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In-Vessel Retention (IVR) is a key technology to retain the molten core in the reactor vessel during severe accidents of Pressurized-water reactors (PWRs). In order to gain the safety margin of IVR, it is crucial to enhance the critical heat flux (CHF) of the reactor vessel, which is submerged in a water pool. To enhance the CHF, we have designed and additive-manufactured porous grid plates with a 3-D printer for design flexibility. We measured the CHF for the porous grid plate on the boiling heat transfer surface and found that the CHF was enhanced by 50 % more than that of the bare surface. The CHF enhanced more with a narrower grid pitch and a lower grid height. The visual observation study revealed that the vapor film was formed at the bottom of the grid plate.

In severe accidents in light-water reactors, the core melt, which falls to the bottom of the containment vessel, is expected to be cooled in the retained water. The particulate debris is generated after the molten material falls from the pressure vessel. For example, the cooling characteristics remain to be further elucidated in a system where particulate debris and structure material coexist and the gas-liquid two-phase flow inside particulate debris. This study investigated air–water two-phase flow inside a particulate bed and near a structure wall using a high-speed camera with a refractive index matching the transparent bed with purified water and a wire-mesh sensor (WMS). Particles with diameters of ø 3, 5, and 10 mm were used and subjected to air–water two-phase flow tests with a superficial gas velocity of 4.0–2.0×103 mm/s and superficial liquid velocity of 0.5–75.3 mm/s. Visualization results of bubble behavior near the wall and inside the bed showed the advection of bubbles from within the bed to the vicinity of the structure wall. This finding explains the higher void fraction measured in the vicinity of the structure wall using the WMS. Analysis of the void fraction distribution based on the theoretical models indicated that the higher void fraction near the structure wall observed in the case with larger particle size may be attributed to the slip velocity between the gas and liquid phases in the particulate bed. The developed techniques and insights gained in this study contribute to a detailed understanding of the thermal-hydraulic phenomena that accompany two-phase flows in particulate debris.

Ex-vessel molten fuel-coolant interaction is important for evaluating the integrity of the reactor containment vessel during a nuclear power plant accident. When molten metal discharged from the reactor vessel interacts with water in the containment vessel, the oxidation of the melt in water affects the occurrence and magnitude of a steam explosion. This study presents a small-scale experiment in which molten droplets of stainless steel mixed with zirconium for compositions ranging from 0 to 30 wt% zirconium are dropped into a water pool to clarify the oxidation characteristics of the molten droplets in water. The solidified particles with a relatively spherical shape for different metal compositions and initial pool water temperatures are obtained, and the oxygen content of the solidified particles, the thickness of the oxide layer, and the elemental distribution of the solidified particle cross-sections are evaluated. The oxide layer thickness on the surface of solidified particles and the oxygen weight percentage of solidified particles increased with the increase of zirconium content or the increase of initial pool water temperature.

Microdroplet-based fluidic systems have the advantages of small size, short diffusion time, and no cross-contamination; consequently, droplets often provide a fast and precise reaction environment as well as an analytical environment for individual molecules. In order to handle diverse reactions, we developed a method to create organic single-micron droplets (S-MDs) smaller than 5 μm in diameter dispersed in silicone oil without surfactant. The S-MD generation microflow device consists of a mother droplet (MoD) generator and a tapered separation channel featuring multiple side channels. The tapered channel enhanced the shear forces to form tails from the MoDs, causing them to break up. Surface treatment with the fluoropolymer CYTOP protected PDMS fluid devices from organic fluids. The tailing separation of methanol droplets was accomplished without the use of surfactants. The generation of tiny organic droplets may offer new insights into chemical separation and help study the scaling effects of various chemical reactions.

To understand the eutectic reaction mechanism and the relocation behavior of the core debris is indispensable for the safety assessment of core disruptive accidents (CDAs) in sodium-cooled fast reactors (SFRs). This paper addresses reaction products and their distribution of the eutectic melting/solidifying reaction of boron carbide (B4C) and stainless-steel (SS). The influence of the existence of carbon on the B4C-SS eutectic reaction was investigated by comparing the iron boride (FeB)-SS reaction by Raman spectroscopy with Multivariate Curve Resolution (MCR) analysis. The scanning electron microscopy with dispersive X-ray spectrometer was also used to investigate the elemental information of the pure metals such as Cr, Ni, and Fe. In the B4C-SS samples, a new layer was formed between B4C/SS interface, and the layer was confirmed that the formed layer corresponded to amorphous carbon (graphite) or FeB or Fe2B. In contrast, a new layer was not clearly formed between FeB and SS interface in the FeB-SS samples. All samples observed the Cr-rich domain and Fe and Ni-rich domain after the reaction. These domains might be formed during the solidifying process.

In this study, we developed a comb-shaped microfluidic device that can efficiently trap and culture a single cell (bacterium). Conventional culture devices have difficulty in trapping a single bacterium and often use a centrifuge to push the bacterium into the channel. The device developed in this study can store bacteria in almost all growth channels using the flowing fluid. In addition, chemical replacement can be performed in a few seconds, making this device suitable for culture experiments with resistant bacteria. The storage efficiency of microbeads that mimic bacteria was significantly improved from 0.2% to 84%. We used simulations to investigate the pressure loss in the growth channel. The pressure in the growth channel of the conventional device was more than 1400 PaG, whereas that of the new device was less than 400 PaG. Our microfluidic device was easily fabricated by a soft microelectromechanical systems method. The device was highly versatile and can be applied to various bacteria, such as Salmonella enterica serovar Typhimurium and Staphylococcus aureus.

The efficient synthesis of amino acid Schiff base copper(II) complexes using a microfluidic device was successfully achieved. Schiff bases and their complexes are remarkable compounds due to their high biological activity and catalytic function. Conventionally, products are synthesized under reaction conditions of 40 °C for 4 h using a beaker-based method. However, in this paper, we propose using a microfluidic channel to enable quasi-instantaneous synthesis at room temperature (23 °C). The products were characterized using UV–Vis, FT–IR, and MS spectroscopy. The efficient generation of compounds using microfluidic channels has the potential to significantly contribute to the efficiency of drug discovery and material development due to high reactivity.

We have developed a new method for measuring temperature and velocity at a high spatial resolution (minimum 2.56 mm pitch along an optical fiber). The developed method uses the same principle as a hot wire anemometer, where the velocity perpendicular to an optical fiber is estimated as a function of the cooling curve of a gold-coated layer on the optical fiber Joule-heated intermittently. The developed optical fiber sensor demonstrated the ability to acquire a transient velocity profile in airflow experiments with high repeatability and accuracy. This paper describes optical fiber-based velocity measurement in the velocity range of approximately 0–7 m/s with an error of approximately 10% compared to a hot wire anemometer and a new method for simultaneous temperature and velocity measurements. Applicability to velocity distribution measurements and seconds transient velocity changes are also described.

The void fraction distribution of a fuel rod bundle in a boiling water reactor is a critical parameter for accurately predicting the optimal thermal margin in the design of a reactor core. The rod bundle configuration, such as a part-length rod (PLR) and water rod, can affect void distribution. To clarify the influence of PLR on void fraction distribution, a boiling flow experiment was conducted using a 5 × 5 heated rod bundle that partially simulated a boiling water reactor (BWR) rod bundle, and three PLRs were arranged in the corner. The cross-sectional void fraction distribution was acquired using high-energy X-ray computed tomography at six height levels for wide flow conditions, system pressures of 0.1 − 7.2 MPa, inlet subcoolings of 20 - 90 kJ/kg, mass fluxes of 500 - 1250 kg/m2/s, and linear heat generation rates (LHGR) of 3.2 - 8.6 kW/m. In the PLR region, the local void fraction temporarily decreases because the PLRs disappear, and the flow channel rapidly expands. Together with the downstream PLRs, the voids propagate to the PLR region and concentrate in the center. The void fraction in the corner of the PLR region remains lower. A maximum 26% decrease in the subchannel void fraction was observed in the corner of the PLR region at the system pressure of 7.2 MPa, mass flux of 1.25 × 103 kg/m2/s, inlet subcooling of 50 kJ/kg, and LHGR of 8.6 kW/m.

Gas–liquid two-phase flow in a stagnant pool is an important phenomenon in designing and operating industrial facilities. When gas is mixed or boiling occurs in stagnant water, the actual water level appears higher than the original water level. The actual water level is called a two-phase mixture level and largely depends on the flow channel geometries, dimensions, and flow conditions. This study focuses on the influence of channel geometries, circular pipes and rod bundles, on the two-phase mixture level and its fluctuation behavior. An air–water experiment using circular pipes with inner diameters of 50 and 224 mm and 5 × 5 and 10 × 10 rod bundles was conducted, and the two-phase mixture level swell was visually observed. As the inlet gas flow rate increased, the two-phase mixture level basically increased regardless of the channel geometry. The fluctuation amplitude was remarkably increased by formulating the slug bubbles covering the entire diameter in the small pipe with a diameter of up to 50 mm. In the rod bundles and large pipe with a diameter of 224 mm, no slug bubble was sustained, and the two-phase water level and its fluctuation amplitude were relatively small compared with those of the small pipe.

We developed a method for passively controlling microdroplet rotation, including interior rotation, using a parallel flow comprising silicone and sesame oils. This device has a simple 2D structure with a straight channel and T-junctions fabricated from polydimethylsiloxane. A microdroplet that forms upstream moves into the sesame oil. Then, the largest flow velocity at the interface of the two oil layers applies a rotational force to the microdroplet. A microdroplet in the lower oil rotates clockwise while that in the upper oil rotates anti-clockwise. The rotational direction was controlled by a simple combination of sesame and silicone oils. Droplet interior flow was visualized by tracking microbeads inside the microdroplets. This study will contribute to the efficient creation of chiral molecules for pharmaceutical and materials development by controlling rotational direction and speed.

Liquid ZrO2 is one of the most important materials involved in severe accident analysis of a light-water reactor. Despite its importance, the physical properties of liquid ZrO2 are scarcely reported. In particular, there are no experimental reports on the viscosity of liquid ZrO2. This is mainly due to the technical difficulties involved in the measurement of thermo-physical properties of liquid ZrO2, which has an extremely high melting point. To address this problem, an aerodynamic levitation technique was used in this study. The density of liquid ZrO2 was calculated from its mass and volume, estimated based on the recorded image of the sample. The viscosity was measured by a droplet oscillation technique. The density and viscosity of liquid ZrO2 at temperatures ranging from 2753 K to 3273 K, and 3170 K–3471 K, respectively, were successfully evaluated. The density of liquid ZrO2 was found to be 4.7 g/cm 3 at its melting point of 2988 K and decreased linearly with increasing temperature, and the viscosity of liquid ZrO2 was 13 mPa at its melting point.

In recent years, research on the application of microdroplets in the fields of biotechnology and chemistry has made remarkable progress, but the technology for the stable generation of single-micrometer-scale microdroplets has not yet been established. In this paper, we developed an efficient and stable single-micrometer-scale droplet generation device based on the fragmentation of droplet tails, called “tail thread mode”, that appears under moderate flow conditions. This method can efficiently encapsulate microbeads that mimic cells and chemical products in passively generated single-micrometer-scale microdroplets. The device has a simple 2D structure; a T-junction is used for droplet generation; and in the downstream, multi-branch channels are designed for droplet deformation into the tail. Several 1–2 µm droplets were successfully produced by the tail’s fragmentation; this continuous splitting was induced by the branch channels. We examined a wide range of experimental conditions and found the optimal flow rate condition can be reduced to one-tenth compared to the conventional tip-streaming method. A mold was fabricated by simple soft lithography, and a polydimethylsiloxane (PDMS) device was fabricated using the mold. Based on the 15 patterns of experimental conditions and the results, the key factors for the generation of microdroplets in this device were examined. In the most efficient condition, 61.1% of the total droplets generated were smaller than 2 μm.

When light water reactor (LWR) is subject to a cold shutdown, it needs to be cooled with pure water or seawater to prevent the core melting. To precisely evaluate the cooling characteristics in the fuel assembly, a measurement method capable of installing to the fuel assembly structure and determining the temperature distribution with high temporal resolution, high spatial resolution, and in multidimension is required. Furthermore, it is more practical if applicable to a pressure range up to the rated pressure 16 MPa of a pressurized water reactor (PWR). In this study, we applied the principle of the wire-mesh sensor technology used in the void fraction measurement to the temperature measurement and developed a simulated fuel assembly (bundle) test loop with installing the temperature profile sensors. To investigate the measurement performance in the bundle test section, it was confirmed that a predetermined temperature calibration line with respect to time-average impedance was calculated and became a function of temperature. To evaluate the followability of measurement in a transient temperature change process, we fabricated a 16 × 16 wire-mesh sensor device and measured the hot-water jet-mixing process into the cold-water pool in real time and calculated the temperature profile from the temperature calibration line obtained in advance from each measurement point. In addition, the sensors applied to three-dimensional temperature distribution measurement of a complex flow field in the bundle structure. The axial and cross-sectional profiles of temperature were quantified in the forced flow field with nonboiling when the 5×5 bundle was heated by energization.

A subchannel void sensor (SCVS) was developed to measure the cross-sectional distribution of void fraction in a 5×5 heated rod bundle with o.d. 10 mm and heated length 2000 mm, and applied to a boiling two-phase flow experiment under the atmospheric pressure condition assuming at an accident or in a spent fuel pool in a boiling water reactor (BWR). The SCVS comprises 6-wire by 6-wire and 5-rod by 5-rod electrodes. The wire electrodes of 0.2 mm in diameter are arranged in lattice patterns between the rod bundle, while the electric conductance value in a region near one wire and another corresponds to local void fraction in the central-subchannel region. The local void fractions at 32 points (= 6×6-4) can be obtained as a cross-sectional distribution. The local void fractions near the rod surface at 100 points (= 4×25) can be also estimated by the conductance value in a region between one wire and one rod. The devised sensors are installed at five height levels along the axis to acquire two-phase flow behavior. A pair of SCVS is mounted at each level and placed 30 mm apart to estimate the one-dimensional phasic velocity distribution based on the cross-correlation analysis of both layers. The temporal resolution of void fraction measurement is 1600 frames (cross-sections) per second. The axial and radial power profile of the heated rod bundle are uniform, and eight pairs of sheath thermocouples are embedded on the heated rod to monitor its surface temperature distribution. The boiling two-phase flow experiment, which simulated a boil-off process, was conducted with the devised SCVS and experimental data was acquired under various inlet flow velocity, rod bundle power and inlet subcooling conditions. The experimental results were presented by the axial and cross-sectional distributions of void fraction, phasic velocity and bubble-chord length.

Carbon-coated SiO (SiO–C), which is a high-capacity anode material, experiences a significant capacity drop in the initial charge–discharge cycles. In contrast, Li-doped SiO–C (Li–SiO–C), which has been recently developed, exhibits a significantly smaller capacity drop. To explain this difference, we performed a detailed investigation of the structures and phase changes associated with the charge–discharge cycling of these materials by comparing their Si structures and electronic states obtained from solid-state magic-angle spinning nuclear magnetic resonance and Si K-edge X-ray absorption fine structure measurements. The results show that, in the case of SiO–C, the Li4SiO4 generated during charge is partially decomposed during discharge in the initial charge–discharge cycles. These generation and decomposition behaviors are most intense during the first 20 cycles. We believe that this phenomenon is the cause of the increased irreversible capacity observed in the initial cycles of SiO–C. In addition, we confirmed that Li2SiO3, a component of Li–SiO–C, is relatively stable electrochemically, although some of it gradually converts into Li4SiO4 during charge–discharge cycling. The presence of Li2SiO3 at the outset implies that less Li4SiO4 is generated during charging compared to SiO–C, which we believe explains the lack of a significant capacity drop in the initial cycles of Li–SiO–C. [ABSTRACT FROM AUTHOR]