The mission of the SIET company, since it was founded in 1983, is to carry out experimental studies on the thermal-hydraulics of nuclear power plants components and systems. The general goal of the SIET testing activities is to provide data for design certification required in the NPP licensing process. The design development and the experimental qualification of specific plant components, as well as the assessment of thermal-hydraulic codes to be used for NPP safety analysis contribute to reach this goal. The SIET overall capabilities in terms of thermal power and fluid flow-rates are big enough to allow full scale or large scale testing of both single components and systems in a wide range of fluid conditions as shown below. - High pressure super-heated steam flow-rate 8 Kg/s (10 MPa, 500 °C); - High pressure saturated steam flow-rate 40 Kg/s (10 MPa); - High pressure water flow-rate 200 Kg/s (16 MPa, 330 °C); - DC power for Test Section heating 7 MW (70 kA); - AC power for plants 9 MWSince the è90s the large experimental facilities designed, built and operated by SIET to investigate the thermal-hydraulic behaviour of components (performance and/or concept demonstration), systems and plants (integral testing), have been mainly oriented to the advanced LWR and, in particular, to the Passive Safety Systems, the focal point of the SIET business. Four SIET experimental facilities are involved in the Passive Safety Systems testing: PANTHERS, GEST, SPES and IETI. PANTHERS (Performance ANalysis and Testing of HEat Removal Systems ) is a high power facility (20 MW) that allows testing of full scale components at prototypical steady-state and transient conditions. In particular for in-pool heat exchangers, a complete experimental assessment of heat transfer performances can be done in a wide range of operating conditions and with different primary fluids (i.e.: steam only, steam-air and steam-helium mixtures) during steady-state tests. Typical transient tests concern system start-up under full normal operating and ATWS conditions, investigation on gas build-up and pool water level effects.As far as the past tests are concerned, an experimental campaign in support of the licensing of GE-SBWR (Small, Simplified Boiling Water Reactor) was performed in 1994-è95 for two prototypes of in-pool heat exchangers, IC (Isolation condenser) and PCC (Passive Containment Condenser). Both thermal-hydraulic and structural behaviour were investigated. In 2003 the facility was used to successfully test an innovative emergency heat removal system designed by SIET within an ENEA funded research program. The system, called PERSEO (in-Pool Energy Removal System for Emergency Operation), consists of a heat exchanger and two pools connected to each other at the bottom and the top, being the heat exchanger pool empty and the other one full of cold water during stand-by condition. By opening a valve installed on the pool bottom connecting pipe the system enables the heat removal by flooding the heat exchanger. The Relap-5 code was extensively validated against heat transfer phenomena in vertical bundles located in water pools.The GEST (GEnerator Separator Tests) facility plant was originally designed for experiments on the PWR Steam Generators at full conditions. The facility was modified in 1996 for testing injection systems as the PIDS (Passive Injection and Depressurisation System) conceived by ANSALDO Nuclear Division for ALWR applications and the SIP-1 (Sistema di Iniezione Passiva) conceived by SIET. In particular, the SIP-1 passive injection system tests demonstrated its capability of water injection into a pressurised vessel (e.g.: reactor vessel or PWR steam generator vessel), simply unbalancing the passive system circuit by a low water level occurrence due, for example, to a Loss of Coolant Accident. The SPES (Simulatore Pressurizzato per Esperienze di Sicurezza - pressurized simulator for safety experiences) facility, initially simulating a three loop PWR and operated for integral tests on the italian PUN (Progetto Unificato Nazionale - National Unified Project), was upgraded in the è90s in order to carry out an integral test program devoted to the Westinghouse AP-600 design certification, with particular focus on the performances of the passive safety systems of such a reactor. The IETI (Impianto per Esperienze Termo-Idrauliche - facility for thermal-hydraulic tests) loop is a multipurpose-high performance test facility (20 MPa, 430°C, 15 kg/s water and 4 kg/s steam flow-rates). It was initially operated for studying the thermal-hydraulic behaviour of LWR power channels. In 1992 the IETI loop was modified to perform tests on Steam Jet Pumps (SJP) for LWR Passive Safety Systems. The SJP (or Steam Injector) is a static device where steam and subcooled water are mixed together and the delivered sub-cooled water can reach a pressure significantly higher than the inlet steam pressure. Therefore the SJP were studied in the frame of the emergency systems for ALWRs as injection and make-up in the LWRs primary loops as well as emergency feed water in the PWRs secondary loops. Two experimental campaigns, sponsored by ENEL, were successfully completed at SIET on a SJP designed by CISE in the è90s, aimed at measuring the SJP performance at the LWR conditions both in steady-state and transient situations (start-up and shutdown). Subsequently different type of SJP were tested at SIET in the frame of European Projects (SYNTHESIS-1996/1998 and DEEPSSI-2001/2003). More recently (2005-2006) a multi-stage Steam Injector, designed by Toshiba Corporation for the ABWR, was successfully tested at SIET. Thanks to the large number of experimental campaigns run on different facilities and devices, SIET has gathered a significant experience on the Passive Safety Systems for Light Water Reactors which represents an unique asset of this company, even in the context of a forthcoming worldwide renaissance of nuclear power.