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discovered 02_2012

COLLABORATIONS// The HZDR Research Magazine WWW.Hzdr.DE 44 45 hot water and saturated steam. Here, depending on the type of flow of the cold water being fed-in and thus the contact surface between the cold water and the steam, there will be different intensities of condensation. The mixing of the media, however, also depends on the impulse of the cold water flowing in. Additionally, there are other phenomena that play an important role: the shape and turbulence of the cold water jet or the type of steam bubbles and the extent to which they are carried away by the flow but also the turbulence in the saturated water. Furthermore, evaporation and condensation are always associated with questions of heat transfer since, for example, condensing steam gives off a large amount of heat to the water that surrounds it, whereby the water temperature can be raised considerably. Overall, the heat transfer and the type of flow depend in a complex way on the thermo-hydraulic boundary conditions, i.e. the flow rate and the temperature of the emergency core cooling water, the temperature and the filling level of the saturated water in the main coolant pipe as well as the steam density, which is determined by the system pressure. Experimental subject: French pressurized water reactor Within the framework of a consortium project that has been running since 2006 the last two and a half years have seen some very unique investigations conducted in the field of fluid dynamics by the HZDR from Germany, the energy enterprises EDF France and AREVA NP France, the publicly-funded IRSN (National Institute for Radiation Protection and Nuclear Safety) and the CEA (French Alternative Energies and Atomic Energy Commission) in France as well as the Paul Scherrer Institute (PSI) and the ETH Zurich in Switzerland. For conducting experiments with flows of steam and water that are close to reality, the so-called pressure tank at the HZDR’s TOPFLOW facility is particularly suitable: The experimental set-up uses a model with principal hydraulic components of a French pressurized water reactor to a scale of 1:2.5. It comprises a section of the cold leg with a main pipe including the pump casing, the downcomer section of the reactor pressure vessel (i.e. the area where the coolant entering the reactor flows downwards at the reactor wall) as well as the pipe for the emergency coolant. The test status section that is well equipped with measuring instrumentation delivers spatial and temporal high-resolution data regarding temperature distribution and flow pattern in the experimental set-up, for example. Over 200 different measuring points record temperatures inside the main pipe as well as inside the downcomer section of the reactor pressure vessel, an infra-red camera measures the exact temperature distribution of the pipe walls and finally a high-speed camera observes the flow of the emergency water as it enters the main pipe. Apart from that, wire-mesh sensors have been developed at the HZDR that are implemented to provide information on the gas phase - if any - within the liquid phase and record the water velocity inside this pipe. Experiments have been performed for various combinations of parameters at operating pressures of up to 50 bars, corresponding to the pressure at a water depth of 500 meters. Due to the unique technology of the pressure tank no damages were inflicted in spite of extremely high pressures SCENARIO: Following a leak with accompanying loss of coolant, emergency cooler water is fed into the main coolant circuit. If this water mixes only insufficiently with the hot saturated water, a thermal shock on the reactor pressure container’s wall results. additional emergency cooling water main coolant pipe wall of the reactor pressure vessel critical area saturated steam cooling water hot saturated water