The description of the “Dissymmetric Test” performed in the Phénix sodium fast reactor has recently become available as a blind benchmark for thermal-hydraulic modelling within the H2020 SESAME project. The transient consists of a largely asymmetric temperature distribution in the sodium pool resulting from a pump trip in one of the two intermediate circuits, followed by a reactor scram. Although this transient is particularly suitable to validate a 3-D Computational Fluid Dynamics (CFD) model of the thermal-hydraulics in the sodium pool, the computational cost of a relatively long transient analysis with a full-scope CFD model of the whole Phénix reactor system would be huge. The present paper describes the system thermal-hydraulic (STH) model and the multiscale approach adopted by NRG for the simulation of the Dissymmetric Test. The in-house STH code SPECTRA was used to model the complete primary and secondary sides (intermediate loops) and explicitly coupled with the CFD code ANSYS CFX. The latter was used to resolve the details of the flow distribution inside the sodium pools, as well as at the outlet window of the primary side of the intermediate heat exchangers. An STH stand-alone simulation of the transient was carried out for comparison.