• fast reactors
• HLM
• LBE
• lead bismuth eutectic
• liquid metals
• STH code
• thermal hydraulics

The scope of the presented thesis work aimed at investigating the thermal-hydraulic phenomena that take place in Heavy Liquid Metal (HLM) cooled nuclear reactors, which are selected to be one of the proposed Gen-IV reactor designs. The objective of the numerical simulation was to evaluate the capability of the System Thermal Hydraulic (STH) codes to predict the phenomena that occur in HLM pool-type systems that result from postulated accidents, mainly Protected Loss Of Flow Accident (PLOFA).
The thermal-hydraulic phenomena are investigated through a synchronized experimental campaign and numerical analysis. The experimental campaign was carried at CIRCE-HERO facility, built at ENEA Brasimone research center, which is a large LBE pool-type facility, whereas the numerical simulation activity was performed at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa (UNIPI).
The main numerical activity performed in the presented work focused on the analysis of three distinct groups of tests: the first group included heat losses characterization tests, performed in both steady state and transient condition to estimate the external heat losses through the pool; the second group of tests was PLOFA-type experiment in which the loop encountered a transition from forced circulation to natural circulation, while the third group comprised low pressure steady states tests (MYRTE experimental campaign) that were carried out at low power and low pressure to support the development of the Primary Heat eXchanger (PHX) of MYRRHA reactor in Belgium.
The work was conducted using RELAP5/Mod3.3 version modified at the University of Pisa, that was mainly used for STH code standalone calculations and coupling calculations with CFD code ANSYS FLUENT. In particular, the work included designing the most convenient nodalization to model the CIRCE-HERO test facility through shifting from a simplified nodalization to a full detailed one that model the whole test section, then performing RELAP5 standalone simulations by setting up the appropriate boundary conditions for each experiment. Several sensitivity analyses were performed that resulted in estimating the value of the thermal conductivity of the stainless steel powder packed with helium that separates between the LBE side and the water side of the HERO SGBT steam generator and estimating the value of the overall heat transfer coefficient (HTC) between the external surface of the pool and the surroundings. The outcomes of these simulations played an important role in evaluating the capability of the STH to predict the thermal-hydraulic behavior of HLMs following accidental scenarios, besides contributing to the validation process of the utilized code.