ABSTRACT The ENEA Brasimone Research Centre since 1999 is strongly involved in the national and European research programmes performed in the field of heavy liquid metal technology aiming at the development of critical (LFR) and subcritical (ADS) nuclear systems. In particular, in the frame of the IP-EUROTRANS, (6th Framework Program EU), ENEA assumed the commitment to perform an integral experiment with the aim to reproduce the primary flow path of a HLM pool-type nuclear reactor, cooled by Lead Bismuth Eutectics (LBE). This new experimental activity, named ICE "Integral Circulation Experiment", will be performed by an appropriate test section designed to be installed in the CIRCE facility. In order to support the ICE activity, as well as characterise the natural and gas enhanced circulation flow regimes in a HLM loop, qualify test procedures, components nuclear relevant, a new facility was designed and built up by Brasimone Research Centre, named NACIE "NAtural CIrculation Experiment". The paper reports a detailed description of the loop and the main experimental results carried out from the natural circulation tests already performed on the NACIE loop. Numerical simulations have been performed in collaboration with the University of Pisa, adopting the RELAP5/Mod3.3 system code modified to allow for LBE as a cooling fluid. The aim of the performed post-test calculations is to compare the code response with the experimental results under the natural circulation flow regime, allowing to qualify the adopted nodalisation as well as the performance of the code when employed on HLM loop
