related system capable of removing heat from the steel containment vessel (SCV) to the atmosphere and preventing the containment from exceeding the design pressure and temperature following a postulated design-basis accident. The PCCS heat removal mechanisms include condensation on the internal SCV surface, heat conduction, natural convection, evaporation of water film, and radiative heat transfer. In two basic postulated scenarios, the reactor decay heat can ultimately be removed from the SCV only by air natural convection. The first scenario occurs 72 h following a large-break loss-of-coolant accident (LBLOCA) when the passive containment cooling water storage tank becomes unavailable. The second scenario occurs following a postulated loss of shutdown decay heat removal event. Hence, investigating the thermal-hydraulic behavior of the containment under transient conditions is essential to ensure its safety and integrity. In this study, a simplified three-dimensional model using ANSYS FLUENT is developed to investigate the cooling capability of air natural convection outside the SCV during a LBLOCA event. Because of the lack of experimental data, code-to-code validation was performed using the actual results of AP1000 alongside other research findings. The results show good agreement with available data, which can be used for future research.