Sodium-cooled fast reactors achieve higher operating temperatures (core outlet exceeding 500 degree-C) and higher power generation efficiency than light water reactors thanks to their excellent thermal conductivity and high boiling point. On the other hand, because sodium temperature is high and it reacts chemically with air and water, design countermeasures should be adopted. For example, an upper plenum of a reactor vessel (RV) is filled with argon gas to shield upper structures from sodium heat, the RV has a free liquid surface consequently. One of the multiphase flow phenomena posing a challenge is gas entrainment in which the cover gas is drawn in from the liquid surface. They may become factors disturbing core reactivity and affecting the heat transfer performance. Furthermore, although sodium does not boil during normal operation, two-phase flow phenomena are critical subjects in safety which include boiling behavior under accident conditions, high-pressure steam ejection from steam generators due to tube damage, and hydrogen gas generation resulting from water-sodium reactions.

In this series, we examine why fast reactors are interested today; and outline the challenges associated with multiphase flow which is a critical factor in realizing fast reactors with superior characteristics.

In the design of sodium-cooled fast reactors, cover gas entrainment at the free surface inside the reactor vessel is one of the key thermal-hydraulic issues. Cover gas entrained into the coolant behaves as a non-condensable gas and can disturb core reactivity while passing through the core. In addition, large amounts of cover gas entrainment may degrade the heat transfer performance of heat exchangers inside the reactor vessel. Therefore, fast reactor design usually requires suppression of gas entrainment from the liquid surface and control of the total amount of non-condensable gases (e. g., argon) bubbles entrained into the primary coolant system to acceptable levels. This paper presents a numerical analysis–based evaluation approach for cover gas entrainment phenomena in sodium-cooled fast reactors, including an evaluation method for gas entrainment induced by free-surface vortices using the evaluation tool StreamViewer and an analysis method for the transport behavior of non-condensable gases within the primary coolant system using the SYRENA code.

While liquid sodium has excellent properties as a coolant for fast reactors, it has chemical reactivity with oxygen and water. Evaluating the effects of sodium combustion that occurs when sodium leaks from a cooling system and the effects of sodium-water reaction that occurs when water or vapor leaks from a heat transfer tube of steam generator is one of the major challenges in safety assessment. The multiphysics numerical analysis methods for sodium combustion and sodium-water reaction have been developed from state-of-the-art computational science and experimental techniques. In the field of sodium combustion, the computational code, SPECTRA, was validated through the analysis of the experiments. The code was applied to the example problem of design optimization of a containment vessel. In the field of sodium-water reaction, the knowledge that contributes to phenomena elucidation and the data for code validation were obtained through the systematic experiments. The experiment on water-vapor discharging into sodium was analyzed by the multidimentional mechanistic computational code, SERAPHIM. The code predicted high-temperature region formed by a reacting jet successfully.

Sodium-cooled fast reactors (SFRs) have merits of high natural circulation capacity thanks to high thermal conductivity of liquid sodium, and lower reactor pressure than that of lightwater reactors (LWR), near atmospheric pressure, making it easy to maintain the liquid level inside the reactor vessel (RV) even in the event of coolant leak, and making it less likely to reach the loss-of-coolant scenarios in LWR. On the other hand, SFRs have the characteristic that reactivity is introduced when coolant boiling or movement of core constituents occurs within the core. Therefore, in the event of a severe accident involving core meltdown, there is a possibility of recriticality leading to mechanical energy release. To avoid this possibility, a safety design involves installing flow paths within the core to rapidly discharge molten fuel from the core, as well as a core catcher at the bottom of the RV to retain it. It is believed that molten fuel discharged outside the core interacts with sodium, and as a result of this “molten fuel-sodium interaction,” the molten fuel solidifies into fine particles and accumulates on the core catcher. This paper presents the current status of assessment of molten fuel-sodium interactions in severe accidents.

With the ultimate goal of enabling early detection and improved understanding of coolant boiling transitions in the cores of sodium-cooled fast reactors, this study seeks to establish the fundamental knowledge required for developing anomaly detection technologies associated with local core abnormalities and to demonstrate their basic feasibility. We developed a deep learning–based method and evaluated its performance in detecting the onset and transition of subcooled boiling using acoustic measurements. Acoustic data were obtained during subcooled boiling of ultrapure water, and characteristic features of the boiling phenomena were extracted using time–frequency representations. A convolutional neural network (CNN) model was constructed for classification tasks. Visualization of the model’s decision criteria using gradient-weighted class activation mapping (Grad-CAM) identified the acoustic frequency bands that the model considered important. This analysis demonstrated that the model could detect the onset of nucleate boiling with improved interpretability. Furthermore, a regression-based deep learning model was developed, demonstrating that boiling heat flux can be predicted with high accuracy.

We present an Image-Processing-based Ultrasonic Velocimetry (IPUV) technique for measuring flow velocity distributions using extremely weak acoustic scatterers, including submicron particles. Unlike conventional Doppler or time-domain correlation methods, IPUV applies two-dimensional Fourier transforms to spatiotemporal echo intensity images, achieving high sensitivity to weak echo signals. Mixing flow experiments were conducted using a variety of dispersed systems as flow tracers, including solid microparticles with different sizes and shapes, microbial suspensions, and liquid–liquid colloidal systems such as homogenized milk. The flow velocity measurement performance of IPUV was evaluated through a direct comparison with a commercially available ultrasonic velocity profiler. The results demonstrate that IPUV consistently provides stable velocity distributions under low-voltage and low-scattering conditions where conventional ultrasonic methods fail, highlighting its applicability to complex and weakly scattering dispersed media without the need for externally added tracer particles.

Jet Grouting is the ground improving method that creates solid soil columns by mixing soil with cement slurry which is injected into soil with high pressure. To maintain and improve column quality, jet performance is important. In this study, we used pressure mapping sensor and CFD simulation to analyze impact pressures of waterjet. First, we measured actual impact pressure by using mapping sensor. Second, we set up and ran CFD simulations. Third, we compared them to adjust CFD settings. As a result, we succeed in detailed measurement and highly precise simulation.

Recently, microchannel heat exchangers have begun to be used in air-conditioning units instead of ordinary fin-tube type heat exchangers. Although the microchannel heat exchanger offers several advantages, such as a large specific surface area, it also has disadvantages. One of them is the maldistribution of flow among parallel channels. To understand these phenomena, measuring the flow rate in each channel is quite important, but direct measurement has been very difficult. In this paper, the flow rate in each channel is evaluated using visualization images obtained by neutron radiography. The results provide information on flow characteristics, such as maldistribution and flow oscillation, and also confirm the validity of methods for estimating the flow rate.

Two-phase flow in a sphere-packed bed has been extensively studied to simulate the cooling process of residual core fuel and fuel debris generated during severe accidents in a nuclear reactor. Recently, pore-network models have been developed to take into account for local porosity structures in packed beds. However, experimental data remains insufficient for the further development of these models, in particular with respect to three-dimensional void fraction distributions. In this study, two-phase flow in a sphere-packed bed was investigated using X-ray tomography, and the porosity and void fraction distributions were evaluated. The porosity distribution exhibited a clear radial dependence, with relatively higher porosity observed in the near-wall region compared to the central region. Consistent with this porosity distribution, the void fraction in the near-wall region was higher than that in the central region of the packed bed. These results suggest that the liquid and gas distributions within the sphere-packed bed are strongly influenced by the local porosity distribution.

This study investigates cavitation inception caused by the backscattering of pressure waves from the gas phase, utilizing the 6-equation two-phase flow model. Although the initial conditions are established with uniform pressure and velocity fields, numerical oscillations in pressure and velocity were observed at the gas-liquid interface prior to the pressure wave reaching it. The primary objective is to identify the source of these spurious oscillations and to propose improvements to the computational algorithm. The mechanism underlying these oscillations is further analyzed from the standpoint of the model’s fundamental principle, the Abgrall criterion. Employing the improved scheme, the results demonstrate that cavitation inception near the interface - induced by the interaction between a pressure wave and a gas region - can be reliably simulated while effectively mitigating spurious oscillations. In addition to the conventional and improved schemes, a third, intermediate approach is introduced, and a comparative analysis of all three methods is conducted, emphasizing conservation of volume fraction and other pertinent factors. Ultimately, the three schemes are applied to a cavitation inception scenario characterized by opposing velocities at the boundaries. Based on these findings, a discussion is provided concerning the selection of the most appropriate method, considering the specific characteristics of the problem.

This study investigates the potential of Electrical Impedance Spectroscopy (EIS) to monitor the structural development of whipping cream. EIS and rheological measurements were conducted during the churning of cream with 30% and 40% fat contents. The data were analyzed using electrical equivalent circuit and mechanical equivalent circuit (Maxwell model). Results indicated that solution resistance R_s exhibited a strong positive correlation with overrun, demonstrating that R_s effectively reflects air incorporation. Crucially, the high-frequency resistance R_h correlated significantly with the partial elastic modulus G_p. This correlation implies that fat globule network formation via partial coalescence, which drives stiffening, also increases electrical resistance. Additionally, high-frequency capacitance C_h correlated with viscosity η_p, particularly in 30% fat cream, reflecting the increase in effective volume fraction due to fat globule networking.

Effective drainage design to prevent water droplet entry is essential for the reliable operation of automotive electronic control units (ECUs). Numerical simulations that can predict and visualize droplet behavior inside products are effective. Although previous studies have reported simulations of droplet pinning at step (edge), such approaches often require modification of internal algorithms, making them difficult to reproduce using commercial software. In this study, as an industry-friendly computational approach, we investigate a pinning prediction method based on user-controllable contact angle boundary conditions. We experimentally examine the pinning behavior of water droplets on inclined planes and at step edges, derive an empirical relationship between the Bond number (Bo) and the static contact angle θ_eq, and implement this relationship within a numerical simulation framework. We further evaluate the critical tilt angle for droplet detachment under varying droplet volumes and step-edge angles and show that the simulations reproduce the experimental results. These results confirm that the proposed combined experimental–numerical approach can predict the pinning effect induced by step-edge geometry.

The effects of ultrafine bubbles (UFBs) on water absorption and growth of hydroponically cultivated plants were investigated using sweet basil (Ocimum basilicum) and Japanese mustard spinach (Brassica rapa var. perviridis). Plants cultivated with UFB-treated nutrient solutions exhibited larger total water absorption and greater fresh weight than those grown with untreated nutrient solutions. In both plant species, increases in water absorption corresponded closely with increases in fresh weight, and the difference between UFB-treated and untreated (standard, STD) conditions became apparent during the middle stage of cultivation. However, osmotic pressure experiments using a semipermeable membrane showed that UFB application inhibited osmotic water transport. These results indicate that the increased water absorption observed in UFB-treated plants is not due to direct enhancement of water uptake processes, but rather, due to an increase in transpiration area and consequently greater water consumption. The findings suggest that UFBs contribute to hydroponic cultivation primarily through indirect growth-promoting effects. Ultrafine bubbles, Hydroponic cultivation, Water absorption, Osmotic pressure