• FPS
• Gen-IV
• STH/CFD
• NACIE-UP
• LBE
• HLM

In the frame of GEN-IV/ADS nuclear systems, the correct evaluation of the temperature distribution in the fuel pin bundles are of dominant interest. In particular, the use of lead or lead-bismuth eutectic (LBE) as coolant for new generation fast reactors is one of the most promising choices due to its high density and high thermal conductivity. For this reason, a detailed analysis of the thermal-hydraulic phenomena of the heavy liquid metal (HLM) inside the sub-channels of a fuel rod bundle is thus necessary for the development of GEN-IV/ADS nuclear reactors. In this frame, the interaction between the numerical analyses by STH/CFD and data coming from experimental facilities seems to be crucial to assess the feasibility of the primary loop components for purposes of design and safety. An important experimental facility named NACIE-UP (NAtural CIrculation Experiment UPgraded), is located at ENEA Brasimone research center and it is dedicated to study HLM free, forced, and mixed convection in loops, adopting LBE as a coolant.
The research provided in this thesis work is a part of an experimental campaign investigating the 19-pins wire-spaced fuel pin simulator installed inside the previously mentioned NACIE-UP loop-type facility. In particular, the present work is devoted to study the thermal-hydraulic behaviour of the NACIE-UP loop during the transition from forced to natural circulation for three tests (ADP10, ADP06, ADP07) by using RELAP5/Mod3.3. In addition, some simulations have been carried out using computational fluid dynamics (CFD) with the intention of getting information about local temperature distribution inside the FPS. The aim is to analyze the effects of the non-uniformity in the heater section on the local temperatures and on the overall system behavior. Furthermore, several sensitivity studies were performed with the aim to obtain better predictions of the experimental data.
A good agreement has been achieved between experimental and numerical data by adopting different assumptions and approaches for the considered computational tools. Moreover, by comparing the experimental data obtained directly from the NACIE-UP facility with the numerical results collected from RELAP5/CFD calculations, this work tries to qualify the addressed numerical codes for Heavy Liquid Metal (HLM) systems applications.