The SPES3 facility simulates with 1:100 volume scale and 1:1 height scale, the IRIS (International Reactor Innovative and Secure) primary, secondary, containment and safety systems. It will be built at SIET laboratories and operated at the IRIS nominal conditions. A RELAP5 nodalization, based on the 'Conceptual design', has been developed and used during the design phase for obtaining feedback information on the facility design verifying SPES3 capability to simulate the physical phenomena of the postulated accidents. At this end five base test transient cases were chosen among the tests specified in the SPES3 test matrix. That document reports a detailed analysis of the results of the base transient cases simulation.