Highly scalable meshless multigrid solver for 3D thermal-hydraulic analysis of nuclear reactors

To predict the thermal hydraulic behavior in nuclear power plants, traditional system codes such as RELAP, MARS, and CATHARE, designed for one-dimensional two-phase flows, have been employed. However, these codes face limitations in simulating unknown phenomena or new geometries, highlighting the need for advanced three-dimensional simulations for the safety analysis of reactors, especially small modular reactors with passive safety systems. Recent efforts in multidimensional thermal hydraulic analysis have employed various codes for specific scenarios, such as pressurized thermal shock and hydrogen distribution in containment vessels. Consequently, the Korea Atomic Energy Research Institute (KAERI) has developed the CUPID code since 2007, which utilizes a two-fluid, three-field model on unstructured grids, enabling efficient large-scale simulations through MPI-based parallelization. Despite its successes, the computational efficiency of pressurebased solvers, which consume a significant portion of simulation time, is identified as a crucial area for improvement. This study introduces and verifies the meshless Geometric Multi Grid (GMG) technique, a novel approach to enhance the efficiency of solving the pressure correction equation on unstructured grids. The GMG technique, aimed at reducing computational time and increasing scalability, is compared to the conventional Bi-Conjugate Gradient method through various numerical simulations, demonstrating its effectiveness and potential as a superior alternative for thermal hydraulic simulations in nuclear power plant analysis.