The reactor cavity cooling system (RCCS) improves the transfer of the decay heat from the reactor to the reactor cavity by passively removing the heat by means of natural circulation. Air- and water-cooled variations exist of this system for the fourth-generation nuclear reactor designs. The effectiveness of the designs needs to be validated. This is sometimes done by constructing scaled down models of the design. The University of Wisconsin (Madison) constructed an experimental facility of such a scaled down model of the RCCS. This work focused on the simulation of this experimental facility. The computational model consisted of the primary loop of the experimental facility and used a 3D CFD code (ANSYS Fluent) and a systems CFD code (Flownex SE) to simulate the phenomena in the system. The 3D CFD code was used to simulate the heated section of the experimental facility, while the 1D code was used to simulate the remainder of the network. This coupled/co-simulation methodology combined the strengths of the 3D CFD and 1D systems CFD to have an optimum tradeoff between detailed results and computational time. The results that were considered of importance were the temperature distributions that were induced by the natural circulation. The convection heat transfer coefficients of the natural circulation were also calculated for the riser tube inner surfaces.