Formation and destruction of thermal stratification can occur under certain flow conditions in the upper plenum of sodium cooled fast breeder reactors (SFR). The flow patterns in the hot sodium pool of the upper plenum are very complex, including zones of free and wallbounded jets, recirculation and stagnation areas. The interaction of the sodium flow and thermal stratification has been analyzed experimentally at CEA in the years 1980-1990 in the SUPERCAVNA facility. The facility consists of a rectangular cavity with temperaturecontrolled heated walls where the flow is driven by a wallbounded cold jet at the bottom of the cavity. Experimental data of the temperature distribution in the cavity are available for steady-state and transient flow conditions. The experiments are analyzed with the CEA CFD reference code TrioCFD and the commercial code FLUENT by using Reynolds Averaged Navier-Stokes (RANS) equations. It is shown that a two-dimensional treatment is sufficient for the analysis of steady-state SUPERCAVNA experiments. It is necessary to take into account correctly the conjugate heat transfer between walls and cavity. Turbulence modelling with k-s models, in either standard, realizable or RNG formulations, does not lead to significant differences in the calculated temperature fields which are in good accordance to the measurements. Different wall treatments also do not change these results. Thus, it seems that turbulence modelling is not a predominant factor in a successful simulation of the mixed convection experiments. However, using temperaturedependent physical properties is a very important factor in simulating the experiments correctly, although the Boussinesq approximation is justified. Finally, it is shown that a three-dimensional treatment is necessary for the analysis of transient experiments.