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Severe accident in a water-cooled nuclear power plant may lead to the production and release of hydrogen. If hydrogen forms flammable gas with air and steam in the containment, it may trigger more serious hydrogen explosion accident, threaten the integrity of the equipment and the containment. The behavioral characteristics of hydrogen under severe accident conditions become primarily responsible for the assessment of combustion risk. This review paper first identifies and describes various key physical phenomena associated to hydrogen behavior in the containment during a severe accident from four main aspects, including the source of hydrogen, hydrogen transport, combustion, and risk mitigation. The typical release process of hydrogen, the combustion limits of hydrogen and main risk mitigation measures are clarified comprehensively. Moreover, representative experimental facilities, related tests and key conclusions are introduced emphatically, which can provide specific guidance for future and ongoing related experimental research. Additionally, through the analysis of corresponding typical simulation studies based on LP method and CFD method, numerical methods suitable for various key phenomena are summarized and recommended. Currently, associated models realized in codes have limitations for predicting hydrogen behavior under certain conditions, which are mainly derived from the coupling effect of complex factors such as condensation, jets, and flame propagation, etc. The applicability and uncertainty of the models in these situations still need to be further evaluated and developed.

The small PWR has been paid more and more attention due to its diversity of application and flexibility in the site selection. However, the large core power density, the small containment space and the rapid accident progress characteristics make it difficult to control the containment pressure like the traditional PWR during the LOCA. The pressure suppression system has been used by the BWR since the early design, which is a suitable technique that can be applied to the small PWR. Since the configuration and operating conditions are different from the BWR, the pressure suppression system should be redesigned for the small PWR. Conducting the experiments on the scale down test facility is a good choice to reproduce the prototypical phenomena in the test facility, which is both economical and reasonable. A systematic scaling method referring to the H2TS method was proposed to determine the geometrical and thermohydraulic parameters of the pressure suppression containment response test facility for the small PWR conceptual design. The containment and the pressure suppression system related thermohydraulic phenomena were analyzed with top-down and bottom-up scaling methods. A set of the scaling criteria were obtained, through which the main parameters of the test facility can be determined.

Aerosol is one of the main forms of fission products in nuclear reactor accidents, which are derived from direct leakage of solid fission products and homogeneous and heterogeneous nucleation of gaseous fission products. Pool scrubbing may reduce the leakage of radioactive aerosol, and it is vital for the prediction of radioactivity to accurately grasp the retention efficiency. In this paper, experimental research on aerosol retention characteristics in jet regime was carried out. Titanium dioxide and cesium iodide were used to simulate radioactive aerosols with different hydrophilicity, and the retention efficiency under different inlet pressure and aerosol size was obtained. The results show that the retention efficiency of aerosol increases significantly while the inlet pressure increases from 43 kPa to 110 kPa. While the inlet pressure is higher than 110 kPa, the retention efficiency still increases with the increase of inlet pressure, although the increasing trend is not obvious. The most easily permeable range of cesium iodide aerosol is smaller than that of titania aerosol, and the retention efficiency of cesium iodide aerosol is always higher than that of titania aerosol, which may be caused by hydrophilicity. However, with the increase of stokes number, the difference of retention efficiency between the two aerosols gradually decreases. The difference between different aerosols was analyzed in this paper and it was found that the influence of hydrophilicity was negatively correlated with stokes number. The results of this research can be used to assist the validation of pool scrubbing model.

As a new filter material, metal fiber has good high-temperature resistance, moisture resistance, and radiation resistance. To explore the influence of the fiber’s primary structural parameters on the fiber’s performance, this paper experimentally studied the relationship between the flow resistance, filtration efficiency, and dust holding capacity of the metal fiber filter material and the thickness and solid volume fraction (SVF). Under the condition of ensuring uniform deposition of NaCl aerosol, we found that the deposition law of NaCl polydisperse aerosol is the same as that of monodisperse aerosol. The results show that for the same fiber diameter, increasing the fiber’s thickness and SVF will increase the flow resistance of the fiber under the same dust holding capacity; that is, reducing the dust holding capacity of the fiber. In the depth filtration, the efficiency increases rapidly due to the aerosol deposition in the fiber, while the efficiency changes gently when the filter cake is covered on the filtration surface. At the same time, for each high filtration efficiency fiber, the variation characteristics of resistance with dust holding capacity in the deep filtration stage are the same. After the filter cake is completely covered on the filter surface, the growth trend of resistance is the same, independent of the fiber’s structural parameters.

As an important part of the ventilation system of nuclear facilities, the performance of filter is of great significance to the environment and personnel safety. With the operation of the filter, the filtration mode will change from depth filtration to surface filtration. In this paper, a numerical simulation method is developed for the unsteady deposition characteristics of particles in the surface filtration. The parcel injection model is applied to accelerate the formation and growth process of dust cake on the surface of fiber filter. A series of subroutines running in FLUENT environment are developed to enhance the capabilities of computational fluid dynamics code in ANSYS. The Brownian force is added to the DPM model to calculate the particle trajectory more accurately. The function that aerosol particles are intercepted and deposited on the surface of fiber filter to form dust cake is added. The porous media model is used to simulate the fiber filter and dust cake. The porosity and resistance coefficient change with the unsteady deposition of particles. Combined with the subroutine with FLUENT software, a numerical simulation method which can be used to predict the unsteady deposition characteristics of particles in the surface filtration is formed. The numerical simulation results are compared with the experimental results, and they are basically consistent. So the developed numerical simulation method can be used to predict the unsteady deposition characteristics of particles in the surface filtration, and can also provide a theoretical basis for the structural design of pleated filter.

Under the severe accident of the nuclear power plant, hydrogen-steam mixture gas will be released into the containment compartments. In this process, there is a strong coupling relationship between mixture gas diffusion and steam condensation on the compartment wall surface. Especially for local compartments with poor connectivity, steam condensation is likely to aggravate the risk of hydrogen accumulation and combustion. This coupling phenomenon, which has not been systematically studied in previous numerical simulations or experiments, is especially complex and needs to be further studied. Moreover, the applicability of mechanistic numerical models for predicting condensation-induced transient gas transport processes also should be further confirmed. In this study, the mechanistic numerical model was evaluated through the transient hydrogen mixing experiment of the THAI facility. Based on the validated model, the transient mass transfer process of the mixture gas induced by condensation in a typical single compartment during the steam-hydrogen release process is studied. Furthermore, the influence of release condition of gas source term, wall sub-cooling on the hydrogen accumulation phenomenon in the compartment was discussed. The conclusion is helpful to further understand the transport behavior of hydrogen-air-steam mixture gas in the local single compartment under the severe accident.

In order to study the influence of the water level of the water tank on the flow instability and heat transfer character istics,which is of the low-pressure two-phase open natural circulation system. The open natural circulation system was numerically simulated and experimentally verified. Numerical simulations are calculated by using a homogeneous flow model in this study, The flow rate and heat removal power of the system under different water level conditions were calculated and recorded, and the trend of changing with the water level was obtained. The results were then compared with the experimental results. It shows that the calculated open natural circulation flow and heat removal power results are consistent with the experimental. As the liquid level decreases, the flow state of the system gradually changes from the periodic flashing state inside the adiabatic section to the stable flashing operating state, and the heat exchange power also increases gradually. times. Therefore, in the low-pressure system, reducing the liquid level of the water tank can achieve a good effect of enhancing the flow stability of the low-pressure two-phase open natural circulation, and also has a good effect on the improvement of the heat removal power of the system.

Bubble scrubbing is recommended as a potential way to remove aerosol under accident conditions not only because of its sufficient specific transfer area and considerable residence time but also the feasibility of the concept. The purpose of this paper is to illuminate the effect of operating parameters such as liquid height, gas flow rate on micro-nano soluble aerosol deposition characteristic under bubble scrubbing condition. The results indicate that a strong hygroscopic growth phenomenon happened under bubble scrubbing condition. A direct effect is change in deposition mechanism and the deviation of MPPS. Liquid height can significantly improve the removal efficiency of micro-nano aerosol and present a more obvious influence on inertial-dominant aerosol than Brownian one. The increase of gas flow rate can weaken the deposition efficiency in range of 0–0.6 m3/h but will reverse to enhancement effect when exceeding 0.6 m3/h.

The transport phenomena in packed beds are sensitive to porosity variations. However, empirical correlations in the literature are not in agreement regarding the prediction of the mean porosity for packed beds, resulting in challenges for reactor design. In this study, the mean porosity variations in a densely packed bed with 1.2

The bubble growth in boiling water-cooled packed bed reactor were determined via experiments. Oxidized carbon steel balls were heated by electromagnetic induction method and packed in the glass tube with water as coolant. A high-speed camera was employed for the visualization study of bubble growth process. The effects of corner region, inlet temperature and mass flux on the bubble growth curve were analyzed, and bubble growth curve were proposed based on certain parameters. The results indicated that value predicted by bubble growth model is good agree with experimental data.

Bubble coalescence reduces specific area and weakens the work performance of bubble column. The bubble coalescence near gas sparger which is caused mainly by bubble growing is different from the ones occurring in major liquid. Bubble coalescence efficiency near gas sparger is influenced by many factors including sparger configuration, gas flow rate, bubble deformation, solution composition, etc. This work has conducted a set of visual experiments to study the coalescence characteristics near multi-orifice plate. The experiment parameters cover a wide range of conditions including large scope of gas flow rate, different kinds of solution and orifice configurations. The experimental results suggest that coalescence time is applicable to reflect the influence of the pitch of orifices and gas flow rate on bubble coalescence efficiency. As the number of orifices increases, bubble coalescence efficiency is reduced by the disturbance from the bubbles at adjacent orifices. A hindering coefficient is used to consider the hindering effect of additives on bubble coalescence efficiency. Finally a new calculation expression is established to predict bubble coalescence efficiency near multi-orifice plate whose fundamental form is based on the logistic curve of binary response. The calculated values that refer to this calculation expression are well consistent with the experimental results.

A series of experiments have been performed to investigate thermal stratification induced by steam-air mixture vertical injection, in the range of steam mass flux 3–30 kg/m2s, air mass fraction 0–5% and submergence depth 100–300 mm. Experimental results show that: thermal stratification occurs at the region where none of chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume is strong enough. For pure steam experiments, thermal stratification occurs when the steam mass flux above 15 kg/m2s. The pool is mixed well firstly, then thermally stratified, and then re-mixed in sequence, as the water temperature increases. A little air can make thermal stratification and re-mixing occur ahead of time and more air can completely remove thermal stratification. Gas bypass induced by shallow submergence depth makes thermal stratification occur earlier and more severe. The submergence depth has little effect on thermal stratification in chugging regime.

Packed beds with large tube-to-particle diameter ratios (D/d) suffer from poor lateral heat transport. Consequently, for applications with internal heat generation, fluid with high temperature concentrates in the center of packed beds, which is unfavorable to the design of high efficiency reactors. To address this problem, a novel method is developed: a free channel—flow guiding conduit (FGC)—is constructed in the center of a packed bed using highly fluid-penetrable material. Therefore, in the presented work, CFD simulations are carried out to study the influences of FGC to a conventional packed bed on packing structure, fluid flow distribution as well as heat transfer. It shows that cold fluid with high velocities is guided directly into the original hot zones where fluid temperature is generally higher in a conventional packing. Fluid flow in the lateral direction can be promoted with the Venturi effect. Moreover, with the FGC in proper sizes, pressure drop can be reduced about 16–26%; heat transfer coefficients merely drop 4–6%. However, although favorable effects could be achieved using the FGC, it is also observed that if an oversized flow guiding conduit was imposed into a packed bed, loss of effective cooling for particles would be also resulted since excess fluid could bypass through the conduit. Consequently, unexpected fluid temperature distribution could get some recovery. These findings are helpful for understanding of transport phenomena in packed beds as well as the design of high efficiency reactors.

Thermal stratification induced by steam direct contact condensation is a significant technical concern to consider in the energy industry. Thirty experiments have been performed to investigate the effect of steam mass flux, bulk water temperature and air mass fraction on temperature distribution in the water pool. The condensation behavior and time evolution of temperature distribution are recorded and analyzed. Two condensation regimes of chugging and bubbling are observed. Three flow patterns, namely chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume, are discussed. Three thermal patterns of mixing, stratification and re-mixing are identified. A small amount of air (1–2%) could make stratification pattern occur earlier and severer. The maximum vertical temperature difference is about 30 °C in pure steam experiments and 60 °C in mixture gas experiments. Re-mixing pattern occurs when the bulk water temperature is close to 80 °C. In addition, thermal penetration depth, the velocity of thermocline moving down and the thickness of thermocline are estimated through temperature data.

The channeling effect at the container wall results in difficulties for heat transfer in the central part of packed beds with small tube-to-particle diameter ratios. To overcome this shortcoming, a new kind of packed bed is developed with radial layered configuration: a fluid-penetrable wire mesh is used to divide the container into two parts in the radial direction, which allows particles of different sizes for regional loading. Therefore, following the general rule of fluid flow distribution in packed beds, small particles could be filled in the zone at the container wall to restrain the channeling effect and large particles in the center to promote local heat transfer capacity. In the present work, to facilitate the proper functionality of new equipment using the novel method, a CFD simulation combined with the DEM method for random packing generation is conducted to study the characteristics of the new packed bed on the packing structure, fluid flow, and particles-to-fluid convective heat transfer under a high fluid velocity regime (Re ∼ 104). It finds that the new packing method could exhibit a much regular structure than the conventional packed beds. And outstanding performance is observed in pressure drop reduction and heat transfer efficiency improvement. Moreover, the well-known equations for pressure drop and convective heat transfer predictions are found not applicable to the new bed and distinctive features of fluid velocity and temperature distribution are identified. These findings are helpful for the design of high efficiency reactors.

Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure.

Steam direct contact condensation in subcooled water is an inevitable process in pressure suppression system of advanced light water reactor. Ten pure steam experiments have been performed in the range of steam mass flux 5–80 kg/m2 s and water temperature 19–96 °C. The steam condensation behave, fluid oscillation and pressure oscillation are recorded and analyzed. Prominent small bubbles are observed on the bubble surface due to the Rayleigh-Taylor instability. Vortex ring appears after aspherical bubble collapses. The upper threshold of chugging is identified by temperature data acquired continuously. The amplitude and frequency of fluid oscillation are obtained and compared with previous correlations. Two typical pressure wave shapes of continuous oscillation and pulse oscillation are identified. The pulse oscillation is induced by steam bubble collapse and observed at low water temperature. The maximum pressure pulse peak is about 1 MPa measured in the nozzle and 0.1 MPa in the pool. The pressure oscillation intensity in the water pool increases first, then decreases rapidly, and then increases slowly. It has a maximum at water temperature about 55–75 °C and the related temperature increases with the increment of the steam mass flux. The pressure oscillation dominant frequency is analyzed and compared with available correlations.