Verification and Validation (V&V) of CFD codes and turbulence models has been an on-going research area for decades. One of the sectors where CFD V&V plays an important role is the nuclear industry where simulation tools must have a sufficient degree of maturity and reliability. The aim of the present work is to carry out an assessment of three different CFD codes in simulating ascending mixed convection flows, a representative flow in the core of gas-cooled nuclear reactors under 'post-trip' conditions. For validation purposes, comparison is made against the Direct Numerical Simulation (DNS) data. The CFD codes used in the present study are 'CONVERT', 'STAR-CD', and 'Code_Saturne', which are respectively in-house, commercial, and industrial packages. Computations are carried out using two advanced Reynolds-Averaged Navier-Stokes (RANS) models, namely the k-ω-SST model and the non-linear eddy-viscosity model of Suga. The results of CONVERT and STAR-CD are compared using the Suga model, while the k-ω-SST model is used when comparing the computations of Code_Saturne and STAR-CD. In general the two models were found to perform poorly in capturing the laminarization phenomenon present at certain heat loading. However, mean flow profiles showed that there is good agreement between the codes despite using different numerical procedures. Some discrepancies were found when comparing the turbulence profiles which are mainly associated with the implementation of the turbulence models.