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.
Figure 1: Operating principle of KERENA emergency condenser.
The KERENATM reactor is a boiling water reactor 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. Figure 1 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, 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.
Figure 2: Schematic view of the test section.
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 table 1.
|maximum pressure||65 bar||diameter||43,3 mm|
|maximum temperature||281 °C||length||approx. 3000 mm|
|steam mass flow||1 kg/s||declination||0,76 °|
|cold water mass flow||30 kg/s|
Table 1: Maximum operating conditions of the test set up.
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 thermocouples varied in azimuthal position connected to the wall of the condensation pipe. With this data the local heat flux will be derived.
Figure 3: Experimental set up for high pressure condensation including the x-ray tomograph.
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.
The project is carried out in the frame of a current research project in collaboration with AREVA NP GmbH and Paul Scherrer Institut (PSI).