Pre-test CFD simulations on TOPFLOW-PTS experiments with ANSYS CFX 12.0

Pressurized Thermal Shock (PTS) has been identified as one of the most important industrial needs related to nuclear reactor safety. The PTS analysis requires the simulation of the thermal mixing of cold Emergency Core Cooling (ECC) water injected to the cold leg and flowing to the downcomer with the hot coolant, which is present in the primary circuit. The simulation of single-phase and two-phase PTS situations including e.g. stratification of the flow and direct contact condensation is a challenge for CFD methods and requires careful validation against experimental data. In the frame of the NURISP project attempts are made to improve the CFD modelling for two-phase PTS situations. For this purpose, two reference cases with and without mass transfer due the condensation were defined, which are related to the TOPLOW PTS experiments. The present paper focuses on numerical investigations of thermal mixing process in the cold leg and the downcomer using homogeneous and inhomogeneous models for the resolution of momentum equations. Numerical simulations were performed by using the commercial CFD code ANSYS CFX 12.0.