During a severe accident of a pressurized water reactor (PWR), fission products (FPs) may be released and transported to the containment. The containment spray can be utilized to remove the aerosols of FPs, which contributes to retaining the FPs in the sump. Therefore, it is important to develop an analytical model for predicting the aerosol removal efficiency by the containment spray for nuclear safety. The containment spray is required to have a very high spray coverage ratio where the droplets are expected to impact the containment side walls. In such condition, the gas flow induced by the droplets will behave differently from that in the other condition without the droplet impaction where a stable gas circulation is expected between the boundary of the spray and the side walls. Since the aerosol removal efficiency also depends on the gas flow behavior, to develop the aerosol removal model applicable for the containment spray from this viewpoint, several aerosol removal experiments were carried out in the TOSQAN IRSN facility. Based on the droplet mass flux measurements, it was confirmed that the spray coverage ratios were comparable to that of the containment spray and many droplets impacted the side wall. According to the aerosol measurement results, it was found the aerosol removal was more effective in higher spray water flow rates. Since larger particles were removed more quickly, the mean particle diameter was decreasing during the spray operation and approached to a value. Based on a CFD simulation for the experiment, gas velocity field was investigated. The measured and calculated particle number concentration decays agreed well.

The dependence of pool scrubbing performance on particle number density is studied through numerical simulation of experimental results. The DF values obtained from the authors’ experiments (Sun et al., Sci. Technol. Nucl. Inst., Article ID 1743982, 2019) indicate a sharp decrease with an increase in the inlet particle number density beyond 10 11 /m 3 . The mechanisms underlying such dependence is yet to be studied. In this paper, a simple model is developed to study the factors affecting the experimentally observed dependence of DF. The test results suggest that the condensational growth of particles plays an essential role in the inertial capture. The vapor condensation on the particles has an effect to deplete the vapor supersaturation in the bubble by both lowering the vapor concentration and raising the temperature. This effect will become important at high particle number densities. The bubble mass and energy balance is calculated to derive the particle growth and the inertial DF as a function of the bubble rise distance through the pool water. The balance is assumed to be quasi-steady, and the vapor concentration and the temperature to be uniform in the bubble. It is shown that the model reproduces the tendency observed in the experimental DF. The model predicts that the degree of supersaturation is affected when particle concentration exceeds 10 11 /m 3 , curbing the condensational growth of particles, and thereby retarding the inertial capture.

During a severe accident of a nuclear reactor, radioactive aerosols may be released from degraded nuclear fuels. Pool scrubbing is one of the efficient filters with a high aerosol removal efficiency, in other words a high decontamination factor (DF). Because of its high performance, many pool scrubbing experiments have been performed and several pool scrubbing models have been proposed. In the existing pool scrubbing experiments, an experimental condition of aerosol number concentration was seldom taken into account. It is probably because DF is assumed to be independent of aerosol number concentration, at least, in the concentration where aerosol coagulation is limited. The existing pool scrubbing models also follow this assumption. In order to verify this assumption, we performed a pool scrubbing experiment with different aerosol number concentrations under the same boundary conditions. The test section is a transparent polycarbonate pipe with an inner diameter of 0.2 m. 0.5 μm SiO 2 particles were used as aerosols. As a result, DF was increasing as decreasing the aerosol number concentration. In order to ensure a reliability of this result, three validation tests were performed with meticulous care. According to the results of these validation tests, it was indicated that DF dependence on the aerosol concentration was not because of our experimental system error including measurement instruments but a real phenomenon of the pool scrubbing.

Pool scrubbing is one of the effective mechanisms to filter out radioactive aerosols in a severe accident of a nuclear reactor. A lot of work has been done on the pool scrubbing models and experiments. However, large discrepancies still exist between the simulation and experimental results. To improve the pool scrubbing model, an accurate decontamination factor (DF) evaluation by an aerosol measurement and a detailed two-phase flow structure measurement is inevitable. A pool scrubbing experimental apparatus was constructed by the thermohydraulic safety research group in Japan Atomic Energy Agency. The test section is a transparent pipe with the inner diameter of 0.2 m and the length of about 4.5 m. The aerosol laden air flow was injected upwardly into the pool water. The aerosol particle diameter distribution was measured by a light scattering aerosol spectrometer. White polydisperse BaSO 4 particles were used as the aerosol test particles. In the first step, we focused on investigating and reducing the error of DF experimentally. Several problems resulting in the error and their solutions for the error reduction were summarized in this paper. Based on the error reduction methods, the DFs of pool scrubbing were measured in two water submergences. The results showed that the DFs for the aerosol with small diameter were independent of the injecting air velocity in the submergence of 0.3 m. In addition, it was found that the DFs increased with increasing the air flow rate in the submergence of 2.9 m. It was presumed that the increase of DF was dominated by the increase of bubble surface area and/or turbulence intensity with the air flow rate increase, while the effect of the reduced bubble traveling time in the water, which may reduce the DF, was smaller than the increasing effect.

As for the turbulent two-phase flow in the non-circular duct, the flow could show an anisotropic turbulence feature in liquid phase. In this study, the air-water bubbly turbulent upward flow experiment in the large square duct with the inside cross-section of 136mm×136mm has been conducted. Since the bubble size is very important for air-water bubbly flows, the bubble generating method was improved to get more uniform bubble size. After confirming the flow symmetry in the measuring cross-section, the distributions of local void fraction, bubble frequency and primary liquid velocity were measured by a hot-film anemometry, and the bubble behaviors were also investigated by using the high-speed video camera. The results show that the bubbles tend to accumulate to the wall region, where the liquid primary velocity shows the maximum especially at the corner.