TOPFLOW-PTS air-water experiments on the stratification in the ECC nozzle and the ECC water mixing during PTS scenarios

Pressurized Thermal Shock (PTS) occurs in Pressurized Water Reactors (PWR) during hypothetical Small Break Loss of Coolant Accidents (SBLOCA) with the action of the Emergency Core Cooling System (ECCS). For reactor designs where cold water is injected into the cold leg, insufficient mixing can lead to critical thermal stress in the hot RPV wall. The mixing process involves several thermal hydraulic phenomena, such as direct contact condensation (DCC), entrainment of steam bubbles and multi-scale momentum transfer, and is therefore difficult to model. Conventional 1D system codes are not designed to resolve flows, which are governed by 3D multiphase phenomena. Therefore Computational Fluid Dynamics (CFD) codes are currently being qualified for such problems.
For the validation of CFD models and simulations related to PTS problems, the TOPFLOW-PTS project is running in cooperation with AREVA, CEA, EDF, ETHZ, IRSN and PSI. There, an experimental installation was implemented inside the TOPFLOW pressure tank to run scaled down steam-water and air-water experiments. The PTS assembly is a model of a French 900 MWe PWR including downcomer, cold leg with ECC injection and a pump simulator - all scaled at 1:2.5. (See [1] for details) The setup is highly instrumented with sophisticated special instrumentation. Temperature profiles in the flow domain are measured with about 200 distributed thermocouples and the cold leg wall is observed by an infrared camera system. Furthermore high-speed camera observation and wire mesh sensors record the flow structure in the cold leg.
In a first project phase several air-water experiments have been carried out which were intended to give a general insight into the flow structures for some parameter variations. These experiments indicated the flow stratification phenomena in the ECC line and thermal mixing in the cold leg in dependence on the ECC mass flow rate, the system pressure (gas density) and other parameters. One of the most important determinants for thermal mixing is the jet momentum at the impact position, which mainly depends on the jet’s mass flow rate. It turned out that there can be different flow regimes in the ECC line which need to be considered in boundary condition definitions for CFD calculations. Therefore this effect was particularly studied utilizing an ultrasound sensor together with simultaneous high-speed camera observations of the impingement point and the water surface. A digital bubble tracking velocimetry method was then applied to compare different flow regimes in the cold leg.