The Fukushima Daiichi accident underscored the urgent need to understand complex thermal-hydraulic phenomena governing containment integrity and gas mixture distribution during a severe accident. In response, the Japan Atomic Energy Agency (JAEA) established the CIGMA (Containment InteGral Measurement Apparatus) facility, a flagship large-scale installation capable of high-temperature, high-pressure experiments with a steamair-helium gas mixture. This paper presents key findings from a comprehensive experimental campaign with CIGMA. The JT-SJ series demonstrated the effectiveness of external surface cooling in suppressing top head flange overheating. The CC-SP series revealed spray-induced mixing mechanisms that rapidly homogenize flammable stratifications. The CC-PL series identified condensation processes of the gas mixture that are decisive for containment cooling strategies. Finally, the CC-SJ series provided insights into inter-compartment gas transport relevant to the multi-stage explosions in Unit 3 of Fukushima Daiichi. These results establish a highfidelity experimental database, offering benchmarks for CFD validation and advancing the development of robust hydrogen mitigation and accident management strategies worldwide.

The onset of significant void (OSV) is a critical parameter in the thermal-hydraulic design and safety analysis of nuclear reactors. However, existing OSV prediction models suffer from a fundamental lack of consensus regarding the underlying trigger mechanism, with various mechanistic models relying on conflicting assumptions such as bubble detachment, thermal-hydrodynamic limits, or global bubble coalescence. This study develops a novel, simplified prediction model by leveraging an artificial neural network (ANN) to resolve these uncertainties and identify the most influential physical drivers. Initially, an ANN framework was employed as a robust analytical tool to evaluate the complex interdependencies among various thermal-hydraulic parameters across an extensive experimental database. Through this systematic feature selection, the complexity of OSV prediction was distilled from numerous variables down to only two primary dimensionless parameters that capture the core physics of the phenomenon. The resulting model provides a direct prediction that transcends the limitations of conflicting theoretical assumptions while maintaining high physical consistency. Benchmarking results demonstrate that the proposed model achieves a substantial improvement over established classical correlations. Furthermore, the model exhibits exceptional geometric robustness, maintaining high predictive accuracy across round tubes, annular channels, and rectangular channels. By providing a highly accurate and physically grounded tool, this research offers a superior alternative for estimating axial void fraction profiles and enhancing safety margins in light water reactors.

This study experimentally investigates non-condensable gas transport induced by steam condensation using the CIGMA facility, simulating reactor building conditions of Fukushima Daiichi Unit 3 during a severe accident. Steam and helium, used as a hydrogen surrogate, were continuously injected into the CIGMA's vessel equipped with partition plates representing the hierarchical structure of the reactor building. Parametric experiments were conducted by varying flow path ratio, steam-to-helium mass ratio, and cooling conditions. The results show that steam condensation is the dominant mechanism controlling non-condensable gas accumulation by increasing the relative concentration of helium. The highest helium concentrations generally occur below the injection point rather than at the injection elevation, indicating downward transport followed by condensation-driven accumulation. Shapiro ternary diagram analysis indicates that condensation-driven changes in gas composition lead to prolonged residence within flammability and detonation regions. These findings highlight the critical role of condensation in hydrogen distribution and provide experimental insight relevant to hydrogen risk assessment and mitigation in reactor buildings during severe accidents.

This paper presents a study on an advanced wall function model applicable to steam condensation in the presence of air. Since condensation flows encountered in a nuclear reactor containment vessel experience suction and buoyancy forces near the wall, more general and cost-efficient wall treatments are needed to overcome the limitations of conventional wall functions relying on the log-law of the wall. In this study, we propose a subgrid-based analytical wall function, extending an original model developed for single-species flows to address suction effects and buoyancy induced by temperature variations and accumulation of non-condensable gas near the condensing wall. Validation against separate-effect test data for channel flows (WINCS, CONAN, and SETCOM) demonstrates that the proposed model generally provides reasonable agreement and low mesh sensitivity in the predicted boundary layer profiles and condensation heat flux under various conditions for suction and buoyancy effects.

The interaction of turbulent jets plays a critical role in heat and mass transfer across a variety of engineering applications. In particular, the merging phenomena of jets in nuclear reactors are important for validating the credibility of the core exit temperature (CET) as an indicator of core heat-up. CET serves as a key criterion for accident management and ensuring the safe operation of the reactor. Understanding the dynamics of these jet interactions is essential for improving reactor safety and efficiency. Furthermore, in the field of containment thermal hydraulics during severe accidents, the behavior of a jet interacting with a grid-type obstacle has attracted significant attention. This study utilized Large Eddy Simulation (LES) to gain a deeper understanding of the flow characteristics of a jet affected by such an obstacle, with particular focus on the turbulence production mechanism. To improve the accuracy of turbulence modeling, the Coherent Structure Smagorinsky Model (CSM) was employed. Validation against experimental data confirmed that LES is a valuable tool for investigating jet flow behavior. The vortex structure of the jet was visualized using the Q-criterion. Additionally, the impact of the grid-type obstacle on the turbulence kinetic energy production rate was analyzed through radial profiles at various locations and its cumulative production.

Environmental problems have become increasingly evident in post-revolutionary Iran. As a result, the field of environment has come to be a focus of research studies and technical management in the country. The recent proliferation of scientific analyses of the so-called environment indicates that the schemes of modern science are developing to combat the problems therein. Research findings also suggest, however, that environmental discourses and practices are not entirely reducible only to the terms of the natural sciences; distinctively, differing ideas of “nature” are drawn on to conceptualize differing schemes of environmental activities. This article explores how the materiality of national symbols brings to light particular social histories that reflect on and unfold through environmental discourses and practices encountered in Tehran.

This study was motivated by previous analysis by TEPSYS, which indicated that different temperatures in the 5th and 4th floors of the reactor building (R/B) of Fukushima Daiichi Unit 3 (1F3) could significantly influence the distribution of non-condensable gases during the severe accident in 2011. Understanding this effect is crucial for assessing the risks associated with non-condensable gas accumulation in R/B, particularly considering that the hydrogen explosion in 1F3 may not have been a single-stage event but could have involved a multi-stage explosion. To investigate the distribution and transport on non-condensable gas under varying cooling conditions, an experimental study was conducted using the CIGMA facility, a large-scale test vessel with segmented sections created by partition plates, designed to replicate the structure and conditions of the R/B. Steam and helium, used as non-condensable hydrogen substituted, were continuously injected at the top of the CIGMA vessel for 10,000 s to replicate the leakage of steam and hydrogen through the shield plug. Two cooling conditions were tested: Case 1 with a cooling temperature of 50 degrees C, and Case 2 with a cooling temperature of 90 degrees C. The experiments aimed to explore how varying cooling temperatures impact helium accumulation in different regions of the R/B structure. The study found that in both cases, the highest concentration of non-condensable gases may not always be near the injection point but rather in downstream regions. In these regions, steam and non-condensable gases move downward and continue to condense, significantly enhancing the concentration of helium. Specifically, in Case 1, after 10,000 s, the helium concentration reached 65 % in the middle region (representing the 4th floor of R/B) and 45 % in the top region (representing the 5th floor of R/B). Analysis using the Shapiro ternary diagram showed that, under these conditions, the gas mixture in the middle region posed a potential detonation risk. This study provides important insights into gas distribution within a nuclear reactor building during severe accident conditions, which are crucial for developing more effective safety measures and risk mitigation strategies in nuclear reactor designs.

The wall function (WF) enables analyzing condensation flow in a nuclear reactor containment vessel with reasonable computational costs. However, conventional wall treatments rely on the logarithmic laws for velocity, temperature, and concentration, limiting applicability. In this paper, we applied the analytical wall function approach to the condensation flow analysis of steam/air mixtures. This approach features the analytical integration of transport equations considering the buoyancy, the material property change, and the convective terms. We conducted CFD analysis with the analytical wall function models for the forced, mixed, and natural convection and confirmed good prediction, especially when the log law does not hold.

When analyzing containment thermal-hydraulics, computational fluid dynamics (CFD) is a powerful tool because multi-dimensional and local analysis is required for some accident scenarios. According to the previous study, neglecting steam bulk condensation in the CFD analysis leads to a significant error in boundary layer profiles. Validating the condensation model requires the experimental data near the condensing surface, however, available boundary layer data is quite limited. It is also important to confirm whether the heat and mass transfer analogy (HMTA) is still valid in the presence of bulk condensation. In this study, the boundary layer measurements on the vertical condensing surface in the presence of air were performed with the rectangular channel facility WINCS, which was designed to measure the velocity, temperature, and concentration boundary layers. We set the laminar flow condition and varied the Richardson number (1.0–23) and the steam volume fraction (0.35–0.57). The experimental results were used to validate CFD analysis and HMTA models. For the former, we implemented a bulk condensation model assuming local thermal equilibrium into the CFD code and confirmed its validity. For the latter, we validated the HMTA-based correlations, confirming that the mixed convection correlation reasonably predicted the sum of wall and bulk condensation rates.

Immersed boundary methods (IBMs) have been developed as complementary methods for computational fluid dynamics (CFD). They allow a flow simulation in a mock-up model that includes complex-shaped inner structures and/or boundaries with a non-body conformal mesh. Such a model might force us to create a complicated body-fitted mesh with a high cost in the conventional CFD (CCFD) approach. We focus on the Brinkman penalization (BP) method and its extended version, which we call here the extended Brinkman penalization method (EBP), among the different types of IBMs, aiming to apply them to the phenomena that occur during severe accidents in a nuclear reactor containment vessel and explore the possibility that the methods can partially replace the CCFD. In this paper, as a preliminary step to validate the applicability of these methods, we measure the jet flow rectified by a grating-type structure used for the validation of numerical techniques and apply them to simulate the behavior of an upward jet rectified by a horizontally placed grating-type structure modeled as an immersed body. This type of structure is generally used in reactor buildings, and it is crucial to evaluate their influence on gaseous flows because the behaviors of hydrogen produced during severe accidents may be influenced by them. The structure is selected as our subject because it has moderate complexity, enabling us to examine the effects of the IBMs and compare them with CCFD. We investigate whether these methods can reproduce a result of corresponding CCFD in which the grating is modeled as body-conformal mesh and show that the former can produce the latter with equivalent accuracy. All these results are also compared with the experimental data on the flow velocity distributions downstream of the grating measured using particle image velocimetry.

During a severe accident in a nuclear power plant, jets released from the primary system exhibit complex thermohydraulic behavior due to buoyancy effects and impingement on internal obstacles such as inner walls and floors. Thus, the obstacle-influenced jets are of interest in recent research activities. This paper describes an experimental investigation of the behavior of jets passing through a grid-type obstacle. The flow field was acquired by a particle image velocimetry system. The experiment captured the jet fragmentation by the grid-type obstacle and the jet recoupling after passing through the obstacle. The mean velocity field obtained by postprocessing indicates a “Rectifying effect,” with the axial velocity increasing at the center and the magnitude of the radial velocity decreasing. The meandering flow was suppressed due to this effect. In the near grid-obstacle region, the axial turbulence intensity was relatively large at the edge of each fragmented region due to shear stress. Moreover, the spatial distribution of the radial turbulence fluctuation became more complex. Further investigation is required to clarify the budget of the transport equation for turbulence fluctuation. In addition, the experimental data shown in this paper is useful for computational fluid dynamics validation.

Pool scrubbing is an important filtering process that prevents radioactive aerosols from entering the environment in the event of severe accidents in a nuclear reactor. In the process of transporting aerosol particles using bubbles, bubble hydrodynamics plays a crucial role in modeling pool scrubbing and significantly affects particle removal in a bubble. Our future research strategy is to apply the three-dimensional Computational Fluid Dynamics (CFD) approach to understand the detailed bubble interaction, which is difficult to be assessed experimentally. This study validates the applicability of the CFD simulation to bubble hydrodynamics at the flow transition from a globule to a swarm region. Two types of solvers based on the Volume Of Fluid (VOF) and Simple Coupled Volume Of Fluid with Level Set (S-CLSVOF) methods were used to capture the gas-liquid interface in the CFD simulation. We use the experimental data for validation. As a result, the VOF and S-CLSVOF methods accurately predict the bubble size and void fraction distributions. In addition, we confirmed that the bubble rise velocity of the S-CLSVOF method almost agrees with the experimental results. The validated code is expected to play a critical role in evaluating the constitutive equations in the stand-alone pool scrubbing code - SPARC-90.

The hydrogen behavior in a nuclear containment vessel is a significant issue when discussing the potential of hydrogen combustion during a severe accident. After the Fukushima-Daiichi accident in Japan, we have investigated in-depth the hydrogen transport mechanisms by utilizing experimental and numerical approaches. Computational fluid dynamics is a powerful tool for better understanding the transport behavior of gas mixtures, including hydrogen. This paper describes a Large-eddy simulation of gas mixing driven by a high-buoyancy flow. We focused on the interaction behavior of heat and mass transfers driven by the horizontal high-buoyant flow during density stratification. For validation, the experimental data of the Containment InteGral effects Measurement Apparatus (CIGMA) facility were used. With a high-power heater for the gas-injection line in the CIGMA facility, a high-temperature flow of approximately 390 °C was injected into the test vessel. By using the CIGMA facility, we can extend the experimental data to the high-temperature region. The phenomenological discussion in this paper helps understand the heat and mass transfer induced by the high-buoyancy flow in the containment vessel during a severe accident.

Pressurized Thermal Shock (PTS) is induced potentially by the rapid cooling of the cold-leg and downcomer wall in the primary system of a Pressurized Water Reactor (PWR) due to the initiation of Emergency Core Cooling System (ECCS). Thus, fluids mixing in a horizontal cold-leg and downcomer should be predicted accurately. However, turbulence production and damping often hinders this prediction due to the presence of the density gradients. Hence, the Fifth International Benchmark Exercise, "the cold-leg mixing Computational Fluid Dynamics (CFD) Benchmark," was conducted under the support of OECD/NEA. The experiment was designed for visualization of the mixing phenomena of two liquids with different densities. The heavy liquid was a simulant of cold water from ECCS, in a horizontal leg and downcomer. We used the Large-eddy Simulation (LES) to investigate the time fluctuation behaviors of velocity and liquid concentration. The CFD simulation was performed with two turbulence models and three different numerical meshes. We investigated the characteristics of the appearance frequency of the heavy liquid concentration with the statistical method. Based on our findings, we propose further experiments and numerical investigations to understand the fluid mixing phenomena related to PTS.

This paper describes the computational fluid dynamics (CFD) analysis and validation works from the previous experimental study on the natural convection driven by outer surface cooling in the presence of density stratification consisting of air and helium (as a mimic gas of hydrogen). The experiment was conducted in the Containment InteGral effects Measurement Apparatus (CIGMA) facility at Japan Atomic Energy Agency (JAEA). CIGMA vessel is a large cylindrical stainless steel with an inner diameter of the main cylindrical part 2.5 m and an overall height of 11 m. The mass fraction proportion of helium in the whole vessel was 11% and the helium molar fraction at the top vessel was 48%. Two experiments were performed and the numerical simulation was carried out to analyze the detailed effect of the cooling region on the erosion of the helium stratification layer. First test was named CCLP30 or case 1 and second test was named CCPL34 or case 2. The main difference between case 1 and case 2 was the cooling area of case 1 was narrower than case 2. In case 1, cooling area was only located at the one-fourth of outer vessel. Whereas, in case 2, cooling area was located at one-half of outer vessel. The temporal and spatial evolution of the helium concentration and the gas temperature inside the containment vessel was predicted and validated against the experimental data. The results indicated that the numerical predictions fairly agreed with the experimental data. However, the predicted erosion rate showed discrepancies compare with the experimental data. The relative errors time required for the complete dissolution of the helium gas were within 15%. In addition, two stratification behaviors that depend on the cooling location were presented and discussed. The CFD simulation confirmed that an upper head cooling caused two counter-rotating vortexes in the helium-rich zone. Meanwhile, the upper half body cooling caused two counter-rotating vortexes in the helium-poor zone. These findings are important to understand the mechanism of the density stratification process driven by natural convection in the containment vessel.

It is essential to improve computational fluid dynamics (CFD) analysis accuracy to estimate thermal flow in a containment vessel during a severe accident. Previous studies pointed out the importance of the initial and boundary conditions on the analysis. The purpose of this study is to evaluate the influence of initial and boundary conditions by numerical analysis of natural convection experiments. A density stratification layer was initially formed, and natural convection was induced by the external cooling of the vessel. Mixing by natural convection eroded the density stratification. We applied the RANS model with dynamic turbulent Schmidt number model. We used the measured temperature and gas concentration as the boundary and initial conditions. The erosion velocity did not change much with the initial gas distribution. The temperature boundary condition of small internal structures significantly influenced the fluid temperature distribution, and we evaluated this influence of the small internal structure quantitatively. (c) 2021 Elsevier Ltd. All rights reserved.

This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

For estimating thermal flow in a nuclear reactor during an accident accurately, it is important to improve the accuracy of computational fluid dynamics simulations. The temperature and flow velocity are not homogeneous and have large variations in a reactor containment vessel because of its very large volume. In addition, Kelm’s work pointed out that the influence of variations of initial and boundary conditions was important. Therefore, it is necessary to set the initial and boundary conditions taking into account the variations of these physical quantities. However, it is a difficult subject to set such complicated initial and boundary conditions. Then, we can obtain realistic initial and boundary conditions and an accurate flow field by data assimilation, and we can improve the accuracy of the simulation result. In this study, we applied data assimilation by a local ensemble transform Kalman filter to a simulation of natural convection behavior in density stratification, and we performed a twin model experiment. We succeeded in estimating the flow fields and improving the simulation accuracy by the data assimilation, even if we applied the boundary condition with error for the true condition.

Y Heat transfer in the rod bundle is augmented by the mixing vanes on the spacer grid. We conducted a computational fluid dynamics (CFD) simulation with three isotropic turbulence models - standard k-epsilon, realizable k-epsilon, and SST k-omega models - to investigate the relationship between heat transfer and turbulence behaviors downstream a simulant spacer (with four vanes) in a single tube under single-phase flow conditions. Quantitatively, the predicted heat transfer coefficient (HTC), secondary flow intensity, and turbulence intensity with the SST k-omega model displayed a better agreement (than the other isotropic models) with the experimental data in previous studies. Moreover, the turbulence production was localized in the near-spacer region (z/D < 10, where D is the inner diameter), which corresponds with the HTC augmentation region. These results indicate that examining the turbulence production when discussing the HTC augmentation downstream the spacer is essential. (C) 2021 The Author(s). Published by Elsevier Ltd.