Contact Author:
Dr. Sanjeev Gupta
gupta@becker-technologies.com
P:+49 6196 936-115
F:+49 6196 936-100
Koelner Strasse, 6
Eschborn Sued, 65760
Germany
Experimental investigations of BWR Suppression Pool Behaviour Under Loss of Coolant Accident Conditions
Sanjeev Gupta, Benjamin Balewski, Karsten Fischer, Gerhard Poss (Becker Technologies GmbH)
In the case of a hypothetical loss-of-coolant accident (LOCA), high pressure steam venting from a pipe break within the drywell is injected through downcomer pipes into the suppression pool. The course of accident follows distinct transient phases of the downcomer pipe clearing followed by steam discharge together with noncondenslble air into the suppression pool. Due to the released mass and energy, pressure suppression pool may be subjected to short term dynamic loads and long term thermal transients which may in turn affect its overall capacity. Since the pressure suppression pool acts as the primary BWR safety system, a better understanding of the phenomenon occurring during the progression of LOCA followed by transient release of air, steam or air/steam mixture is of vital importance.
The present paper provides an overview of the experimental series performed as part of an extensive experimental program on containment safety research, to investigate the hydrodynamic behaviour of BWR suppression pool under simplified conditions for the purpose of Computational Fluid Dynamics (CFD) and Lumped Parameter (LP) code validation. The experiments have been performed in THAI test facility, which is a cylindrical vessel of 9.2 m height, 3.2 m diameter and the gas volume of 60 m3.
The first experimental series considered the starting phase of LOCA in which air was injected through the downcomer pipe resulting into pool swelling and associated dynamic phenomenon in the suppression pool. Experiments were performed with vertical downcomer pipes of diameter 100 mm and 200 mm. Experiments also quantified the effect of an orifice installed in the blowdown pipe on bubble growth and associated dynamic loadings. Upstream air blowdown pressure was varied between 2 bara and 10 bara. The grids of fast pressure transducers were installed to capture the dynamic pressure loadings on bottom rigid surface, vessel walls, and gas space induced during the clearing of downcomer pipe and pool swelling process. Additionally, a high speed camera (1000 fps) was installed to visualize the formation and propagation of air bubble in the suppression pool and the resulting pool swelling phenomena. The second test series more specifically investigated the slow steady release of air or air/steam mixture in the suppression pool. The hydrodynamics of bubble column was measured by applying sophisticated flow visualization techniques, such as, Particle Image Velocimetry (PIV) and high speed imaging. In order to develop reliable predictive tools for bubble column design, this experimental series emphasized to predict the bubble ring formation and break-up mechanisms at the blowdown pipe outlet and the bubble size distribution near the pool surface. The test parameters varied were pool temperature, air mass flow rate and steam content. Third experimental series investigated a late phase of LOCA. Weak mixing in the pool due to low mass flow rate of steam can cause development of thermal stratification and reduction of pressure suppression pool capacity. The prediction of thermal stratification phenomena is necessary in order to develop confidence in the safety analysis of the pressure suppression pool behavior with the support of LP or CFD codes. Experiments were performed with a specific attention to study the establishment of thermal stratification in water pool as a function of pool temperature, steam mass flux, pipe submergence depth and initial pressure in the gas space of the suppression pool. Thereafter, the cooling/mixing effects of the water spray on the thermally stratified water pool were investigated. An overview of the investigated test series with main outcomes will be discussed in the frame of the present paper.