The best-estimate nuclear system thermal hydraulic code, RELAP5, and computational fluid dynamics (CFD) code, FLUENT, are coupled to develop a coupled CFD-system code, permitting parts of a system, where flow is largely one-dimensional, to be analyzed using the system code and other parts, where 3D effects are important, to be analyzed using the CFD code. User-defined functions (UDFs) in FLUENT are used to couple the two codes. CFD code is the master code in the coupling topology, while nuclear system code is the slave. To overcome the sonic Courant limit, semi-implicit coupling method is employed. The coupled CFD-system code is validated by comparing the results obtained using coupled simulations, the CFD only results, and results obtained using only the system code. Minor difference in the transient mass flow rates among the simulation results is observed. The potential of the coupled code for thermal hydraulic analysis of nuclear power system is demonstrated by an application to a pressurized water reactor (PWR) model. The flexibility of the coupled code makes it an excellent tool to model other nuclear systems including GEN IV reactors.