Guillaume Mignot

Break-Up of Gas Stratification in LWR Containment Induced by Negatively Buoyant Jets and Plumes

For the creation of an experimental database related to physical phenomena relevant for LWR containment safety, tests are performed in MISTRA (CEA, France) and PANDA (PSI, Switzerland) facilities in the frame of the OECD/SETH-2 project. The specific purpose of these tests is to obtain data suitable to improve and validate advanced Lumped Parameter (LP) codes as well as codes with 3D capabilities with respect to the prediction of post-accident containment thermal-hydraulic conditions. The experimental data is related to hydrogen transport within containment compartments. In particular, the effect of mass sources (the release of steam and hydrogen), heat sources (hydrogen-oxygen recombiner), and heat sinks (condensation of steam caused by containment coolers and sprays or “cold” wall) on the break-up/erosion of an initially gas stratified configuration characterized by a layer with a high hydrogen content. Helium is used to simulate hydrogen in the PANDA facility. This paper presents the result of a series of SETH-2 PANDA tests attributed to “vertical fluid release” (plumes or jets). Two large containment compartments (∼180 m3 ) connected by a bended pipe of ∼1 m diameter are used for these tests. For all the tests, a helium-steam mixture having a thickness of 2 m is created in the upper volume of one compartment while the remaining volume is filled with steam. During the tests, steam jets or plumes are created by injecting steam from a vertical pipe located at the center of the vessel 2 m below the helium-steam mixture. The jet or plume is initially positively buoyant and becomes negatively buoyant once it reaches the helium-steam layer. These transient tests show the degradation of the helium-steam layer for different jet Reynolds numbers. The initial Froude number at the injection pipe varied in the range of ∼3 to ∼9, while the estimated Froude number at the helium-steam mixture/steam interface varied from ∼0.70 to ∼2.Copyright

Large Scale Gas Stratification Erosion by a Vertical Helium-Air Jet

Containment conditions after certain postulated severe accident scenarios in nuclear power plants might result in the accumulation of hydrogen in the vessel dome. Inspired by these accident scenarios an experiment for the OECD/NEA benchmark exercise (2014) was carried out in the large scale PANDA facility at the Paul Scherrer Institut in Switzerland. The benchmark experiment was conducted at room temperature and under conditions characterized by an initially positively buoyant jet which becomes negatively buoyant while interacting with a helium layer. The experiment addresses (i) the initial conditions especially at the tube exit and (ii) the details of the entrainment of the helium stratification into the jet and the transport of the mixture towards the lower parts of the vessel. For the tube exit velocity mean and fluctuating quantities we find a reasonable agreement with pipe flow data, but a lack of agreement between past tube exit measurements and our results. It is shown that the axial velocity of the jet experiences a strong deceleration in the vicinity of the helium-rich layer and is finally stopped. Fluid accumulates in this zone and part of this fluid is flowing back in a narrow annular region around the upward flowing jet. Consequently, part of the annular flow is reentrained into the rising jet. During the layer erosion, the flow structure changes from a more downwards oriented annular type to a more horizontally oriented mushroom type of flow. It is found that locations for which we record considerable turbulent kinetic energy extends above the region where the velocity magnitude has decayed to almost zero, indicating that the jet deceleration and redirection introduces considerable turbulence in the helium stratification.

Infrared film thickness measurement: comparison with cold neutron imaging

Near InfraRed FILM thickness PROfile (NIR-FILMPRO) is an optical technique for non-intrusive measurement of water film thickness. The technique is based on absorption of NIR light. A passband filter centered at 1612 nm was selected to measure isothermal gravity-driven films flowing on a vertical wall with sand blasted surface. Non-intrusive 2D mapping of the film thickness was acquired at 200 fps, with an image size of 320×256 pixels and a spatial resolution of 0.677 mm. Theoretical developments were brought to consider multiple reflections of light in the liquid film providing a more accurate model to compute the film thickness. Further improvements were made regarding the calibration procedure and the image processing. The measurements were compared against cold neutron imaging, an established technique which provides images of the time averaged thickness. An excellent correspondence between the two methods was found. The root mean square of the deviation between the two techniques taken over a region covered by a wavy film was found to be 2.3% of the measurement with film thicknesses fluctuating between approximately 100 and 500 µm. The spatial comparison with cold neutron imaging complements the previous comparisons and validates the application of the technique.

Effect of Thermal Stratification on Full-Cone Spray Performance in Reactor Containment for a Scaled Scenario

The results of an experimental study on the nuclear reactor containment spray system are presented. Depending on the initial conditions, the spray nozzle configuration and flow rates, the spray may cause higher hydrogen concentration during depressurization due to steam condensation, or it may erode the hydrogen stratification by enhanced mixing. To investigate these phenomena, the tests are performed using a full-cone spray nozzle in PANDA facility at Paul Scherrer Institut, Switzerland. Temporal evolution and spatial distribution of the fluid temperature and the fluid concentrations are measured using thermocouples and mass spectrometers. Two tests are performed with initial vessel wall temperatures of 105°C and 135°C, which create condensing and non-condensing environments respectively. The different initial conditions lead to different density stratifications. The effect of these different density stratification on the flow patterns and mixing of gases in the vessels due to the action of the spray is revealed by these tests.© 2014 ASME

Large-Scale Containment Cooler Performance Experiments under Accident Conditions

Computational Fluid Dynamics codes are increasingly used to simulate containment conditions after various transient accident scenarios. This paper presents validation experiments, conducted in the frame of the OECD/SETH-2 project. These experiments address the combined effects of mass sources and heat sinks related to gas mixing and hydrogen transport within containment compartments. A wall jet interacts with an operating containment cooler located in the middle (M-configuration) and the top (T-configuration) of the containment vessel. The experiments are characterized by a 3-phase injection scenario. In Phase I, pure steam is injected, while in Phase II, a helium-steam mixture is injected. Finally, in Phase III, pure steam is injected again. Results for the M-configuration show helium stratification build up during Phase II. During Phase III, a positively buoyant plume emerging from the cooler housing becomes negatively buoyant once it reaches the helium-steam layer and continuously erodes the layer. For the M-configuration, a strong degradation of the cooler performance was observed during the injection of the helium/steam mixture (Phase II). For the T-configuration, we observe a mainly downwards acting cooler resulting in a combination of forced and natural convection flow patterns. The cooler performance degradation was much weaker compared with the M-configuration and a good mixing was ensured by the operation of the cooler.