Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

0In 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 the reliable removal of the decay heat from the reactor core. During a hypothetical small break LOCA with failure of the high pressure emergency core cooling system, the decay heat has to be released to the secondary circuit over the steam generators. Therefore, the primary circuit is designed to forward a natural circulation if the main coolant pumps are not available. Furthermore, if steam is generated in the primary circuit due to its depressurisation, stratified two-phase flow regimes can occur in the main cooling lines, which could be relevant for the reactor safety. It is intended that a computational fluid dynamics (CFD) approach could increase the simulation accuracy of such transient accident scenarios compared to the state of the art system codes.

In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, a model of the hot leg of a pressurised water reactor was built at Forschungszentrum Dresden-Rossendorf (FZD). The hot leg is the line connecting the reactor pressure vessel (RPV) to the steam generator (SG) and is composed of a horizontal pipe, a 50° upward bend and an inclined riser (Figure 1). The hot leg model is operated in the pressure chamber of the TOPFLOW facility of FZD (Figure 1), which is used to perform high-pressure experiments under pressure equilibrium with the inside atmosphere of the chamber. Therefore, the test section does not have to support overpressures and can be designed with thin materials. Consequently, parts of the test section could be equipped with big size windows for the application of optical observation techniques, also at reactor typical boundary conditions. In order to provide optimal observation possibilities, a flat test-section design was chosen with a width of 50 mm.

Co-current flow experiments were performed in the hot leg model, simulating a two-phase natural circulation in the primary circuit of a PWR. The experiments were done with air and water at 3.0 bar and room temperature as well as with steam and water at pressures up to 50 bar and the corresponding saturation temperature (i.e. up to 264°C). Over this range of boundary conditions, the main fluid properties vary significantly. The frequency distribution of the water level measured in the RPV simulator was used to characterise the flow in the hot leg (Figure 2). It was found that the form of the distribution informs about the stationarity of the water flow to the steam generator: the flatter the distribution, the more discontinuous the transport of water over time. This tendency was confirmed by the high-speed video observations (Figure 3), which were also used to identify the flow regime. Furthermore, Figure 2 shows a comparison between the frequency distributions obtained from the air/water and the steam/water experiments. Generally, the distributions are flatter for the cold experiments than for the hot ones. This shows that, due to the lower surface tension and viscosity, the transport of water induced by the gas is more constant in time for the steam/water flow.