Benchmark simulations on TOPFLOW-PTS experiments

The EU NURISP project (Nuclear Reactor Integrated Simulation Project) continued to improve the CFD modelling for two-phase PTS situations. For this purpose, two reference cases from the TOPFLOW-PTS experimental programme were defined: one for steady air-water and one for steady steam-water flow. The numerical simulations of the reference experiments were done using three different CFD codes: NEPTUNE_CFD, ANSYS FLUENT and ANSYS CFX. Due to technical and time constraints, three different meshes have been used with the three codes. In the air-water test, there is a temperature gradient only in the region upstream from the ECC injection point. ANSYS CFX predicted the temperature in the ECC upstream region well, while NEPTUNE_CFD and ANSYS FLUENT considerably underestimated the temperature there. In the NEPTUNE_CFD computation, a stratified flow was found in the ECC line, contrary to the experimental observation and causing higher velocity of jet, thus influencing the mixing in this area. In the ECC downstream and in the dowmcomer the fluids are well mixed. All the codes managed to predict this prefectly mixed temperature well. From the steam-water pre-test simulations, we could see that only NEPTUNE_CFD and ANSYS CFX predicted a thermal stratification at the entrance into the downcomer. The results of the steam-water test could not be validated due to lack of the experimental data. In the downcomer, NEPTUNE_CFD and ANSYS FLUENT predicted a homogeneous temperature which is very close to the saturated temperature. ANSYS CFX calculated an inhomogeneous temperature there which is considerably lower than in NEPTUNE_CFD and ANSYS FLUENT cases. Moreover, in the ANSYS CFX simulation we could see a downward flowing cold-water plume in the downcomer.