Several safety-related transients have been identified for non-light water reactors (LWR), advanced reactor designs where 3D flow patterns are important and a 1D system-level code cannot capture all relevant phenomena as compared to a 3D Computational Fluid Dynamics (CFD) code can. As a result, research efforts have been dedicated to couple 1D system-level codes with 3D CFD codes. Most efforts have been based on the separate domain approach, in which CFD is used to model the system components where 3D flow effects requires more attention, while the rest of the system is modeled with a 1D system-level code. However, this approach suffers from poor numerical robustness once not only the coupling becomes unstable but also convergence is difficult to achieve.

More recently, a novel overlapping-domain approach has been implemented where the entire system is modeled by the 1D system code, including the portion modeled also by the CFD code. This method was proven to be significantly more numerically robust than conventional separate-domain approaches. Previous coupling efforts considering both techniques has been subject of study using the system code TRACE and the CFD code STAR-CCM+.

The present work extends these strategies for targeting applications for advanced reactors. Here, the system-level code SAM and the GPU-compatible, CFD code NekRS are employed. The proposed coupling methodology between these two codes relies on a lumped parameter approach and a time-lagging scheme that couples the momentum equation of which are detailed in this study. Results are presented for both steady and transient test cases and assessment is made upon previous studies on this topic as well as standalone simulations of each code being considered. Furthermore, verification of the CFD model is also provided. Finally, the present work is in agreement with previous findings as it was found that the overlapping-domain approach provides substantial benefits when compared to the separate-domain approach.