Implementation of high-fidelity neutronics and thermal-hydraulic coupling calculations in HNET

To perform nuclear reactor simulations in a more realistic manner, the coupling scheme between neutronics and thermal-hydraulics was implemented in the HNET program for both steady-state and transient conditions. For simplicity, efficiency, and robustness, the matrix-free Newton/Krylov (MFNK) method was applied to the steady-state coupling calculation. In addition, the optimal perturbation size was adopted to further improve the convergence behavior of the MFNK. For the transient coupling simulation, the operator splitting method with a staggered time mesh was utilized to balance the computational cost and accuracy. Finally, VERA Problem 6 with power and boron perturbation and the NEACRP transient benchmark were simulated for analysis. The numerical results show that the MFNK method can outperform Picard iteration in terms of both efficiency and robustness for a wide range of problems. Furthermore, the reasonable agreement between the simulation results and the reference results for the NEACRP transient benchmark verifies the capability of predicting the behavior of the nuclear reactor.