IRIS (International Reactor Innovative and Secure) is an integral modular, medium size PWR, under development by an international consortium with approximately 20 partners led by Westinghouse. The licensing process required by the U.S. Nuclear Regulatory Commission (NRC) foresees a series of experimental tests on properly built facilities suitable to verify the behaviour of the new plant and its safety system capabilities to cope with postulated accidents. The SPES3-IRIS integral test facility, which will be built at SIET research area, will be able to simulate the IRIS primary, secondary, containment and safety systems with 1:100 volume scale and 1:1 height scale. Moreover the facility will be suitable to perform both integral and separate effect tests and will allow to investigate the thermal hydraulic interaction among the various systems. This document reports the conceptual design of the SPES-3 facility.