Different scenarios of small break Loss of Coolant Accident (LOCA) for pressurised water reactors (PWR) lead to the reflux-condenser mode in which steam enters the hot leg from the reactor pressure vessel (RPV) and condenses in the steam generator. A part of the condensate flows back towards the RPV in counter current to the steam. During the reflux-condenser mode, a counter-current flow limitation (CCFL) must be prevented because this would limit the core cooling. The simulation of CCFL conditions, which is dominated by 3D effects, requires the use of a computational fluid dynamics (CFD) approach. These methods are not yet mature and have to be validated before they can be applied to nuclear reactor safety. Therefore, dedicated experimental data is needed with high resolution in space and time. In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, the “hot leg model” was built at Forschungszentrum Dresden-Rossendorf (FZD). This setup is devoted to optical measurement techniques, therefore, a flat test-section design was chosen with a width of 50 mm. The test-section outlines represent the hot leg of a German Konvoi PWR at a scale of 1:3, which corresponds to a channel height of 250 mm in the straight part of the hot leg. The test-section is mounted between two separators, one simulating the reactor pressure vessel and the other is connected to the steam generator inlet chamber. This allows to perform co-current as well as counter-current flow experiments. Moreover, the hot leg model is built in the pressure vessel of the TOPFLOW facility of FZD, which is used to perform high-pressure experiments under pressure equilibrium with the inside atmosphere of the vessel. Therefore, the test section can be designed with thin materials and equipped with big size windows like in the hot leg model. The presented air/water experiments focus on the flow structure observed in the region of the riser and of the steam generator inlet chamber at room temperature and pressures up to 3 bars. The performed high-speed observations show the evolution of the stratified interface and the distribution of the two-phase mixture (droplet and bubbles). Counter-current flow limitation, or the onset of flooding, was found by analysing the water levels measured in the separators. A confrontation with the images indicates that the initiation of flooding coincides with the reversal of the flow in the horizontal part of the hot leg due to high air velocities. Afterwards, bigger waves are generated, which develop to slugs. Furthermore, the CCFL data was compared with similar experiments and empirical correlations available in the literature. The agreement of the CCFL curve is good and indicate that the data is relevant for CFD validation purposes. The zero penetration was found at lower values of the Wallis parameter than in most of the previous work, which can be attributed to the rectangular geometry of the hot leg model.

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