THE HYDRODYNAMICS OF TWO-PHASE FLOW IN A TUBE BUNDLE AND A BARE CHANNEL

摘要

The focus of the presented work is a severe accident scenario in which a steam generator tube rupture combined with core damage occurs in a Pressurized Water Reactor (PWR) primary loop. The core damage is accompanied by the release of radioactive aerosols and iodine species from the fuel elements. If the secondary side of the steam generator is filled with water, the aerosols and iodine may be partially removed due to interactions with the water. The test facility TRISTAN (Tube Rupture In Steam generaTor multi-phAse flow iNvestigations) was built for the experimental investigation of the hydrodynamics of the described scenarios. TRISTAN is a mockup of a steam generator bundle that is operated at ambient temperature and pressure. For bare pool tests, the bundle can be removed. The liquid inventory of the flooded steam generator is modeled using demineralized water. Two layers of a wire-mesh sensor (WMS) (120 × 120 wires) are built directly into the bundle, which provides high-resolution data for the flow structure. The goal is to study the dynamics of the interfacial area between gases and liquid, which is a key parameter for the characterization of transport phenomena in two-phase flow. The experiments allow us to study the hydrodynamics for an increasing distance from the break location and different gas flow rates and to make comparisons between the flows in tube bundles and pools. The significance of the presence of a tube bundle on the retention of radioactive aerosols was investigated previously. Aerosols are retained more efficiently when a tube bundle is present. Without a detailed understanding of the underlying hydrodynamics especially in the interfacial area, final conclusions on the effects that cause an increase in retention cannot be determined. Additionally, this work develops the data analysis methodology for the WMS data. Therefore, a novel technique was used for the first time for the measurement of the interfacial area concentration (IAC). In this paper, a comparison between the tube bundle and the bare pool experiments is presented. Because the WMS is a flow imaging technique with high temporal and spatial resolution, information regarding the averaged void fractions and its standard deviation as well as kinetic information regarding the interface velocity and interfacial area concentration can be measured. The selected cases cover one distance between the injection location and the sensor, namely, 625 mm, and two gas flow rates of 50 kg/h and 250 kg/h.