Complex flow structure interaction and heat transfer process take place in the nuclear reactor cores. Given the extreme pressure/temperature and radioactive conditions in the core, numerical simulations offer an attractive (and sometimes more feasible) approach to study the related flow and heat transfer phenomena compared to experiments. Meanwhile, the rapid development and deployment of high-performance computing (HPC) facilities are helping unlock the even greater potential of massively parallel numerical simulations. Under the Exascale Computing Project, this study aims to develop suitable momentum sources that can reproduce the effects of spacer grid and mixing vanes (SGMV) on the core coolant flow. The ultimate project goal is to simulate the full core of a small modular reactor (SMR) with coupled thermal-hydraulics and neutronics. Modeling the SGMV effect without body-fitted computational grid avoids the excessive costs in resolving the local geometric details, and supports all the simulation to be scaled up to larger domain sizes. Thus, it is a crucial steppingstone to achieve the full core simulation on upcoming exascale supercomputers. This paper presents the preliminary results on the momentum source modeling as well as the roadmap for future research.