Multi-Mesh Approach for Geometry-Independent Neutronic/Thermal-Hydraulics Coupling Analyses Using RMC and Ansys Fluent

Abstract This paper presented a multi-mesh approach for high-fidelity and geometry-independent neutronic/thermal-hydraulics (N-TH) coupling calculation capable of various nuclear reactor analyses using Reactor Monte Carlo code (RMC) and Computational Fluid Dynamics (CFD) code Ansys Fluent. Verifications and coupling analyses were performed on various models. Customized RMC/Fluent interfaces were developed for direct data transfer through hdf5 files without text files to achieve higher transfer efficiencies and precisions. This study presented a multi-mesh approach to realize mesh mapping between solvers by separating the data statistical Q-mesh and T-mesh from the computational cells. Q-mesh tallied the power distributions in 3D structured forms using mesh tallies of RMC, while T-mesh tallied the temperatures and densities in Fluent model by the UDFs. Both solvers read data through Q-mesh and T-mesh without directly interacting with each other, and thus the geometry-independent coupling was realized. This study also considered relaxation method during iterations to stabilize coupling solutions and improve convergence. Verifications on a unit cell model showed great agreement to previous studies with a maximum fuel temperature deviation less than 0.6%. Coupling calculations performed on a light water-cooled unit cell model and a helium-cooled assembly model both converged in 3 iterations. The results showed that the coupling effects were dominated by the moderator density variation. The maximum fuel temperature of coupled results was 17.6% higher than that of decoupled results, and 7% higher than that of the coarse model, showing the conservative nature of high-fidelity coupling, and thus the high-fidelity coupling was beneficial for reactor safety.