EFFECT OF SUPPRESSION POOL CONDITIONS ON RISING BUBBLE PARAMETERS DURING WETWELL VENTING

Abstract

The severe accident in Fukushima Daiichi NPP increased the need for better understanding of an accident progression. The events after the main earthquake followed by tsunami brought the attention to the flaws in the understanding and analysis of severe accident and made it clear which key points require more studies. One of the important factors became the containment thermal hydraulics and aerosol transport. In Fukushima Daiichi Unit 1 and 3 wetwell venting was utilized as a way to minimize the release of Fission Products (FPs) to the environment. However, the monitoring post radiation measurements showed that it was not the case. The retention of FPs in the Suppression Pool due to the pool scrubbing effect is affected by the changing conditions in the pool. Thermal stratification and depressurization may influence steam and non-condensable gas carrying FPs bubble hydrodynamics while bubbles are rising through the pool. The FPs transport from the bubbles to the water pool depends not only on the retention mechanisms inside the bubble, but also on the bubble hydrodynamics. Therefore, data base of bubble diameter and bubble rising velocity under thermal stratification and depressurization conditions is important for the improvement of Pool scrubbing code hydrodynamic models. Experimental data were collected in the test facility consisting of two pressure vessels – Drywell and Wetwell – connected via downcomer pipe, steam generator, and air compressor. The mixture of steam and non-condensable gas (air) was directed from Drywell to Wetwell, while the flow was governed by the pressure difference between the vessels slightly higher than the water head in the downcomer pipe. As the first step, experiments were conducted changing the conditions in suppression pool, varying the pool temperature (uniform and thermally stratified pool), air content, and downcomer submergence under atmospheric pressure. The rising bubble hydrodynamics was analysed adopting backlit shadowgraphy technique capturing the images with high speed camera. From the recorded data, influence of varied parameters was analysed on bubble size distribution represented by Sauter Mean Diameter (SMD) and average bubble rising velocity. Results show that bigger bubbles reached the pool surface with increasing downcomer submergence. Air content had small effect on SMD, while bubble rising velocity decreased with higher air content. With increasing suppression pool temperature both SMD and bubble rising velocity were decreasing. Thermal stratification did not have significant influence for both SMD and bubble rising velocity under constant pressure condition.