URANS and LES simulations of a test facility have been performed to investigate the thermal hydraulic phenomena during a step-down thermal transient, which is typical of some important accident scenarios in sodium-cooled fast reactors (SFRs). Wall-modelled URANS simulations were used to study the entire transient duration, while LES was used to provide detailed information about its early stages, with both validating well against experimental data from the facility. The results showed thermal stratification developed above the outlet, with the lower parts of the test section becoming well-mixed after a short period, mainly due to the presence of a mean flow circulation and the unsteadiness of the jets, which resulted from a feedback loop between the flow from the inlets and the recirculating flow. Comparing different URANS models with LES showed that all tested could capture the basic flow behaviour, but only the more advanced turbulence model could capture the unsteadiness of the jets or the irregular stratification interface. It was also found that for the present case, different formulations of turbulent Prandtl number did not significantly change the overall transient response.