An implicit numerical method for the three-dimensional two-fluid model with large phase changes in pressurized water reactors

An implicit numerical method is proposed for solving the three-dimensional two-fluid model with a large phase, which is widely used in two-phase flow analysis of Pressurized Water Reactors (PWRs). The phase change process is implicitly modeled by coupling it with the pressure equation derived from the momentum and mass conservation equations. Simultaneously, the convection and diffusion terms of the governing equations are treated implicitly. To reduce the size of the system matrix, the computation procedure is divided into two separate steps: the “phase-link” step and the “space-link” step. This implicit scheme has been implemented in CUPID, a three-dimensional two-phase flow analysis code developed specifically for PWR applications. The present method was verified using a conceptual problem that simulates two-phase flow blowdown and refill phenomena occurring during a Loss of Coolant Accident (LOCA). The calculations demonstrated stability even with a large Courant–Friedrichs–Lewy (CFL) number. Subsequently, the implicit scheme was validated against the FLECHT-SEASET reflood experiment. The robustness and accuracy of the implicit scheme were confirmed by comparing the simulation results with experimental data across different CFL numbers. The differences between the results obtained at various CFL numbers were negligible.