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The SPES3 facility was designed and it is being built at the SIET laboratories, in the frame of an R&D program on Nuclear Fission, led by ENEA and funded by the Italian Ministry of Economic Development. The facility is based on the IRIS reactor design and it is suitable to test the plant response to postulated Design and Beyond Design Basis Events, providing experimental data for code validation and plant safety analysis. The IRIS reactor is an advanced medium size, integral layout, pressurized water reactor developed by an international consortium of utilities, industries, research centres and universities. It is based on the proven technology of PWR with an innovative configuration and safety features suitable to cope with Loss of Coolant Accidents through a dynamic coupling of the primary and containment systems. The SPES3 facility reproduces the primary, secondary and containment systems of the reactor with 1:100 volume scale, full elevation, prototypical fluid and thermal-hydraulic conditions. The RELAP5 code was extensively used as a support tool in the design of the facility to identify criticalities and weak points in the reactor simulation. A design-calculation feedback process, based on the comparison between the IRIS simulations by FER, with the GOTHIC and RELAP5 coupled codes, and the SPES3 simulations by SIET, with the RELAP5 code, led to reduce the differences in the two plants behaviour, versus a 2 inch equivalent Direct Vessel Injection line Double Ended Guillotine break, considered the most challenging LOCA for the IRIS plant. Once available the final design of SPES3, further calculations were performed to investigate Beyond Design Basis Events, where the intervention of the Passive Containment Condenser is fundamental for the accident recovery. Sensitivity analyses showed the importance of the PCC actuation time, to limit the containment pressure peak, to reach an early balance between the primary and containment pressure and to allow the passive water transfer from the containment to the reactor vessel for core cooling enhanced by the ADS Stage-II opening. This paper deals with the SPES3 facility description and the results of the BDBE DVI line break with no Emergency Heat Removal System (EHRS) available.