Abstract

In some scenarios in severe accidents (SAs) of nuclear power plants (NPPs), non-condensable gas containing vapor and small particles of fission products (FPs) will be released from the reactor vessel, pass through the wet well of a suppression chamber. In these conditions, water inside the wet well will trap FPs before released to the atmosphere. This trapping effect is called pool scrubbing. Since pool scrubbing is assumed to have a good decontamination effect, researches about the pool scrubbing are important from the viewpoints of NPPs safety. Therefore, it is necessary to understand the two-phase flow behavior in pool scrubbing.

The mechanism of the FPs removal behavior in pool scrubbing is not clearly understood due to the complicated hydrodynamic phenomena. In addition, the evaluation of the validity of physical models used in pre-existing SA analysis codes are not enough. Therefore, a reliable model to evaluate particle decontamination efficiency by pool scrubbing is highly regarded.

Currently, decontamination factor (DF) of pool scrubbing is calculated by analysis code such as MELCOR. In the MELCOR code, some assumptions and empirical formula are used, e.g. bubble diameter and rising velocity. However, the correlation between DF and flow structure characteristics are not understood, and dominant factor of decontamination behavior in two phase flow is required.

Therefore, the aim of this study is to elucidate the dominant factor of decontamination by measuring DF and flow structure of bubbly plume. As to understand two phase flow characteristics during pool scrubbing, we focus on bubble diameter and swarm rise velocity. In Air-particle mixture flow injected into the experimental facility gas velocity, void fraction and bubble diameter distribution were measured by a wire mesh sensor (WMS) in each height. In addition, DF by pool scrubbing was derived from aerosol measurements by an aerosol spectrometer. The measured warm rise velocity were compared with equation of single bubble terminal velocity. The swarm rise velocity is higher than single bubble velocity. Furthermore, Large bubble are identified to be still remained at swarm region defined in MELCOR. In addition, dominant factor of decontamination behavior is defined as particle diameter, neither aerosol solubility nor particle density.

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