Abstract: The reactor cavity cooling system (RCCS) is an important safety facility for high-temperature gas-cooled reactor pebble-bed module (HTR-PM). The RCCS relies on two coupled natural circulations to passively remove the reactor cavity heat to the ultimate heat sink (atmosphere), i.e., the air natural circulation within the cooling tower and the cooling-water natural circulation. It does not depend on any active equipment for example pumps, fans, and diesel engines. Even under accident conditions, it also does not require operator intervention and control signals. Therefore, the RCCS is called a fully passive cooling system. During normal operation, the RCCS is responsible for cooling the reactor cavity. Under accident conditions, the RCCS can stably and passively remove the core decay heat to ensure the thermal integrity of the reactor pressure vessel (RPV) and reactor cavity. For each reactor module of the HTR-PM, the RCCS is divided into three independent units. According to the design requirements, any two units operating normally can provide the required heat removal power under any operation condition. Based on the accident analysis of HTR-PM, the reactor cavity heat about 500 kW and 1 200 kW is needed to be removed under normal operation and accident conditions, respectively. Correspondingly, the maximum concrete temperature should be controlled below 65 ℃ and 180 ℃, as mentioned in the division 2 (Code for Concrete Containments) of ASME BPVC-Ⅲ (Rules for Construction of Nuclear Facility Components). In the current investigation, the computational fluid dynamics (CFD) method was used to carry out a 2D full-scale simulation of RCCS convection and radiation heat transfer. Specifically, the symmetric boundary conditions were used for a local one twentieth model (18° chamfer), including 15 RCCS water cooling pipes. There were three types of the simulated operation conditions, such as the shielding cooling system failure, one RCCS unit failure and the pressurized loss of forced cooling (PLOFC) accident. The results show that under any operation condition, the RCCS has sufficient heat removal power to achieve the effective cooling for the reactor cavity, meeting the inherent safety characteristics of HTR-PM. As for the severe accident, the maximum concrete temperature is about 111℃, which is far lower than the design limitation. Meanwhile, the atmosphere temperature has a significant impact on the heat removal power of RCCS. When the atmosphere temperature increases from 25 ℃ to 42 ℃, the maximum concrete temperature increases from 86 ℃ to 111 ℃. The time required for thermal disturbance to pass through the concrete is about 48 hours, while the concrete temperature distribution tends to stabilize after 120 hours. Moreover, the heat transfer rate distributions of the concrete inner and outer walls are determined. This work can provide reference for RCCS design and accident analysis of HTR-PM.