Contact

Matthias Beyer
Experimental Thermal Fluid Dynamics
m.beyerAthzdr.de
Phone: +49 351 260 - 3465, 2865
Fax: 13465, 2818

Dr. Dirk Lucas
Head Computational Fluid Dynamics
d.lucasAthzdr.de
Phone: +49 351 260 - 2047
Fax: +49 351 260 - 12047

Experiments in the hot leg model of the TOPFLOW facility

Background and motivation

The reflux condenser mode and the counter-current flow limitation

In the event of a loss-of-coolant-accident (LOCA) in a pressurised water reactor (PWR), emergency strategies have to be mapped out in order to guarantee a safe removal of the decay heat from the reactor core - also in case of a component breakdown. During a hypothetical small break LOCA with failure of the high pressure emergency core cooling system (ECC) and of the main feed pumps, a natural circulation starts in the primary circuit. This allows the heat removal, also if steam is generated in the reactor core due to the depressurisation of the primary circuit. But if during the further run, the water level in the reactor pressure vessel (RPV) falls below the hot-leg inlet, only steam will flow to the steam generator. Therefore, the natural circulation breaks down and switches to the reflux condenser mode.

In the reflux condenser mode, the steam coming from the RPV condenses in the vertical U-tubes of the steam generator. In each half of the steam generator, the condensate flows down the tube in which it has been formed. Therefore, about one half of the condensate flows as usual over the pump to the downcomer, whereas the other half flows over the hot-leg back to the upper plenum. In the hot-leg, the condensate has to flow in counter-current to the steam.

The horizontal stratified counter-current flow of condensate and steam is only stable for a certain range of flow rates. If the steam flow increases too much, the condensate is clogged in the hot-leg. This is the beginning of the counter-current flow limitation (CCFL): the liquid is deflected by the steam and partially entrained in opposite direction to the steam generator. As a consequence, the hot-leg and steam generator are flood, which decreases the water level in the RPV and reduces the core cooling. In case of an additional increase of the steam flow, the condensate is completely blocked and the cooling of the reactor core from the hot-leg is impossible.

Motivation

The flow conditions governing the reflux condenser mode or the counter-current flow limitation cannot be predicted with the required accuracy and spatial resolution by the state of the art one-dimensional system codes. In order to improve the modelling of these flow regimes, computational fluid dynamics (CFD) codes are currently under development. In CFD, closure models are required that must be validated, especially if they are to be applied to reactor safety issues. The aim of our experimental investigations of stratified two-phase flows is mainly to deliver high resolution data that is needed for the validation of CFD codes.

The hot-leg model

The hot leg model
Fig. 1: Scheme of the hot-leg model installed in the pressure vessel of the TOPFLOW facility

The "hot-leg model" allows the experimental investigation of stratified two-phase flow in a complex geometry by variation of the system pressure. In fact, the hot-leg model is built in the pressure vessel of the TOPFLOW facility, which is used to perform high-pressure experiments, but under pressure equilibrium with the inside atmosphere of the vessel (Fig. 1). For steam/water experiments at pressures up to 50 bars and temperatures up to 264 °C, a special heat exchanger condenses the exhaust steam from the test section in the test vessel. Its cold end is permanently connected to the inner atmosphere of the vessel in order to guarantee full pressure equilibrium at all times. Therefore, the test section does not have to support overpressures and can be designed with thin materials.

In the hot-leg experiment, the test section is a steel frame with glass side walls for high-speed video observations. Its outlines represent a 1:3 hot-leg model of the German Konvoi-reactor and corresponds to a channel height of 250 mm (straight part of the hot-leg). The test section is mounted between two separators simulating the reactor pressure vessel and the steam generator of a pressurised water reactor. The experiments focus on the flow regimes observed in the region of the elbow and of the steam generator inlet chamber.

Experiments

Instrumentation

The boundary conditions (e.g. inlet flow rates, pressures, temperatures, or water levels in the separators) are measured with the data aquisition system of the TOPFLOW facility. Furthermore, high-speed video observation was applied from the side of the hot-leg model. An algorithm was developed to recognise the interface in the images. This allows the extraction of quantitative values from the images (e.g. the water level in any cross-section). Further statistical treatments of the data are used for comparisons between experiments and CFD.

Types of experiments

The conducted experiments can be classified as follows:

  • Simulation of the co-current two-phase flow natural convection;
  • Simulation of the reflux condenser mode up to counter-current flow limitation;
  • Generic investigation of counter-current flows for CFD validation purposes.

Results

The high-speed video observation of the flow shows the phase distribution in the bended region of the hot-leg and in the steam generator inlet chamber. As an example, the flow conditions during counter-current flow limitation with steam and saturated water at a system pressure of 50 bar and about 264°C are shown in Fig. 2. Under these conditions the flow is basically stratified, but highly mixed zones are visible: the interface has a pronounced 3-dimensional shape, bubbles are entrained in the liquid phase and droplets detach from the interface, especially at wave crests. This kind of high-resolution visualisation of steam/water flows over large windows at reactor typical boundary conditions is a world premiere.

Flow conditions observed in the hot leg model during steam/water CCFL at 50 bar
Fig. 2: Flow conditions observed in the hot-leg model during steam/water counter-current flow limitation at 50 bar

Selected references

Related links

Acknowledgements

This work is carried out in the frame of a current research project funded by the German Federal Ministry of Economics and Technology, project number 150 1329.

Contact

Dr. Dirk Lucas

Contact

Matthias Beyer
Experimental Thermal Fluid Dynamics
m.beyerAthzdr.de
Phone: +49 351 260 - 3465, 2865
Fax: 13465, 2818

Dr. Dirk Lucas
Head Computational Fluid Dynamics
d.lucasAthzdr.de
Phone: +49 351 260 - 2047
Fax: +49 351 260 - 12047