Ralf Kapulla

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.

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.

Pool stratification and mixing induced by steam injection through spargers: analysis of the PPOOLEX and PANDA experiments

Spargers are mutli-hole injection pipes used in Boiling Water Reactors (BWR) and Advanced Pressurized (AP) reactors to condense steam in large water pools. A steam injection induces heat, momentum and mass sources that depend on the steam injection conditions and can result in thermal stratification or mixing of the pool. Thermal stratification reduces the steam condensation capacity of the pool, increases the pool surface temperature and thus the containment pressure. Development of models with predictive capabilities requires the understanding of basic phenomena that govern the behavior of the complex multi-scale system. The goals of this work are (i) to analyze and interpret the experiments on steam injection into a pool through spargers performed in the large-scale facilities of PPOOLEX and PANDA, and (ii) to discuss possible modelling approaches for the observed phenomena. A scaling approach was developed to address the most important physical phenomena and regimes relevant to prototypic plant conditions. The focus of the tests was on the low steam mass flux and oscillatory bubble condensation regimes, which are expected during a long-term steam injection transient, e.g. in the case of a Station Black Out (SBO). Exploratory tests were also done for chugging and stable jet conditions. The results showed a similar behavior in PPOOLEX and PANDA in terms of jet induced by steam condensation, pool stratification, and development of hot layer and erosion of the cold one. A correlation using the Richardson number is proposed to model the erosion rate of the cold layer as a function of the pool dimensions and steam injection conditions.