A small scale water-cooled experimental facility was built in order to study the complex thermohydraulic phenomena taking place in the Reactor Cavity Cooling System (RCCS) during the normal operation (steady-state case) an during accident scenario when forced convection is lost. The facility represents a portion of the reactor vessel with nine stainless steel coolant risers. The pipes are connected via cold and hot manifolds to a water tank located on top of the cavity. Due to the complexity of the expected thermal hydraulic phenomena, a RELAP5-3D input deck was prepared in order to predict the main thermohydraulic parameters, mainly coolant flow rate and temperatures. During the facility shakedown, the coolant flow was constantly monitored in order to observe the natural circulation startup phase and some interesting features of the coolant behavior were observed. The comparison of the preliminary experimental results from a test run with the prediction of the RELAP5-3D simulations helped validating the assumptions and simplifications adopted in the model for future simulations of steady-state and transients, and confirmed the potentiality of the system code for analysis of such systems. In the present paperwork, a detailed description of the experimental facility and the RELAP5-3D model are provided. Preliminary experimental results from different test runs are described and compared with the RELAP5-3D simulation results.