An advanced new fully implicit numerical method for two-phase flow subchannel analysis based on the Drift Flux Model

Highlights: • A new fully-implicit numerical scheme is presented to simulate two phase flow in rod bundle subchannels. • The Drift Flux Model (DFM) discretized equations are solved based on the Newton method. • A set of equations is derived to calculate the exact Jacobian matrix. • The BFBT-BWR is simulated at steady-state and transient for benchmarking case studies. • Transients are simulated at different time scales without any stability limit. - Abstract: In this paper, a new and efficient fully implicit numerical model is introduced using the Drift Flux Model (DFM) for simulation of the reactor core thermal-hydraulics using subchannel analysis. Although several fully implicit models have been developed to simulate two-phase flow in a single channel, but development of an efficient model for more realistic conditions, i.e. reactor core, would be useful. Based on the benefits of the Newton method, a procedure for the accurate approximation of the inverse of the Jacobian matrix and a fully implicit numerical scheme is developed. To benchmark the present model, a well-scaled 8 × 8 rod bundle was simulated and fifteen steady-state test series and two transient cases were selected to analyses the subchannel grade void distributions in NUPEC (Nuclear Power Engineering Corporation) 8 × 8 rod bundle test facility. The steady-state void distributions predicted by the model are in agreement with the measured data for a wide range of thermal-hydraulic parameters investigated. Transient calculations were also performed with different time scales and it is concluded that the model is not subject to any time step restrictions.