Shuichiro Miwa

Research activities on nuclear reactor physics and thermal-hydraulics in Japan after Fukushima-Daiichi accident

ABSTRACT Research and development in nuclear reactor physics and thermal-hydraulics continue to be vital parts of nuclear science and technology in Japan. The Fukushima accident not only brought tremendous change in public attitudes towards nuclear engineering and technology, but also had huge influence towards the research and development culture of scientific communities in Japan. After the Fukushima accident, thorough accident reviews were completed by independent committees, namely, Tokyo Electric Power Company (TEPCO), the Japanese government, the Diet of Japan, the Rebuild Japan Initiative Foundation, and the Nuclear and Industrial Safety Agency. Reactor physics and thermal-hydraulics divisions of Atomic Energy Society of Japan (AESJ) also issued the roadmaps after the accident. As a result, lessons learned from the accident were made clear, and a number of new research activities were initiated. The present paper reviews ongoing nuclear engineering research activities in Japanese institutes, universities, and corporations, focusing on the areas in reactor physics and thermal-hydraulics since the Fukushima accident to the present date.

Analytical Evaluation of Debris Cooling and Spreading Behaviors at Molten Core in Severe Accident

On March 11, 2011, severe accident occurred at Fukushima Daiichi Nuclear Power Plant, and Units 1 to 3 of the plant have led to core melt. That is to say, melted fuel rods and core internals fell to the bottom of the Reactor Pressure Vessel (RPV). It is also believed that molten core has leaked into the reactor containment vessel. In order to plan for a safe molten core removal from the reactor, it is important to estimate the conditions of molten core by conducting analysis. Particular importance of the analysis is to understand the mechanisms of molten core spreading-cooling processes. However, sufficient understanding of this process has not been obtained yet.The main purpose of this study is to evaluate molten metal spreading-cooling phenomena and subsequently, estimate the conditions of the molten metal. In order to achieve the purpose, the Computational Fluid Dynamics (CFD) for thermal fluid analysis, STAR-CCM+ was utilized. In the simulation of the unsteady two-phase flow, the volume of fluid model was applied for the spreading and interfacial surface formation of molten metal with the surrounding air. The key parameter for the molten metal spreading is the temperature dependent viscosity of molten metal. To assess the validity of this model, the analysis of the VULCANO VE-U7, molten metal spreading experiment, has been compared with simulation results.© 2014 ASME

Development of one-dimensional two-fluid model with consideration of void fraction covariance effect

ABSTRACT Accurate evaluation of gas-liquid two-phase flow behavior within rod bundle geometry is crucial for the safety assessment of the nuclear power plants. In safety assessment codes, two-phase flow in rod bundle geometry has been treated as a one-dimensional flow. In order to obtain the reliable one-dimensional two-fluid model, it is essential to utilize proper area-averaged models for governing equations and constitutive relations. The area-averaged interfacial drag term utilized to evaluate two-phase interfacial drag force is typically given by the drift-flux parameters which consider the velocity profile in two-phase flow fields. However, in a rigorous sense, the covariance due to void fraction profile is ignored in traditional formulations. In this paper, the rigorous formulation of one-dimensional momentum equation was derived by taking consideration of void fraction covariance, and a new set of one-dimensional momentum equation and constitutive relations for interfacial drag was proposed. The newly obtained set of formulations was embedded into TRAC-BF1 code and numerical simulation was performed to compare against the traditional model without covariance. It was found that effect of covariance was almost negligible for steady-state adiabatic conditions, but for high void fraction condition with added perturbation, the traditional model underpredicted the damping ratio at around 8%.

Hydrodynamic Characterization of Nickel Metal Foam—Effects of Pore Structure and Permeability

The structural characterization of chemical vapor deposition (CVD) nickel metal foam is presented in this study. Scanning electron microscope and post image processing were utilized to analyze the surface of the nickel metal foams. Measured data on foam unit cell, ligament thickness, projected pore diameter, and averaged porosity were obtained. The unit cell and projected pore diameters of CVD nickel metal foam possess Gaussian-like distribution. Characteristics of pore structure and its effect on permeability in the Darcian flow regime were analyzed. Results indicate that the permeability and the viscous conductivity of the CVD processed metal foam are highly affected by the porosity and ligament thickness.

Code performance with improved two-group interfacial area concentration correlation for one-dimensional forced convective two-phase flow simulation

ABSTRACT In gas–liquid two-phase flow simulation for reactor safety analysis, interfacial momentum transfer in two-fluid model plays an important role in predicting void fraction. Depending on flow conditions, a shape of the two-phase interface complicatedly evolves. One of the proposed approaches is to quantify the gas–liquid interface information using interfacial area transport equation. On the other hand, a more simplified and robust approach is to classify bubbles into two-groups based on their transport characteristics and utilize constitutive equations for interfacial area concentration for each group. In this paper, interfacial drag model based on the two-group interfacial area concentration correlations is implemented into system analysis code, and void fractions were calculated for the evaluation of numerical behaviors. The present analysis includes (1) comparison of one-group and two-group relative velocity models, (2) comparison with separate effect test database, (3) uncertainty evaluation of drag coefficient, (4) numerical stability assessment in flow regime transition, and (5) transient analysis for simulating the prototypic condition. Results showed that utilization of interfacial drag force term using constitutive equations of two-group interfacial area concentration yields satisfactory void fraction calculation results. The proposed solution technique is practical and advantageous in view of reducing the computational cost and simplifying the solution scheme.

Microscopic Fluid Dynamic Simulation of the Metal Foam Using Idealized Cell Structure

Simple, yet accurate representation of cell structure is essential when conducting a multidimensional thermo-fluid simulation on porous medium in microscopic scale. Presented in this paper is a study of the fluid dynamic simulation of the nickel metal foam’s unit cell domain using idealized cell structure. Commercially available multi-physics package, COMSOL, was utilized to conduct numerical simulation. Simplified methodology to create an idealized cell structure of metal foam is presented, and simulation results on pressure drop are discussed. Nonlinear solver in COMSOL was utilized to solve the unidirectional pressure drop and permeability across the cell structure. Obtained results showed confirmed agreement to the data obtained from the experiment and previous researchers, verifying the practicality and applicability of the proposed unit cell structure.

Experimental study of gas–liquid two-phase flow through packed bed under natural circulation conditions

Dry-out phenomena in packed beds or porous media may cause a significant digression of cooling/reaction performance in heat transfer/chemical reactor systems. One of the phenomena responsible for the dry-out in packed beds is known as the counter-current flow limitation (CCFL). In order to investigate the CCFL phenomena induced by gas–liquid two-phase flow in packed beds inside a pool, a natural circulation packed bed test facility was designed and constructed. A total of 27 experimental conditions covering various packing media sizes (sphere diameters: 3.0, 6.4 and 9.5 mm), packed bed heights (15, 35 and 50 cm) and water level heights (1.0, 1.5 and 2.0 m) were tested to examine the CCFL criteria with adiabatic air–water two-phase flow under natural circulation conditions. Both CCFL and flow reversal phenomena were observed, and the experimental data including instantaneous and time-averaged void fraction, differential pressure and superficial gas–liquid velocities were collected. The CCFL criteria were determined when periodical oscillations of void fraction and differential pressure appear. In addition, the Wallis correlation for CCFL was utilized for data analysis, and the Wallis coefficient, C, was determined experimentally from the packed bed CCFL tests. Compared to the existing data-sets in literature, the higher C values obtained in the present experiment suggest a possibly higher dry-out heat flux for natural circulation debris systems, which may be due to the water supply from both top and bottom surfaces of the packed beds. Considering the effects of bed height and hydraulic diameter of the packing media, a newly developed model for the Wallis coefficient, C, under natural circulation CCFL is presented. The present model can predict the experimental data with an averaged absolute error of ±7.9%.

Performance Evaluation of SCS for AHTR and Time Assessment of Operation Procedures

The Advanced High-Temperature Reactor (AHTR) is a new nuclear power reactor concept being investigated in some countries including the United States. The coolant is a liquid salt with a melting point of 460°C and a boiling point of 1430°C. The AHTR uses Silo Cooling System (SCS) as the decay heat removal system in a Beyond-Design-Basis Accident (BDBA). SCS has two accident mitigations. The first component is low-cost, and thick steel rings which conduct heating up the silo wall for BDBA. The second component is an annular ring of an inexpensive, solidified BDBA salt, which is heated from the bottom and melts when the temperature of the salt increases above the melting point, then flows into the silo, and floods the whole silo to its top level. SCS could make AHTR free from catastrophic accidents, where core melting or vessel failure never takes place since the BDBA salt near the top of silo passively absorbs decay heat. On the other hand, AHTR decreases its heat removal ability to avoid freezing of the salt and blocking the flow of the liquid when the temperatures are low. We performed the numerical calculation of AHTR heat removal system and evaluated whether it has the ability to remove decay heat with the robustness for a long-time cooling operation after BDBA. Furthermore, we need to build up and optimize the operation plan of SCS in AHTR, taking its thermal characteristics of this system into account. It is essential to avoid severe accidents which we can suppose as the possible catastrophic scenario.In this paper, we calculated temperature distributions using the thermal-hydraulics code developed for AHTR, and assessed the performance in a long term cooling period under BDBA conditions. Finally, we investigated the temperature distributions of the whole plant, predicting the accident scenario without air-cooled passive decay heat-removal system. We obtained important conclusion about SCS of the AHTR that its heat removal ability was enough to avoid catastrophic accidents under Loss of Heat Sink (LOHS) conditions.Copyright

Two-Phase Flow Induced Force Fluctuations on Pipe Bend

In this study, fluctuating force induced by both upward and horizontal gas-liquid two-phase flow on 90 degree pipe bend at atmospheric condition was investigated. First, the database comprised of dynamic force signals and two-phase flow parameters such as volumetric fluxes, area averaged void fraction and pressure fluctuations covering entire two-phase flow regimes was developed for both flow orientations. Then, study was conducted to develop a model which is capable of predicting the force fluctuation frequency and magnitudes particularly for the slug flow regime. The model was fundamentally developed from the local instantaneous two-fluid model which was applied to the control volume around the elbow test section. Main contribution of the force fluctuation of two-phase flow is from the momentum and pressure fluctuations for most of the flow regimes. For slug flow regime, however, water-hammer like impact was produced by the collision of liquid slug against the structure surface. In order to consider that effect, the liquid slug impact force model was developed. The model utilizes two-group interfacial area concentration correlation to treat the flow regime transition without an abrupt discontinuity. It was found that the newly developed model is capable of predicting two-phase flow induced force fluctuation and dominant frequency range with satisfactory accuracy for flow regimes up to churn-turbulent.Copyright