The design of Advanced Light Water Reactors is characterized by the implementation of innovative passive safety systems in order to improve and simplify safety. These systems rely on natural phenomena (gravity injection, natural circulation, etc.) to improve their reliability and performance. Decay Heat Removal (DHR) is one of the main safety functions covered in a passive way by implementing in-pool heat exchangers, which require the actuation of isolation valves on the primary or secondary loop of LWRs to initiate their function. The important thermal-mechanical constrains that affect these valves can impair the reliability of such systems.ENEA (Ente per le Nuove Tecnologie, l'Energia e l'Ambiente) in collaboration with SIET (Società Informazioni Esperienze Termoidrauliche) developed an innovative device, so-called PERSEO (in-Pool Energy Removal System for Emergency Operation), that enables the control of the removed power by modifying the thermal exchange conditions in the pool, thus avoiding the installation of mechanical valves on the primary circuit and improving the system reliability. This device represents an evolution of the Thermal Valve device, already studied by ENEA and CEA within the framework of a EU FISA Project.PERSEO consists of two pools connected at the bottom and top, one containing the heat exchanger and the other full of cold water. During normal operation the former one is empty as the valve on the bottom connecting line is closed; thus inhibiting the heat removal from primary side. The valve opening starts the Decay Heat Removal function that relies on the natural circulation establishing between the two pools. The steam produced in the first pool, through a steam duct, is driven and condensed in the second one, which guarantees the water reserve for long term energy removal. An injector is installed at the exit of the steam duct inside the second pool for improving the water mixing and avoiding thermal stratification phenomena. Full-scale component and integral tests were performed at SIET thermal-hydraulic research center in Piacenza (Italy) in order to study specific phenomena and to verify the validity and performance of PERSEO.A complete assessment of the reliability and efficiency of a DHR system implementing the PERSEO device requires a numerical analysis of the overall plant response to a selection of accidental scenarios. Industrial system code like CATHARE (best estimate French code) can be used to this purpose. The experimental data acquired in the test campaign have allowed the development and qualification of a CATHARE one-dimensional model of PERSEO. This paper after describing the full-scale tests and the CATHARE model presents the satisfactory results of the post-test analysis and provides a better understanding of the limitations of the one-dimensional model in predicting all the physical phenomena involved.