IRIS 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 D.S. Nuclear Regulatory Commission (NRC) foresees an experimental campaign to verify the behaviour of the new plant and its safety system capabilities to care with postulated accidents. To this end an experimental facility, so-called SPES3-IRIS, will be built at SIET laboratories (Piacenza, Italy) simulating, with l:100 volume scale and l:1 height scale, the primary, secondary, containment and safety systems typical of the IRIS reactor. This document deals with the basic of SPES3-IRIS facility reference design.