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The thermal-hydraulic (TH) system code ATHLET is developed by the Gesellschaft fur Anlagen- und Reaktorsicherheit (GRS) mbH in the last decades for the analysis of the whole spectrum of design basis and beyond design basis accidents for PWRs and BWRs. Recently, ATHLET was coupled with the commercial 3D computational fluid dynamics (CFD) software package ANSYS CFX to improve the system code simulation capabilities for flows with pronounced 3D phenomena such as flow mixing, thermal stratification, etc. The aim of the current development of the coupled code ATHLET – ANSYS CFX is focused on the extension of the physical models of the computer program for the application to innovative reactor concepts. Within the European project THINS (Thermal Hydraulics of Innovative Nuclear Systems), validation activities for coupled thermal-hydraulic codes are carried out. The Swedish experimental facility TALL, operated by KTH, Stockholm is currently being refurbished for thermal-hydraulic experiments with lead-bismuth eutectic alloy (LBE) coolant at natural and forced circulation conditions. Simulations with ATHLET – ANSYS CFX will be carried out and the results will be compared with experimental data. The paper gives an overview on models’ extensions and the implementation of different physical properties of liquid metals in ATHLET and ANSYS CFX. First coupled 1D-3D calculations for simplified configurations are presented. Then, preliminary results of the supporting 3D simulations for the design process of the 3D TALL test section are discussed.Copyright © 2012 by ASME