This work deals with smoke propagation through a multi-compartment assembly in case of a fire event in a nuclear installation. The scientific issues are the understanding of flows involving two modes of propagation (vent and doorway), together with the role of mechanical ventilation and oxygen backflows to the fire. The study is based on the analysis of two scenarios reproduced experimentally at large scale and simulated numerically. The main outcomes concern the comparison of the flow at a doorway and at a vent, the consequence of the smoke propagation for thermal stratification and the combined effect of the fire heat release rate and mechanical ventilation. The results highlight the performance of CFD simulations in predicting these complex scenarios. Low-velocity flow zones are identified, enabling the structure of these flows and their amplitudes to be quantified. This information provides new insights to improve fire risk assessment in nuclear facilities.