CFD - STH Code Coupling for the Thermal Hydraulic Analysis of NACIE-UP Experimental Facility

GEN-IV Lead-cooled Fast Reactors are recognized as an economically competitive solution with intrinsic safe operation. ENEA is a member of the FALCON Consortium, which has the goal to construct the Advanced Lead-cooled Fast Reactor European Demonstrator (ALFRED) in the 2030s. In this framework, computational tools are required to support the design and safety assessment of new facilities and reactors. Computational Fluid Dynamic (CFD) codes are able to reproduce local phenomena (e. g., thermal stratification, fluid mixing and local distributions) by solving directly the three-dimensional Navier-Stokes equations, but at the price of high computational cost. Instead, System Thermal-Hydraulic (STH) codes solve one-dimensional equations and are more suited for system-scale analyses. The goal of this work is to develop, validate and apply a simulation tool able to reproduce the TH behavior of Heavy Liquid Metals (HLMs) through the coupling between STH and CFD codes. The tool aims to exploit the advantages of the two families of codes and adopt a multi-scale approach for improved simulation at component level within system analysis, with an acceptable computational time. The coupling technique is based on FORTRAN user routines implemented in Ansys CFX, i.e. the master CFD code,. The STH code used in this activity is RELAP5/MOD3.3. The user routines take care of data exchange, RELAP5 execution, and error checking. The coupled simulation tool is adopted to reproduce experimental data on a forced-to-natural-circulation transition test, carried out on the NACIEUP facility, with LBE as working fluid. Limitations of the present analysis and plans for future improvements will be discussed.