In severe accidents (SAs) of nuclear power plants, release of gas containing fission products (FPs) from the reactor vessel is thought to be a major issue. As to reduce the leakage of FPs into the environment, gas containing FPs are generally discharged though the wet well, and decontaminated by the transfer effect of FPs from the gas-phase to the liquid phase. This effect is called pool scrubbing. In SA analysis codes such as MELCOR, it is predicted in the model that FP particle motion inside a single bubble, created by the bubbly flow inside the wet well as a major factor in decontamination. However, there are almost no experimental data to investigate the decontamination behavior. Therefore, in our experiment, we used an advanced M/Z interferometer in order to visualize the particle decontamination behavior by adopting Maki prism and installing a high-speed camera. However, since the interferometer experiments are not specialized in non-stable phenomena and, there were several problems to be solved. The first problem was the phase extraction method in the FFT measurement. Since the FFT information of interference is complicated, the existing extract the phase information by hand from the overall amplitude. However, since the high-speed camera visualization provide a large amount of information, this is not a realistic solution in our experiment. Therefore, in order to obtain the threshold between phase information from the overall amplitude quantitatively, we applied the Gaussian mixture model (GMM) as to cluster the data. From the measurement results, we succeed in obtaining a threshold from the fitting results of GMM. The next problem was to obtain a fine image of the bubble interface in order to obtain the decontamination behavior in the bubble interface. However, the interference image contains a stripe on the background which makes it difficult to obtain the interface information. Therefore, in our experiment, we added a LED backlight coaxial to the laser of interferometer in order to obtain the backlight image on the bubble. The interference and backlight image are divided by wave length with a dichroic mirror. We have done a synchronous visualization of interference and backlight image. A fine mask to extract the interface of bubble is obtained from the calibration and comparison of two images. Using the visualization of continuous image of particle decontamination behavior from a single bubble, the decontamination behavior of particle from a single bubble was clearly obtained. Although the existing model predict the decontamination behavior as stable, the non-stationary decontamination of particle from the bubble has been measured.

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