Spontaneous Vapor Explosion Research Articles

Research of mechanisms of interaction of melt and coolant by means of small-sized experiments with solid and liquid metal samples

Several basic fragmentation hypotheses described in the literature for the case of fine-scale spontaneous vapor explosion, when single hot drops are falling in the coolant, are analyzed. It is shown that all of them, depending on the experimental conditions, describe the real physical phenomena. It is highlighted, that the process of fragmentation during the spontaneous vapor explosion is directly connected to the regime of the film boiling collapse of the underheated liquid, which is still underexplored. The experiments on the liquid drops fragmentation are complemented by those with surface boiling at the solid metal samples of semi-spherical form. The connection between the vapor explosion of water and the increase of the surface wettability is confirmed. The video material, indicating the possibility of formation of liquid jets that strike the hot surface during the collapse of the vapor shell, is obtained. The time interval (duration of the contact) between the first contact of the coolant with the hot surface and the explosion boiling of the liquid is experimentally determined. The dependence of the contact duration on the temperature of the overheated wall is determined and its comparison with the theoretical estimations is provided.

Vapor explosion in steel mill industries: Chemical composition analysis on molten slag and coolant

AbstractIt is important to prevent vapor explosion for steel mill industries for the reason that explosions can produce overpressure which cause buildings and facilities to rupture. The spontaneous vapor explosion occurs when a molten slag contacts with coolant. In this research, the molten slag which includes more than 10 chemical compositions produced in actual steel mill industries is employed. Three types of coolant were employed which can be used in real industries such as tap water, freshwater, and yard coolant. Spontaneous vapor explosion occurred when the yard coolant, which has a higher concentration of salt than either tap water or freshwater, was used as coolant. When the vapor explosion occurred, the temperature of the yard coolant was exclusively investigated in the range of 40°C to 55°C. The vapor explosion was independent of the molten slag temperatures employed. The vapor explosion in this work was attributed to the high salt concentration of the yard coolant, which improved the heat transfer to the vapor film around the molten slag droplet. The heat transfer promoted the vapor film to collapse and triggered the spontaneous vapor explosion.

Risk Factors of Vapor Explosion of Molten Slag in Slag Yard for Steelworks

The risk of vapor explosions has always existed in steelworks that deal with a large amount of high-temperature molten slag. Previous studies on vapor explosions caused by the direct contact of high-temperature molten slag with cooling water were mainly focused on nuclear power plants, whereas there is hardly any research on vapor explosions concerning molten slag in steelworks. Therefore, there is a necessity to understand the causes of vapor explosions occurring during the work process at slag yards. This study analyzed risk factors of potential vapor explosions during the work process at slag yards in steelworks and conducted a small-scale experiment based on field conditions. The experiment found the risk factors that cause vapor explosions at the cooling stage and loading stages of the process and found that spontaneous vapor explosions occurred depending on the temperature of cooling water for molten slag and the contained salinity. Thus, it was confirmed that the significant causes of vapor explosions in the steel industry were the salinity of the cooling water concentrated by seawater desalination and high-water temperature at 50°C caused by the circulation structure of cooling water.

10511 溶融銅液滴からのフィラメント生成による膜沸騰蒸気膜崩壊ならびに蒸気爆発の発生(産学における混相研究(3),OS.13 産学における混相研究)

Experiments were conducted in which a molten copper droplet released into water pool. Spontaneous vapor explosion did not occur when water temperature was over 50℃. Spontaneous vapor explosion did, however, occur when water temperature was lower than 50℃. A high-speed video frames explored the stages of vapor explosions: (1) a vapor film was formed and separates the copper droplet and surrounding water, (2) a filament of molten copper grew from the surface and deformed the vapor film, (3) the vapor film collapsed along the filament surface, and finally (4) triggering of vapor explosions occurred from the filament to the whole molten copper droplet. When the filament growth was observed, it triggered the vapor explosion in almost all .cases. When.. not, . vapor explosion was not observed and the vapor film was, therefore, stably formed around the molten copper droplet. We concluded that the filament form the molten copper triggered vapor explosion in a highly subcooled water.

Observations of initiation stage of spontaneous vapor explosions for droplet scale

AbstractIn this study, the initiation stage of spontaneous vapor explosions generated by single droplets of molten tin submerged in water was investigated using a high‐ speed video camera operated with a reflected light system. Photographs of the formation process of vapor film, the process of vapor film disturbance, and the initiation process of the vapor explosions for different masses of molten tin and different nozzle diameters were obtained. The results demonstrate that partial thermal interaction between tin and water does not cause a vapor explosion with fragmentation. The vapor film disappears locally during the formation of the vapor film around the hot liquid droplet. Direct contact between the hot molten tin surface and water is thereby generated. However, the local disappearance of the vapor film does not progress and the vapor film is reconstructed. A vapor explosion occurs when the vapor film collapses at the local area of the bottom or edge of the disk‐shaped droplet. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 41–55, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20185

Effect of Salt Additives to Water on the Severity of Vapor Explosions and on the Collapse of Vapor Film

We proposed ultra rapid solidification and atomization technique, CANOPUS (Cooling and Atomizing based on NOble Process Utilizing Steam explosion), using small-scale vapor explosions to make an amorphous metal. The CANOPUS method is suitable for rapid cooling and atomization process, which utilizing sustainable small-scale vapor explosions. In order to apply the CANOPUS method to a high melting point metal, it is necessary to make a small-scale vapor explosion occur at a high temperature of the molten metal. Small-scale experiment is conducted to develop the vapor explosion promotor in which spontaneous vapor explosion can occur at a high temperature of a molten metal. Spontaneous vapor explosion do not occur when water at 80 o C is used as a coolant. However, spontaneous vapor explosion occurs when water at 80 o C with salt additives is used as a coolant. Specifically, lithium chloride solution generates spontaneous vapor explosions at the highest temperature of the molten tin in the experiment. In order to clarify the triggering mechanism of the spontaneous vapor explosion when the promotor is used as a coolant, a high-temperature solid stainless steel sphere is immersed into a coolant. The interfacial temperature of the stainless steel sphere is measured, and the behavior of a vapor film around the stainless steel sphere is observed with a digital video camera. As a result, salt additives resulted in an increase of quench temperature in all salt solutions. The quenching curves of each coolant indicate that the salt additives improve the film boiling heat transfer. The improvement of the film boiling heat transfer causes an unstable formation of the vapor film and a rise of the quench temperature. It is clarified that the salt additives to water promotes a vapor film collapse. Comparing two experiments, the quench temperature of each solution is in close agreement with the upper limit of the molten tin temperature that causes spontaneous vapor explosion. This result suggests that the vapor film collapse triggers spontaneous vapor explosion.

Observations of Initiation Stage of Spontaneous Vapor Explosions for Droplet Scale

In the present study, the initiation stage of spontaneous vapor explosions generated by single droplets of molten tin submerged in water has been investigated using a high speed video camera operated with reflected light system. The photographs of the formulating process of vapor film, the process of vapor film disturbance and initiation process of the vapor explosions for different mass of molten tin and different diameter of nozzle were obtained. The results show that partial thermal interaction between tin and water does not bring to vapor explosion with fragmentation, and vapor film locally disappears during the formulation term of vapor film around a hot liquid droplet. Direct contact between hot molten tin surface and water is generated in a consequence. However, the local disappearance of vapor film does not progress and the vapor film is reconstructed. A spontaneous vapor explosion begins to vaper film collapse at the local area of the bottom or edge of the disk-shaped droplet.

Thermal Hydraulic Criteria for Base-Triggered Vapor Explosion

Spontaneous vapor explosion can occur when a layer of high temperature molten material is deposited on a pool of water or on a moist floor. This is called a base-triggered vapor explosion. In order to clarify the micro-mechanism of base-triggered vapor explosions, an experimental apparatus was designed and constructed to observe the base-triggered vapor explosion from the bottom upwards. This experimental apparatus also allowed detailed observation of the microscopic behavior at the interface between the molten material and the water. The experiments used U-Alloy95 as a simulant material. As a result, it was found that water was trapped between molten material and the floor when the molten material droplet was released into the water and covered the floor. Particle imaging velocimetry (PIV) analysis and the digital auto-correlation method were applied to the observation images to evaluate the behavior of the molten material. The upward blowout velocity of the molten material was measured from the images observed from the side of the test section and the ratio of the kinetic energy to the thermal energy of the molten material was estimated from the blowout velocity.

A possible mechanism of triggering a vapor explosion

Relations are derived which enable one to assess the size of fragments of a liquid-metal droplet after its fragmentation formed in the case of instantaneous contact between a hot metal surface and a coolant. The obtained experimental data demonstrate that the amplitude of pressure pulses generated during vapor film collapse (second boiling crisis) is several times lower than the values required for triggering a spontaneous vapor explosion. The assumption is validated according to which progress in studying the process of triggering a spontaneous vapor explosion is associated primarily with advances in understanding the mechanism of fragmentation of individual droplet.

4604 高サブクール水中における銅の蒸気爆発トリガリング事象(G06-1 伝熱(1),G06 熱工学)

Experiments were conducted in which a molten copper droplet released into water pool. Spontaneous vapor explosion did not occur when water temperature was 50℃. Spontaneous vapor explosion did, however, occur at a rate of 70%, when water temperature was 20℃. A high-speed video frames explored the stages of vapor explosions: (1) a vapor film was formed and separates the copper droplet and surrounding water, (2) a filament of molten copper grew from the surface and deformed the vapor film, (3) the vapor film collapsed along the filament surface, and finally (4) triggering of vapor explosions occurred from the filament to the whole molten copper droplet. When the filament growth was observed, it triggered the vapor explosion in almost all cases. When not, vapor explosion was not observed and the vapor film was, therefore, stably formed around the molten copper droplet. We concluded that the filament form the molten copper triggered vapor explosion in a highly subcooled water.

Study on Trigger Condition of Base-triggered Vapor Explosion

Spontaneous vapor explosion can occur when a layer of the high temperature molten material lies on the water pool or on the moisture floor. This is so-called base-triggered vapor explosion. The base-triggered vapor explosion is supposed to occur in the case of a severe accident in various industrial facilities. It is very important to clarify the occurrence condition and possibility of the base-triggered vapor explosion from the viewpoints of the prediction and the prevention of the vapor explosion. In order to evaluate the occurrence conditions and to clarify the micro-mechanism of the base-triggered vapor explosion, the experimental apparatus to observe the base-triggered vapor explosion from the bottom of the floor to above is designed and constructed. The experiments using U-Alloy95 as a simulant material are conducted. Consequently, the microscopic behavior at the interface between the molten material and water can be observed in detail with this experimental apparatus. The interfacial behavior of the molten material is quantitatively evaluated by the PIV analysis and the digital auto-colrrelation method with the experimental results. The blowout velocity of the molten material at vapor explosion is evaluated from the visual data obtained on the experiment. The generated pressure at the vapor explosion is estimated by using the blowout velocity. In addition, occurrence condition of the base-triggered vapor explosion is evaluated with the thermal interaction zone (TIZ) theory.

C203 ベーストリガ蒸気爆発における超高速伝熱現象の可視観測

Spontaneous vapor explosion can occur when a layer of the high temperature molten material lies on the water pool or on the moisture floor. This is so-called base-triggered vapor explosion. The base-triggered vapor explosion is supposed to occur in the case of a severe accident in various facilities. It is very important to clarify the base-triggered vapor explosion from the viewpoints of the prediction and the prevention of the vapor explosion. In order to evaluate the behavior of the base-triggered vapor explosion, the experimental apparatus is designed and constructed. The experiments using U-Alloy95 as a simulating substance are conducted. Consequently, the behavior of the molten material can be observed in detail with this experimental apparatus. The behavior of the molten material at the vapor explosion is evaluated by image analysis. Based on the analysis results, conversion ratio, which is defined as ratio of the kinetic energy at the vapor explosion to the initial thermal energy, is evaluated.

A213 ベーストリガ蒸気爆発現象に関する研究

Spontaneous vapor explosion can occur when a layer of the high temperature molten material lies on the water pool or on the moisture floor. This is so-called base-triggered vapor explosion. This base-triggered vapor explosion is supposed to occur in the case of a severe accident in various facilities. It is very important to clarify the base-triggered vapor explosion from the viewpoints of the prediction and the prevention of the vapor explosion. In order to evaluate the behavior of the base-triggered vapor explosion, the experimental apparatus is designed and constructed. The experiments using U-Alloy95 as a simulating substance are conducted. Consequently, the behavior of the molten material can be observed in detail with this experimental apparatus. The digital auto-correlation method and PIV are also applied to the visual observation data obtained on the experiments in order to evaluate the velocity distribution of the molten material. Based on the velocity, the pressure at the vapor explosion is also evaluated.

ICONE11-36606 STUDY ON THE OCCURRENCE CONDITION OF BASE-TRIGGERED VAPOR EXPLOSION

Spontaneous vapor explosion can occur when a layer of the high temperature molten material lies on the water pool or on the moisture floor. This is so-called base-triggered vapor explosion. The base-triggered vapor explosion is supposed to occur in the case of a severe accident in a nuclear reactor and in other facilities. It is very important to clarify the occurrence possibility of the base-triggered vapor explosion from the viewpoints of the prediction and the prevention of the vapor explosion. In the present research, the occurrence conditions of the vapor explosion including the base-triggered vapor explosion are experimentally investigated. In this paper, preliminary results of the experiment of the occurrence condition of the base-triggered vapor explosion are reported besides the comparison with the existing theory.

The Trigger Mechanism of Vapor Explosion

In the present study, trigger mechanisms of the vapor explosion are experimentally investigated. The interfacial behavior between high temperature molten liquid and low temperature water are experimentally investigated by using a molten material droplet and external pressure pulse. As the results, it is indicated that spontaneous vapor explosion hardly occur in high temperature water near saturation temperature since vapor film is stable. The vapor explosion can occur even in high temperature water near saturation temperature in case that the external pressure pulse is applied to high temperature molten material. Vapor explosion can not occur when the interfacial temperature between the molten material and water is lower than the material melting temperature, even if the vapor film around the molten material is collapsed by the external pressure pulse. It is clarified that the impossibility of the trigger process for the vapor explosion can be judged by comparing the interfacial temperature and the molten material temperature. The results obtained in the present experiments are applied to the results of the large-scale experiments using uranium dioxide. The results indicate that the possibility of the vapor explosion of the uranium dioxide and water under the present LWR operational condition is extremely unlikely. It should be noted that the present criteria should be applicable in case that the melting temperature does not decrease by containing the metal component.

Effects of polymer, surfactant, and salt additives to a coolant on the mitigation and the severity of vapor explosions

Effects of mitigation and suppression of vapor explosion have been investigated by adding a surfactant, polymer, and neutral salt into the water droplet impinging onto a molten alloy pool surface. This configuration was selected to attain good reproducibility and visibility, because premixing events prior to the triggering restrained. In addition, an adequate amount of a surfactant can be adsorbed at the vapor/liquid interface compared to that in the case of conventional configurations such as a molten alloy droplet injecting into a water pool. Dilute anionic and nonionic surfactant aqueous solutions did not affect the triggering conditions or the pressure pulse generated by vapor explosion, even if the surfactant was added up to a density 25 times higher than the critical micelle concentration. Polymeric additives, which increase the viscosity of a fluid, have little suppression effect on vapor explosion. Spontaneous vapor explosion was, however, suppressed by a 200 wppm polyethylene glycol (molecular weight of 4×10 6) solution, since deposition of the solute due to cloudy-point phenomenon may stabilize the vapor film and prevent the solution from mixing finely. In order to exert this effect, molecular weight should be heavier so that a cloudy-point temperature is below the boiling point at the tested system pressure. Additionally, this threshold concentration became denser as the impingement velocity increased. Thus, a denser concentration and heavier molecular weight should be used to suppress vapor explosion when the vapor film may be subjected to large inertia and/or external force. When a neutral salt was added, the initial molten alloy temperature range where vapor explosion occurred shifted to a higher temperature and became wider. Vapor explosion was observed on the molten zinc surface, which does not explode spontaneously for water.

The Trigger Mechanism of Vapor Explosion.

In the present study, trigger mechanisms of the vapor explosion are experimentally investigated. The interfacial behavior between high temperature molten liquid and low temperature water are experimentally investigated by using a molten material droplet and external pressure pulse. As the results, it is indicated that spontaneous vapor explosion hardly occur in high temperature water near saturation temperature since vapor film is stable. The vapor explosion can occur even in high temperature water near saturation temperature in case that the external pressure pulse is applied to high temperature molten material. Vapor explosion can not occur when the interfacial temperature between the molten material and water is lower than the material melting temperature, even if the vapor film around the molten material is collapsed by the external pressure pulse. It is clarified that the impossibility of the trigger process for the vapor explosion can be judged by comparing the interfacial temperature and the molten material temperature. The results obtained in the present experiments are applied to the results of the large-scale experiments using uranium dioxide. The results indicate that the possibility of the vapor explosion of the uranium dioxide and water under the present LWR operational condition is extremely unlikely. It should be noted that the present criteria should be applicable in case that the melting temperature does not decrease by containing the metal component.

Self-Triggering Mechanism of Vapor Explosions for the Large-Scale Experiments Involving Fuel Simulant Melt

Vapor explosions are a major safety concern in the severe accident management of nuclear power plants. Here, we investigated the self-triggering mechanism and the occurrence conditions for spontaneous vapor explosions analytically. A spontaneous vapor explosion is assumed to occur when a vapor film between a hot and a cold liquid naturally collapse. We modeled a simple system consisting of a hot liquid droplet in a pool of cold liquid. The stability of perturbed oscillations in a vapor film was then investigated analytically. The model included the effect of thermal radiation on this stability for high temperature melts. The analytical results show that the stability of a vapor film is significantly affected by the emissivity of thermal radiation and by the heat transfer coefficient for the condensation of the vapor to the subcooled cold liquid. Furthermore, as the hot liquid temperature increases, thermal radiation causes the temperature region in which the vapor film is stable to increase in size. Comparisons were then made between our analytical results and those from large-scale experiments in which vapor explosions were studied for water and molten core materials in light-water reactors. They showed that our model explains the tendency of the occurrence of vapor explosions in those experiments.

Self-Triggering Mechanism of Vapor Explosions for a Molten Tin and Water System

The self-triggering mechanism of vapor explosions was investigated analytically and experimentally using molten tin and water. First, we modeled a simple droplet system consisting of a hot liquid droplet in a pool of cold liquid. Then, to model the self-triggering mechanism, we assumed that an instability (i.e. perturbed oscillation) in the vapor/cold-liquid interface produces a collapse of the vapor film, which in turn would produce a vapor explosion. To investigate the stability of perturbed oscillations in a vapor film, we did a linear stability analysis of a vapor film surrounding a hot liquid. We found that there was a region of film stability in the cold-liquid temperature where spontaneous vapor explosions did not occur. To validate our model, we experimentally determined the thermal interaction zone (TIZ) in which spontaneous vapor explosions occur. The occurrence conditions for spontaneous vapor explosions were investigated for molten tin, as the hot liquid, dropped into a water pool, as the cold...

Study on the propagation process of spontaneous vapor explosions and the behavior of induced pressure waves

Single-drop experiments are carried out with the object of studying the propagation process of a spontaneous vapor explosion and the behavior of pressure waves induced by the explosion. Both a molten tin drop-water system and the molten LiNC3 drop-ethanol system are examined. Propagation of an explosion between two tin drops is confirmed in tin-water experiments, although the apparent propagation velocity is only about 5 m/s and the explosion behavior differs largely between the two drops. In the LiNC3-ethanol system, the traveling behavior of the pressure wave in a one-dimensional channel is analyzed and the velocity through the interaction zone is measured. Full-time coherence is confirmed in a 25-drop experiment and it is revealed that the velocity depends largely on the development stage of the vapor explosion. © 1997 Scripta Technica. Inc. Heat Trans Jpn Res. 25 (2): 76–87, 1996