Single effect studies of steam condensation in a declined pipe
The nuclear accident in Fukushima Daiichi in 2011 showed the susceptibility of nuclear power plants towards a station blackout. Due to the absence of the supply with electric energy the cooling of the four reactors was not guaranteed. The decay heat leaded to a dramatically increase in temperature and pressure. This resulted in a malfunction of the containment and an emission of radiation.
Emergency cooling systems in generation II nuclear power plants are mainly active systems. They need external power to operate. This includes for example high pressure injection systems with additional pumps. Future nuclear power plants will be equipped with advanced passive safety systems. Passive safety systems do not need external energy or signals. The driving forces are based on physical laws for example gravity, pressure- and density differences and natural circulation.
The KERENATM reactor design is a boiling water reactor concept of generation III+ with 1250 MWe power. The safety concept of this future reactor design comprises passive safety systems. The heat will be removed from the reactor pressure vessel (RPV) via an emergency condenser and containment cooling condenser to a large water pool outside the containment. The figure shows the operating principle of the first heat transfer step. After a level drop in the RPV (e.g. after a pipe burst) steam enters the emergency condenser. The steam is condensed at the cold pipe wall and flows down into the RPV to provide cooling power. Because of this natural circulation loop heat from the reactor core will be transferred into the core flooding pool.
The phenomena taking place during the operation of the emergency condenser can be simplified as condensation in a declined pipe. Typical properties of this process are a large variety of flow patterns and heat transfer mechanism. The flow will develop from an annular flow, via stratified flow to a bubbly flow. The influence of the gravitational field leads to a strong azimuthal distribution of steam and liquid phase. The water film at the wall (which is important for the heat transfer) varies with the progress of condensation (pipe length) and the azimuthal position (pipe circumference).
State of the art of simulations is a one dimensional modelling. This experiment should make a contribution towards a development of a multidimensional condensation model. The experimental results with a high resolution in space and time can be used as a initial point for code improvement and validation.
Experimental set up
Main part of the test section is a double pipe with the condensation process taking place in the inner pipe. The heat sink is cooling water which is pumped through the outer annulus. At the inlet of the test section a mixing device for steam and saturated water is installed. This permits to start the condensation process with a wide variety of inlet conditions.
Schematic view of the test section with inlet mixer.
The test rig is integrated into the TOPFLOW-facility. The supply with steam and water is maintained by the TOPFLOW steam cycle. The maximum operating conditions of this configuration are depicted in the following table.
The experimental set up is equipped with an x-ray tomograph and a heat flux probe to study flow morphology and heat transfer during condensation. X-ray tomography opens up the possibility to observe cross sectional images of the local flow situation. On the basis of this data the flow pattern and fluid distribution inside the condensation pipe can be studied. The heat flux probe contains pairs of thermocouples varied in azimuthal position connected to the wall of the condensation pipe. With this data the local heat flux will be derived.
The introduced test set up is integrated into the TOPFLOW facility and equipped with advanced measurement technologies with local resolution. Therefore the process of high pressure condensation can be studied for integral characteristics and local phenomena like flow morphology and heat flux distribution.
Industrial scale experiments (pressure up to 65bar, temperature up to 281°C) of condensation in a declined pipe were performed. The underlying passive emergency condenser contains heat exchanger tubes of about 10m length to provide sufficient heat transfer area, meanwhile the TOPFLOW-condensation test facility is limited to a length of 3m. Therefore, consecutive experiments cover the necessary condensation range. Measurements for each pressure level start with pure steam at the inlet. Flowing through the cooled condensation pipe condensate forms and a two-phase flow of water and steam with a certain steam fraction establishes. The following experimental point contains the outlet flow conditions of the previous run as inlet condition. This procedure is repeated until the minimum steam quality at the outlet is reached.
One major performance indicator for passive safety systems is the condensation rate, which describes the amount of steam condensing to liquid water in a given time and thus providing cooling power. The figure provides an overview of measured condensation rates for different pressure levels. Additionally the change of the intensity of condensation during the ongoing process is illustrated. Throughout the condensation, the transferable power decreases due to an increase of liquid condensate in the flow. The liquid accumulates in the lower part of the tube and forms a flume with interfacial waves and plugs. Hence, the heat transfer area available for film condensation of steam at the cold tube wall is decreasing with decreasing steam fraction. Observing the interaction between formation and behavior of the liquid flume and the thickness and morphology of the condensate film is of major importance for the understanding of this particular thermo-hydraulic system.
The height of the forming liquid flow at the bottom of the condensation pipe is determined by a conventional x-ray tomography system. For this purpose, tomographic images at different length positions during one process condition are acquired and reconstructed. Based on the reconstructed cross-sectional void profiles the time-averaged stratification height is accessible. As an example, the stratification heights of six consecutive experiments at a pressure of 65bar are shown versus the steam fraction. Each inlet steam fraction corresponds to up to five data points (respective positions along the condensation tube). The process from pure steam to high contents of liquid condensate is highlighted by an arrow from right to left. Clearly visible is the increasing water level height inside the pipe with decreasing steam content caused by the formation of condensate. Furthermore, the high reproducibility of the mentioned method of consecutive experiments is demonstrated by an overlap between experiments with different inlet conditions.
The shown achievements are carried out in the frame of a research project (until 2012) in collaboration with AREVA NP GmbH, Paul Scherrer Institut (PSI) and the HZDR.
Based upon these findings a joint research project between HZDR, Gesellschaft für Reaktorsicherheit (GRS) and AREVA GmbH is funded by the German Federal Ministry for Education and Research (BMBF) with the grant number PANAS, FKZ: 02NUK041B. Further improvement of the applied measurement equipment and the enhancement of imaging measurement techniques, including fast x-ray tomography, increase the observations depths significant. Additionally transient experiments planned.
The authors assume the responsibility for the content.