The direct coupling of CFD and 1D system codes has become an attractive possibility for a wider range of important reactor applications. In this project we are developing a coupling scheme between the US NRC best-estimate thermal-hydraulic code TRACE and the CFD code STAR-CCM+. We aim at improving the accuracy and efficiency of nuclear power plants simulations and provide the ability to isolate the regulatory importance of specific 3-D phenomena such as stratification, mixing and natural convection.