Ongoing research on innovative and environmentally sustainable systems for energy production, as an alternative to the traditional sources, led ENEA to investigate high temperature heat generation by solar power and the subsequent energy collection and storage. An experimental facility, the Prova Collettori Solari (PCS), was built at the ENEA Casaccia Research Centre to test the most innovative components before industrialization and to determine the effectiveness of the foreseen thermodynamic cycle utilizing a molten salt mixture, heated in a solar collector irradiated by parabolic mirrors. In parallel with the experimental investigation, the development of a numerical tool for thermal-fluid-dynamic simulation of circuits, with such a mixture as the primary heat transfer fluid, was pursued and the RELAP5 code was modified accordingly. Thermo-physical property tables were developed for the liquid and steam phases of the molten salt mixture and inserted into the RELAP5 code. The modified code was benchmarked versus experimental data. The entire PCS facility was then simulated with various operational transients aimed at studying both the dynamic and thermal behavior of the whole system. This paper summarizes the choices and assumptions for the property definition of the molten salts, their introduction in the RELAP5 code, andthe main results of the operational transients simulation in the PCS facility.