Modeling hot channel two-phase flow of PWR NPPs during abnormal condition, using Python

•The developed code has the ability of employing fine meshing and has acceptable accuracy and speed in predicting vertical flow characteristics.•Predicting PWR coolant flow characteristics is highly dependent into the coolant heat transfer phase transition and onset of each region.•The void fraction and vapor quality are two important factors for deciding about the of entrance of coolant into two-phase regime.•The change of two-phase flow pattern based on difference between equilibrium and non-equilibrium values, depend on applied non-equilibrium model. The boiling and equilibrium characteristics of subcooled region would be considered as an important advantage in PWRs from the Thermal-Hydraulic point of view. However, the temperature gradient at some sections of reactor core and high temperature of clad surface lead to two–phase condition especially in transient conditions. The high temperature cause the heat transfer regime to change while observing the micro-vapor bubbles in some parts of the fuel channels, and a part of the heat flux changes the liquid phase to steam. This change requires the use of new models to predict fluid/wall temperature and non-equilibrium thermodynamic characteristics. In this research, the simulating of fuel channels during normal and abnormal conditions (PLOFA) was performed by employment of heat transfer models predicting the equilibrium and non-equilibrium characteristics based on the thermohydraulic code developed in Python. The comparison of the results (e.g. flow pattern and temperature profile) and the experimental data shows a good agreement for two-phase flow.