Experiments and numerical simulations were carried out to study the turbulent mixing behaviour of two opposing flows inside a 1/18th scaled down model of square chimney structure of a pool type research reactor. The chimney design facilitates drawing pool water in the downward direction, thereby suppressing the upward flow of radioactive water jet to limit the radiation field at the reactor pool top. Experiments were performed using the particle image velocimetry (PIV) technique, considering a mass flow rate of 0.5 kg/s and 1.0 kg/s for the upward flowing water from the core, which corresponds to Reynolds number of 20000 and 40000 respectively. Mass flow ratios of the downward flow and the upward flow were 0.00, 0.05, 0.1 and 0.15. PIV data shows the effect of mass flow ratio on the velocity distribution inside the chimney model. The experiments showed that the jet height decreased with increase in downward flow due to the opposing momentum of downward flow against the upward jet. The effects of Reynolds number on the jet height were found to be marginal. It was observed that both κ–ε and Κ–ω turbulence models were able to simulate the experimental data of turbulent mixing.