To avoid accidents due to human error or the failure of components, the reactor cavity cooling system (RCCS) of a high temperature reactor must be able to operate naturally or passively safe. The RCCS can be a buoyancy driven air-cooled or water cooled system. The system under consideration in this investigation is an air-cooled system with an essentially U-tube configuration. It consists of an annular downcomer which is connected via a bottom-header to 220 risers. The downcomer and risers are connected to the atmosphere via inlet and manifold systems respectively. In this study the entire system is simulated using two 1D network codes, GAMMA+ and FLOWNEX, and a section of the reactor cavity is modelled in 3D using the CFD code STARCCM+. The 3D code is used to obtain the heat transfer coefficients for the surfaces in the reactor cavity, whilst the 1D codes are used to study the performance of the RCCS subject to the radiative and convection heat transfer and atmospheric conditions. Both steady-state and transient conditions are modeled using the 1D codes. The agreement between the results obtained with the two codes is good and serves as a verification of the methodologies that are used. In the transient simulations it has been found that the flow may reverse under certain atmospheric conditions. This may lead to the deterioration of the performance of the RCCS and needs to be investigated in more detail.