Two-phase CFD PTS validation in an extended range of thermohydraulics conditions covered by the COSI experiment

• Models for large interfaces in two-phase CFD were developed for PTS.
• The COSI experiment is used for NEPTUNE_CFD integral validation.
• COSI is a PWR cold leg scaled 1/100 for volume.
• Fifty runs are calculated, covering a large range of flow configurations.
• The CFD predicting capability is analysed using global and local measurements.

In the context of the Pressurized Water Reactors (PWR) life duration safety studies, some models were developed to address the Pressurized Thermal Shock (PTS) from the two-phase CFD angle, dealing with interfaces much larger than cells size and with direct contact condensation. Such models were implemented in NEPTUNE_CFD, a 3D transient Eulerian two-fluid model. The COSI experiment is used for its integral validation. It represents a cold leg scaled 1/100 for volume and power from a 900 MW PWR under a large range of LOCA PTS conditions. In this study, the CFD is evaluated in the whole range of parameters and flow configurations covered by the experiment. In a first step, a single choice of mesh and CFD models parameters is fixed and justified. In a second step, fifty runs are calculated. The CFD predicting capability is analysed, comparing the liquid temperature and the total condensation rate with the experiment, discussing their dependency on the inlet cold liquid rate, on the liquid level in the cold leg and on the difference between co-current and counter-current runs. It is shown that NEPTUNE_CFD 1.0.8 calculates with a fair agreement a large range of flow configurations related to ECCS injection and steam condensation.