Abstract

Pool scrubbing is one of the severe accidents (SA) countermeasures in nuclear power plants. In pool scrubbing, gas containing Fission Products (FPs) generated in the reactor during SA is passed through the pool water in the suppression chamber to move FPs into the pool water and prevent them from being released into the surrounding environment. Since FPs have a significant negative effect on the surrounding environment, it is important to predict and evaluate how much FPs will be removed by pool scrubbing.

Currently, SA analysis codes such as MELCOR are used to evaluate FPs removal in pool scrubbing. However, the removal mechanism is not fully understood because the phenomena occurring in pool scrubbing are very complicated. It has been shown that parameters such as pool water depth and FPs size affect FPs removal rate (DF: Decontamination Factor), but there have been few experimental evaluations of the interrelationship between two-phase flow behavior and FPs removal.

Therefore, we focus on the gas-liquid interfacial area, which is one of the most important parameters in FPs transport between gas and liquid and in two-phase flow behavior, and the aim of our study is development of gas-liquid interfacial area concentration using a Wire Mesh Sensor (WMS) and clarification of relationship with DF. First, the void fraction of two-phase flow was measured using WMS, and the bubble interfaces were reconstructed to estimate the interfacial area concentration. The interfacial area concentration diffused to the surrounding as the relative distance from the nozzle increased as well as the void fraction. The overall interfacial area concentration increased with increasing the flow rate and the relative distance from the nozzle. Moreover, the results of DF measurements were compared with those of flow parameters. It was found that there was an inverse trend between the two parameters, indicating that it is necessary to consider not only the above flow parameters but also FPs transport between gas and liquid and decontamination by unsteady behavior of bubbles such as bubble breakup.

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