ENEA is setting up, applying and validating numerical models and an integrated multi-physics approach, based on existing codes and aimed at supporting safety analysis of liquid metal Gen. IV fast reactors. The paper provides an outline of the activity and it is focused on qualification of a three-dimensional thermal- hydraulic model of EBR-II primary and secondary systems using the system code RELAP5-3DO. The nodalization models one by one the fuel assemblies of the core and of the extended core of the reactor for an easy and efficient coupling with a 3D neutron kinetic code. The paper presents the qualification of the nodalization based on the EBR-II test SHRT-17. The analysis of the experimental data, the identification of the thermal-hydraulics phenomena observed in the tests are the basis for assessing the code performances and for discussing its limitations. © Copyright (2015) by American Nuclcar Society All rights reserved.