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In a severe accident, the molten core material enters the lower head with water or the reactor cavity is cooled and forms a debris bed. The types of the debris bed formed under different conditions are different, including loose granular, or agglomerated cake. Regardless of the types of debris bed, it is desirable to be able to be cooled to prevent further melting. Therefore, the debris bed formation experiment is a very worthy study, as it is a preliminary input condition for evaluating the cool ability characteristics, possibility of remelting, and cooling measures of the debris bed. A test facility was built to study the characteristics of formation of the debris bed in situation of PWR severe accident. The test facility could generate 100 kg molten ZrO2 as corium simulant with melting temperature of 2 700 ℃.The core test section included melting device and debris bed formation tank (water tank).The melting device melted the ZrO2 by electromagnetic induction, with the technology of cold crucible. The water tank structure was a stainless steel frame with segmented parts. There were sealed glass windows on both sides of the tank, so the progress of fuel-coolant interaction can be observed. Under these conditions, the experiments of formation of large mass debris bed were carried out. The main research includes different combinations of molten mass, jet diameter, pool depth and subcooling degree of water. So far, four experimental conditions were carried out. Based on these four conditions, the parameters of debris bed structure, debris shape, debris size distribution, and debris bed porosity were obtained. Under the conditions of this experimental device, the main observations and conclusions are obtained through analysis of experimental data: The height to diameter ratio of the debris bed formed in the natural state is less than 0-2, and the spread degree of the debris bed is relatively large. As the mass of molten material increases, the share of large sized debris in the formed debris bed increases. As the water subcooling decreases, the share of large sized debris increases. The average porosity of the formed debris bed is 0.345, and the porosity of the debris bed will increase as the diameter of the leakage port increases and the water subcooling decreases. Due to the sufficient cooling of the molten material in the water, there is no hightemperature impact on the bottom plate of the experimental water tank. These data will support the study of the coolability and remelting of the debris bed.

When the reactor vessel is penetrated in a severe accident of light water reactor, the molten fuel-coolant interaction including the jet breakup occurs and the jet breakup length becomes one of the important parameters. Most numerical studies on jet breakup process have been carried out using dedicated computer codes. Some researchers are trying to apply commercial CFD codes to their investigations on comprehensive jet breakup process. However, the complexity of the phenomena limits the CFD application only to hydrodynamic aspects. In the present study, numerical analysis of jet breakup under vapor generation is pursued using the STAR-CCM + code. The obtained CFD prediction of the MATE09 experiment shows jet breakup progression patterns consistent to the images taken in the experiment. Further, the predicted positions of leading head, which determine the jet breakup length, are in good agreement with the MATE 09 data. The investigation of hydrodynamic effects on the jet breakup with higher jet velocity results in a stronger shear force and earlier jet breakup process even though there exists the vapor pocket around the corium jet. In future studies, the effect of vapor intensity on the jet breakup length would be investigated further by changing other parameters.

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The fuel–coolant interaction process is a common accident in metal smelting industry. The interaction of columnar molten iron with water was experimentally investigated with the influence of mass (100–600 g) and dropping height (20–150 cm) of columnar molten iron, and depth of cooling water (2–14 cm). Results show that a steam cavity incorporating a mixture of water vapor, cooling water, air and molten iron is formed during the interaction. The solidified products are divided into three patterns: non-fragmentation, partial fragmentation and complete fragmentation, and the formation mechanisms of solidified products are analyzed. The behaviors of steam explosion and fragmentation of molten column are affected by the continuity of columnar molten iron in the falling. The pressure peak value and mass percentage of granular products increase initially and decrease afterward with the critical dropping height of 80 cm. The degree of fragmentation of molten iron is decreased by the water depth. The fragmentation behavior and percentage of solidified product are interpreted according to the thermal effect and dynamic effect involving the molten iron–water interaction process. The findings and conclusions provide guiding information for accident prevention and safety evaluation in metal smelting industry. [ABSTRACT FROM AUTHOR]

Fuel–coolant interaction (FCI) has a pivotal role in the development of core disruptive accident in a sodium-cooled fast reactor. The drastic deformation of multiphase interface in the FCI is hard to deal with in the traditional grid method. In this paper, an improved multiphase smoothed particle hydrodynamics (SPH) algorithm corrected with the kernel gradient correction (KGC) technique is presented for multiphase flow with a large density ratio and complex interfacial behaviors. The density discontinuity across the multiphase interface is described with the use of special volume, which only depends on the position information of adjacent particles. This multiphase SPH algorithm, which is troubled by an unstable and mixed-interface problem under a large density ratio, is significantly improved with the KGC technique. The accuracy and robustness of the improved method are demonstrated in the numerical simulations of the deformation of a square droplet, Rayleigh–Taylor instability, and bubble rising in water. The verified corrected multiphase SPH is applied to simulate the hydrodynamic behaviors of the general FCI and the interaction between fuels coated with stainless steel film and coolant. Boundary layer stripping, Rayleigh–Taylor instability, and Kelvin–Helmholtz instability are observed as important mechanisms of hydraulic fracturing. The fragments’ distribution and the influence of stainless steel film are analyzed. The existence of a stainless steel film has been shown to inhibit breakage.

The present study is aimed at investigating the ability of a CFD modeling of liquid–liquid jet breakup to resolve the principal mechanisms relevant to jet breakup as well as submillimeter-scale drop size. It is generally known that jet leading edge breaks up by boundary layer stripping (BLS), and jet lateral surface breaks up by Kelvin–Helmholtz instability (KHI). The jet breakup rate as well as the resulting particle size are important parameters that would largely govern the intensity of a steam explosion in severe reactor accidents. First, a two-dimensional simulation of KHI along the melt-liquid coolant interface was performed using the VOF model in ANSYS Fluent with fine meshes as small as 0.02 mm. The dominant wavelength obtained by FFT analysis of calculated melt volume fractions showed that the fastest growing wavelength from the linear analysis of KHI is seen only at the very early development of the instability, and it increases gradually. Second, a three-dimensional simulation of BLS was performed, and the shapes and sizes of the melt particles were obtained. The particle size distributions from KHI and BLS simulations were compared with COLDJET experimental data of Woods metal and water, and it showed that the finer drops of one millimeter or smaller are produced by Kelvin–Helmholtz instability, and the drops of a few millimeters in diameter are mainly produced by boundary layer stripping. [ABSTRACT FROM AUTHOR]

In this study, in order to explore the effects of seawater properties on the fragmentation behavior of melt jets during a severe core meltdown accident in the light water reactor, visualized fragmentation experiments are carried out by releasing superheated melt into subcooled water at different coolant salinities, melt temperatures, water temperatures, and melt penetration velocities using the VTMCI (Visualized Thermo-hydraulic characteristics in Melt Coolant Interaction) facility at Sun Yat-Sen University. It is found that under the current experimental conditions, as the coolant salinity increases, the size of the debris decreases, while the variation of debris sphericity and debris bed porosity is insignificant. When the water temperature or melt temperature increases, the sphericity of the debris is higher, and the porosity of the debris bed and the size of the debris decrease. When the penetration rate of the melt is higher, smaller particles can be generated, but it has no significant impact on the debris bed porosity and debris sphericity. The Weber number theory can be used to predict the median diameter of debris, while the Stephan number (St) can be used to predict the trends in debris bed porosity and debris sphericity. This study contributes to a deeper understanding of the actual fragmentation process mechanism of molten materials in severe accidents of light water reactors. The experimental data obtained will also contribute to the development, validation, and improvement of relevant physical models in China's pressurized water reactor severe accident analysis codes. • Interaction between molten alloy and subcooled seawater of different salinities is studied. • Compared with deionized water, smaller fragments are generated for seawater. • Melt & water temperature impact fragment size, debris sphericity & porosity. • Melt velocity affects little fragment shape and bed porosity. • Diagrams are developed for predicting melt-jet mode, debris sphericity and bed porosity. [ABSTRACT FROM AUTHOR]

• The RPV integrity under in-vessel steam explosion loads for China third-generation PWR is evaluated. • When the break area increases, the maximum pressure and impulse increase first and then generally decrease. • With the increase of water temperature, the maximum pressure and impulse first increase and then decrease. • When the melt temperature exceeds 2850 K, the steam explosion is not sensitive to the melt temperature. In this study, in order to evaluate the Reactor Pressure Vessel (RPV) integrity under in-vessel steam explosion loads for China third-generation Pressurized Water Reactor (PWR), 2D and 3D numerical simulation of in-vessel steam explosion is conducted using MC3D code. The sensitivity analyses for in-vessel steam explosion are performed, including break area, water temperature, water level, melt temperature, and break height. The results show that the premixing stage cannot cause RPV failure and the possibility of the steam explosion blowing the RPV upper head off is almost non-existent. When the break area increases, the maximum pressure and impulse of the RPV inner wall increase obviously first and then generally decrease slightly. With the increase of water temperature, the maximum pressure and impulse first increase and then decrease. When the water level is higher, the maximum pressure variation is small. When the water level is very low, the maximum pressure decreases. The maximum impulse generally decreases with the decrease of the water level. When the melt temperature exceeds 2850 K, the steam explosion is not sensitive to the melt temperature. With the increase of the break height, the maximum pressure and maximum impulse first increase and then decrease. Furthermore, based on the results of sensitivity analysis, two cases of conservative conditions are selected, and the RPV integrity under in-vessel steam explosion loads for China third-generation PWR is considered. The result shows that the integrity of the RPV can be maintained under the in-vessel steam explosion loads for China third-generation PWR with a large margin. These studies provide some reference to improve the severe accident management strategy in China. [ABSTRACT FROM AUTHOR]

Experimental investigation of two interactions between metallic corium jets and sodium, pertaining to severe accidents in a sodium-cooled fast reactor, have been undertaken at the MELT facility. X-ray imaging and debris analysis reveal rapid formation of a crust at the melt coolant-interface, instigating thermal fragmentation events. Heat transfer calculations at the jet-coolant interface, supported by particle tracking velocimetry characterisation of the jet velocity, imply the formation of a solid crust within milliseconds of contact with the coolant. A mechanism for enhanced thermal fragmentation is proposed, inspired by observations from the X-ray imaging of coolant entrainment into the jet. Elevated pressures at the internal crust wall compete with densification and contraction of the crust, resulting in significant stress in the crust. Modelling of the thermoelastic stress field indicates that fracture is dictated by the tangential stress at the outer crust wall, consistent with 200–300 μ m long fractures observed during SEM imaging of the outer crust surface. The elevated pressure at the internal crust wall appears to dominate on short timescales, imparting tensile stress at the outer crust wall which may initiate fracture. On longer cooling times, the densification and contraction of the jet dominate resulting in tangential compression. Thus, coolant entrainment and vaporisation within the jet appears to impart competing stresses in the crust which may influence the mode of jet rupture. • X-ray imaging of stainless steel-sodium interactions reveals rapid crust formation. • Spontaneous thermal fragmentation instigated by sodium entrainment into the jet is proposed. • The jet surface is predicted to be solid-like within milliseconds of contact with the sodium. • SEM reveals 200–300 μ m long fractures in the outer surface of the debris. • Entrained coolant vaporisation competes with densification and thermal contraction of the crust. [ABSTRACT FROM AUTHOR]

A steam explosion can occur when molten corium falls from the reactor vessel into the water in the reactor cavity. While various research studies have been conducted on steam explosions following the free fall of molten corium in air before entering the water, steam explosions following submerged corium discharge under the ex-vessel cooling condition have received relatively little analysis. The aim of this paper is to compare the progress and consequences of a steam explosion in experiments and simulations for the partially flooded cavity and ex-vessel cooling conditions. Three steam explosion tests carried out in the TROI (Test for Real cOrium Interaction with water) experimental facility were simulated by the TEXAS-V code. Experimental tests were first modeled, followed by a comparison of the experimental and simulation results. The effect of the molten corium mass involved in the steam explosion under water at the moment of triggering on the strength of the explosion was higher than that of the corium composition in the tests and simulations for the condition of a partially flooded cavity. In the test and simulations of different corium injection modes to the water, the maximum pressure and impulse of the steam explosion appeared in the partially flooded cavity condition. In the simulations for the partially flooded cavity condition, the mass of the molten corium fragmented by Rayleigh-Taylor instability (RTI) was higher than that fragmented by Kelvin-Helmholtz instability (KHI). Modeling of KHI fragmentation caused solidification of the fragmented corium particles, and the impulses reduced accordingly. In the simulations for the ex-vessel cooling condition, as melt jet breakup did not occur before the triggering time, simulations with only RTI fragmentation underestimated the impulse of the steam explosion. Otherwise, modeling of KHI fragmentation increased the impulse of the steam explosion due to fragmentation on the side of the corium jet. Steam explosion simulations in the ex-vessel cooling condition require more detailed modeling of the melt jet and premixing area, as well as variable adjustment for the fragmentation by KHI. [ABSTRACT FROM AUTHOR]

The present study is aimed at investigating the ability of a CFD modeling of liquid–liquid jet breakup to resolve the principal mechanisms relevant to jet breakup as well as submillimeter-scale drop size. It is generally known that jet leading edge breaks up by boundary layer stripping (BLS), and jet lateral surface breaks up by Kelvin–Helmholtz instability (KHI). The jet breakup rate as well as the resulting particle size are important parameters that would largely govern the intensity of a steam explosion in severe reactor accidents. First, a two-dimensional simulation of KHI along the melt-liquid coolant interface was performed using the VOF model in ANSYS Fluent with fine meshes as small as 0.02 mm. The dominant wavelength obtained by FFT analysis of calculated melt volume fractions showed that the fastest growing wavelength from the linear analysis of KHI is seen only at the very early development of the instability, and it increases gradually. Second, a three-dimensional simulation of BLS was performed, and the shapes and sizes of the melt particles were obtained. The particle size distributions from KHI and BLS simulations were compared with COLDJET experimental data of Woods metal and water, and it showed that the finer drops of one millimeter or smaller are produced by Kelvin–Helmholtz instability, and the drops of a few millimeters in diameter are mainly produced by boundary layer stripping.

• Sensitivity analysis on main parameters of model for premixed layer formation in stratified configuration is performed. • Model's parameters are varied for simulation of premixing and explosion phase with the fuel–coolant interaction code MC3D. • Effects of melt fragmentation rate, melt drops size and ejection velocity for FCI in stratified configuration are discussed. Experiments of fuel–coolant interaction in stratified geometry at the PULiMS and SES (KTH, Sweden) test facilities resulted in spontaneous steam explosions. Prior to the explosion, a premixed layer of ejected melt drops in the water layer was observed in the experiments. Based on the experimental and analytical knowledge, we have recently developed a model for premixed layer formation in the Fuel-Coolant Interaction code MC3D and applied it to estimate steam explosion energetics. In the paper, a sensitivity study of this model is performed on the three main uncertain parameters of the premixed layer formation model, which define the melt fragmentation rate, the size of the ejected melt drops and the ejected melt drop velocity. The analysis is performed against the SES S1 and the PULiMS E6 experimental results. The chosen tests were selected as in both of them the same material was used, the geometry was similar, and both of them resulted in a spontaneous steam explosion. The effects can be observed in all the performed analyses and they are consistent in simulations of both experiments, affecting the premixed layer height as well as the explosion strength and duration. Some uncertainty of the experimental results is assessed, with the main limitation related to the visual observations. The combination of both analyses provides us with the assessment of future work necessity and prioritization. The presented sensitivity analysis of the premixed layer formation model enables a more reliable assessment of the stratified vapour explosion risk and its uncertainty in nuclear power plants and other industries. [ABSTRACT FROM AUTHOR]

A visualization test of the fuel–coolant interaction in the Test for Real cOrium Interaction with water (TROI) test facility was carried out. To experimentally simulate the In-Vessel corium Retention (IVR)- External Reactor Vessel Cooling (ERVC) conditions, prototypic corium was released directly into the coolant water without a free fall in a gas phase before making contact with the coolant. Corium (34.39 kg) consisting of uranium oxide and zirconium oxide with a weight ratio of 8:2 was superheated, and 22.54 kg of the 34.39 kg corium was passed through water contained in a transparent interaction vessel. An image of the corium jet behavior in the coolant was taken by a high-speed camera every millisecond. Thermocouple junctions installed in the vertical direction of the coolant were cut sequentially by the falling corium jet. It was clearly observed that the visualization image of the corium jet taken during the fuel–coolant interaction corresponded with the temperature variations in the direction of the falling melt. The corium penetrated through the coolant, and the jet leading edge velocity was 2.0 m/s. Debris smaller than 1 mm was 15% of the total weight of the debris collected after a fuel–coolant interaction test, and the mass median diameter was 2.9 mm.

• We modelled melt drop breakup behavior using MPS method. • Numerical models were validated by simulating a water drop breakup in gas flow. • Melt drop breakup was divided into fragmentation and fragments coalescence phases. • Melt fragments amount, size distribution and perimeter were comparatively evaluated. Hydrodynamic fine fragmentation of melt drops is important in increasing melt-coolant contact area, and thus directly affects heat transfer, oxidation and solidification. The interfacial instabilities incurred by relative flow and density gradient are the dominant factors for melt drops deformation and fragmentation. In this study, we modelled melt drop breakup behavior using a meshless Lagrangian MPS method, and validated the numerical model with the experimental results. Then, the melt fragments amount, size distribution and normalized perimeter were analyzed under different Weber numbers. The results indicate that the melt drop breakup process could be divided into two phases: initial deformation and fragmentation phase governed by flow inertial force and the subsequent fragments coalescence phase under the impact of fluid surface tension. Fragments amount increased in fragmentation phase but decreased in coalescence phase. The amount of fragments with smaller size (d / d 0 < 0.04) was significantly larger at high Weber number cases, but the amount of large-scale fragments (d / d 0 > 0.05) was equivalent among different weber number cases. The maximum fragments perimeter reached 20 times of the initial melt drop perimeter for case of We = 2618, which was about 3.3 times of that at We = 291. [ABSTRACT FROM AUTHOR]

Highlights • Literature review addressing steam explosions using Hugoniot adiabats is performed. • Results of calculations using Hugoniot adiabats are compared with experimental data. • Hugoniot analysis is applied to the case of SGTR/L in LFRs. • Mechanical work released in explosive lead-water interaction has a limited value. Abstract Steam generator tube rupture and/or leakage (SGTR/L) is one of the least studied and dangerous safety issues in pool-type lead-cooled fast reactors (LFRs). During this accident, high-pressure water from the secondary circuit is injected into the primary circuit with relatively low-pressure molten lead. One of the possible consequences of SGTR/L is multiphase flow formation consisting water droplets inside vapor bubbles in lead, which could engender a potential explosive coolant-coolant interaction (CCI). The present paper is devoted to the analysis of energetic molten lead-water interaction during SGTR/L in LFRs using Hugoniot adiabats. A review of literature addressing multiphase thermal detonations using Hugoniot adiabats is carried out. Calculations for CCIs are performed which are compared with earlier works and experimental data. Hugoniot analysis is applied to the case of SGTR/L in Russian BREST reactor and detonation velocities and pressures in the Chapman-Jouguet plane are estimated. The mechanical expansion work of the explosion products and the conversion ratios are calculated for typical values of initial void fractions in two-phase water-steam mixture; lead volume fractions and temperatures. Dependencies of the expansion work potential on the void fraction, initial melt volume fraction, and melt temperature are addressed. It is shown that for typical low void fractions for the case of SGTR/L in LFRs the expansion work has a limited value. [ABSTRACT FROM AUTHOR]

Fuel-coolant interaction (FCI) is an important issue for the assessment of severe accident safety for both sodium-cooled fast reactors (SFRs) and pressurized water reactors (PWRs). For the ASTRID SFR demonstrator, FCI is a key phenomenon affecting the relocation of molten fuel in engineered discharge tubes between the core region and the core catcher plenum. FCI controls jet fragmentation and debris bed formation and raises the issue of potentially energetic vapor explosions in the ASTRID lower head. In this frame, experimental data will be necessary to validate SCONE, the fuel-sodium interaction code under development at CEA. For PWRs, one of the configurations of interest lies within the residual case where in-vessel retention would fail. In this case, it is expected that a light metallic layer would be the first to interact with water, before a heavier oxide melt discharge. Here, steam explosion and debris bed formation are the two major points of interest. Based on the experimental expertise gained from the KROTOS facility and its X-ray radioscopic imaging system, new test facilities have been designed to carry out prototypic (depleted uranium-containing) corium interactions with either sodium or water in PLINIUS2, the CEA future large-mass experimental platform dealing with masses above 100 kg. Some test sections have been specially designed to ensure proper visualization of the fuel, liquid coolant, and vapor phases by an improved X-Ray imaging system. This paper presents the future PLINIUS 2 platform as well as the experimental programs foreseen to study both water-corium and sodium-corium interactions. [ABSTRACT FROM AUTHOR]

A steam explosion may occur during a severe accident, when the molten core comes into contact with water. The pressurized water reactor and boiling water reactor ex-vessel steam explosion study, which was carried out with the multicomponent three-dimensional Eulerian fuel–coolant interaction code under the conditions of the Organisation for Economic Co-operation and Development (OECD) Steam Explosion Resolution for Nuclear Applications project reactor exercise, is presented and discussed. In reactor calculations, the largest uncertainties in the prediction of the steam explosion strength are expected to be caused by the large uncertainties related to the jet breakup. To obtain some insight into these uncertainties, premixing simulations were performed with both available jet breakup models, i.e., the global and the local models. The simulations revealed that weaker explosions are predicted by the local model, compared to the global model, due to the predicted smaller melt droplet size, resulting in increased melt solidification and increased void buildup, both reducing the explosion strength. Despite the lower active melt mass predicted for the pressurized water reactor case, pressure loads at the cavity walls are typically higher than that for the boiling water reactor case. This is because of the significantly larger boiling water reactor cavity, where the explosion pressure wave originating from the premixture in the center of the cavity has already been significantly weakened on reaching the distant cavity wall.

After the Three Mile Island accident, numerous studies on the severe nuclear accident have been conducted. In a degraded core accident, the high-temperature melt corium may drop into the low-temperature coolant, which is called the fuel coolant interaction (FCI). Due to the velocity difference between the melt jet and the coolant as well as the violent film boiling around the melt, the melted corium may fragment into small particles. With the increase of the contact area between the melted corium and the coolant, plenty of coolant steam is produced. The timescale for heat transfer is shorter than that for pressure relief, resulting in the formation of shock waves and/or the production of missiles at a later time during the expansion of coolant steam explosion. During a severe reactor accident scenario, steam explosion is an important risk, even though its probability to occur is pretty low, since it could lead to large releases of radioactive material, and destroy the reactor vessel and surrounding structures. This study provides a comprehensive review of vapor explosion experiments, especially the most recent ones. In this review, small- to intermediate-scale experiments related to premixing, triggering and propagation stages are first reviewed and summarized in tables. Then intermediate- to large-scale experiments using prototypic melt are reviewed and summarized. The recent OECD/SERENA2 project including KROTOS and TROI facilities' work is also discussed. The studies on steam explosion are vital for reactor severe accident management and will lead to improved reactor safety. [ABSTRACT FROM AUTHOR]

Studies on local fuel-coolant interaction in a molten pool are important for the improved evaluation of severe accidents that might be encountered for sodium-cooled fast reactors. To clarify the characteristics of this interaction, several years ago a series of simulated experiments was conducted at the Japan Atomic Energy Agency (JAEA) by delivering a given quantity of water into a molten pool formed with a low-melting-point alloy. In this work, motivated by acquiring more reliable database and knowledge, an improved experimental system has been developed recently at the Sun Yat-sen University. Experimental results from various conditions including much difference in water volume, melt temperature, water subcooling, initial water-lump shape along with melt depth, are discussed. It is found that the water volume, melt temperature, initial water-lump shape and melt depth are observable to have remarkable impact on the pressure-buildup characteristics while the role of water subcooling is less prominent. The observed consistent influence of water volume, melt temperature and water subcooling, as compared to previous studies at JAEA, indicates that despite the existence of some uncertainties, the statistical results recognized at JAEA might be overall valid. Owing to much enriched evidence, the present analyses suggest that the most probable reason leading to the limited pressurization as water volume increases for a given melt and water temperature, should be primarily due to the isolation effect of vapor bubbles at the melt-water interface. [ABSTRACT FROM AUTHOR]

During severe accidents at Nuclear Power Plants (NPPs), fuel-coolant interaction (FCI) is a critical event in which the melt released from the core region comes into contact with the coolant. The melt may eject in the form of a melt jet and threaten the integrity of the NPP. Therefore, fragmentation of the melt jet and quenching of particulate fragments from the melt jet are invaluable from the viewpoint of safety assessment. To assess the integrity of an NPP, melt fragmentation phenomena that affects quenching and sustainable cooling of the debris bed are important factors that must be predicted and evaluated precisely. The present review summarizes experimental works on the FCI phenomenon, especially, fragmentation of a melt jet during a severe accident in an NPP. In addition, special attention is paid to solidification effects. Based on the literature survey, we discussed the dominant factors governing the fragmentation mechanisms. Furthermore, we discuss the applicability of various models for estimating these phenomena. [ABSTRACT FROM AUTHOR]

In light water reactor core melt accidents, the potential risk of energetic molten fuel coolant interaction (FCI, steam explosions) in a partially-flooded reactor cavity has drawn substantial attention. However, when the cavity is completely filled with coolant and the reactor vessel lower head is submerged for the purpose of external reactor vessel cooling, the jet breakup behavior in this case could not only be quite different from the case of partially-flooded reactor cavity, but also affect the risk of fuel coolant interactions. In this study, the jet breakup in liquid-liquid system was experimentally investigated for a fully-flooded and a partially-flooded cavity, and the existing jet breakup models were evaluated against the experimental data. The jet breakup experiment, named as COLDJET, was carried out using the Woods metal whose melting point was 72 °C. The melt jet temperature was lower than the boiling point of coolant so that only the hydrodynamic aspect of jet breakup was investigated. The jet diameter was 50 mm and the coolant depth was 1 m. To investigate the effect of air space of free-falling jet, the experiment was conducted for both fully-flooded and partially-flooded cavities. In the latter case, the free-fall distance in air space was 1 m. A high-speed video camera (typically 4000 fps) was used to visualize the jet breakup and there was an obvious difference in jet breakup behavior between the fully-flooded and the partially-flooded cavities. The jet breakup lengths which were identified in the jet front falling speed were compared with existing models and found that the agreement with the Epstein and Fauske’s model was remarkable. The solidified debris were collected, dried, and sieved to obtain the debris size distribution. It was observed that the average debris size in case of the fully-flooded condition was much smaller than that of the partially-flooded condition at the same jet injection velocity. This is because the entrained air layer between the melt and water acted as like vapor film in boiling condition. The prediction of most probable debris size by the model based on Kelvin-Helmholtz instability showed a good agreement with the experimental data. The jet breakup tests were also simulated using the TRACER-II code, which is a two-dimensional, transient, four-fluid code that can calculate the mixing and propagation of energetic fuel-coolant interactions. The test results were used to improve the jet breakup models of the code as well as to validate the code. [ABSTRACT FROM AUTHOR]

With the renewal of interest for sodium-cooled fast reactors, looking at what is known from the past, it appears – in the frame of severe accident studies in general and FCI in particular, that very few is known about sodium film boiling around hot fuel droplets which is a condition allowing premixing. The past Farahat experiment (Reynolds et al., 1976), performed in 1971, in which hot solid spheres were transferred into sodium has been revisited. Looking in the detailed results, it appears that a phenomena has been ignored, i.e. the existence of two film boiling regimes as observed in similar experiments with water (Honda et al., 1995; Bradfield, 1966). These two film boiling regimes have been analyzed since the transition between these regimes could lead to the onset of spontaneous explosions if the melt is still liquid at this transition. A simple model has been built for the estimation of this transition point. As this model is able to describe the transition observed in Sn-H 2 O experiments (Sher, 2012), it has been used for UO 2 -Na systems. [ABSTRACT FROM AUTHOR]

• Premixed layer formation in stratified fuel–coolant configuration is presented. • Mechanisms for the premixed layer formation are identified and a model for the melt-coolant premixed layer formation in stratified configuration is presented. • Simulations of premixed layer formation and explosion in the PULiMS E6 and SES S1 experiments are performed with MC3D code. A hypothetical severe accident in a nuclear power plant can lead to significant core damage, including melting of the core. The interaction between the molten core and the coolant water is known as a fuel–coolant interaction. One of the consequences can be a rapid transfer of a significant part of the molten corium thermal energy to the coolant in a time scale smaller than the characteristic time of the pressure relief of the created and expanding vapour. Such a phenomenon is known as a vapour explosion. Given possibly a large amount of thermal energy, initially stored in the liquid corium melt at about 3000 K, and pressure peaks of the order of 100 MPa, vapour explosion can be a credible threat to the structures, systems and components inside the reactor containment. It can also threaten the integrity of the reactor containment itself, which would lead to the release of radioactive material into the environment and threaten the general public safety. In analyses of severe accidents in nuclear power plants, a fuel–coolant interaction was mostly addressed in a geometry of a melt jet poured into a coolant pool. Based on some experimental and analytical work from the past a geometry with a continuous layer of melt under a layer of water, called stratified configuration, was believed to be incapable of producing energetic fuel–coolant interaction of sufficient magnitude to likely fail the containment. However, the results from recent experiments performed at the PULiMS and SES facilities (KTH, Sweden) with corium simulants materials contradict this hypothesis. In some of the tests, a premixing layer of ejected melt drops in water was clearly visible and was followed by strong spontaneous vapour explosions. The purpose of our research is to improve the knowledge, understanding and modelling of the fuel–coolant interaction and vapour explosion in stratified configuration. Based on the past experimental and analytical research, mechanisms for the premixed layer formation are identified and a model for the melt-coolant premixed layer formation in stratified configuration is presented. The analyses on the PULiMS and SES experimental results demonstrate the model's capability to describe the premixed layer formation. [ABSTRACT FROM AUTHOR]

Even though extensive researches have been performed for steam explosion, due to its complex phenomenon and inherent uncertainties, establishment of optimum severe accident management guideline and strategy is still one of state-of-the arts in nuclear industry. The goal of this research is primarily to examine applicability of coupled analysis with trinitrotoluene model under a typical fuel-coolant interaction-induced explosive condition. In this context, Jones-Wilkins-Lee equation of state was employed and relevant parameters were calibrated through trial and error technique. Parametric coupled FE (Finite Element) analyses employing four concrete yield criteria and structural assessment were also carried out to investigate behaviors of major components and reinforced concrete structures in a nuclear power plant. As a result, almost same maximum pressures were calculated comparable to those obtained from a complicate sequential FE analyses suggested by the authors. Among the concrete yield criteria, CDP (Concrete Damage Plasticity) model provided the highest stress and strain values. Also, it was proven that the major components and reinforced concrete structures sustained their intended function as pressure boundaries in all parametric analysis cases. [ABSTRACT FROM AUTHOR]

In the paper the applicability of the heat transfer models for the typical fuel-water and fuel-sodium interaction conditions are analysed and discussed. The important differences of sodium properties compared with water require not just adequate modelling of the film boiling regime but also adequate modelling of the transition boiling regime. The aim of the paper is to discuss the heat transfer modelling approaches applicable for the fuel-coolant interaction codes. First, the applicability of the adjusted Epstein-Hauser correlation for the film boiling modelling is analysed either comparing the correlation to the experiments with water or analysing the sodium properties. In the paper two modes of the film boiling regime are discussed. Indeed, experiments performed with the sub-cooled water have indicated an increase in the heat transfer between the stable and unstable mode. The numerical correction factors of the adjusted Epstein-Hauser correlation are proposed for the stable and unstable modes of the film boiling regime. Second, for the stable mode of the film boiling regime a modified Epstein-Hauser correlation is proposed to enable better general matching with the experimental data at significant sub-cooling. Next, for the unstable mode an alternative correlation is proposed and assessed. Finally, the empirical correlation for defining the transition from the film boiling regime towards the transition boiling regime is used to discuss the relevance of the unstable mode or transition boiling regime modelling in sodium. For sodium, additional experiments should be performed. [ABSTRACT FROM AUTHOR]

Steam explosions by the interaction of molten corium with water have been studied extensively because they may have the potential to impact the integrity of the containment. Since breakup and fragmentation processes during premixing are important mechanisms that influence steam explosion behavior, the particle she distribution characteristics on fuel-coolant interaction (FCI) have been investigated in the TROI (Test for Real cOrium Interaction with water) test facility. The data characteristics indicate that FCI characteristics depend upon the composition of the prototypic corium material, and the particle size of the debris is related to the intensity of the dynamic pressure produced by an explosion. The mass mean diameters of the debris produced by explosive compositions were less than that of the non-explosive compositions. A mass mean diameter of 2 mm was found to be a boundary size produced by a steam explosion of corium. The particle sizes of the molten corium involving a steam explosion were shown to be mainly 3 to 6 mm depending on the material and composition, but the size distribution shifted to smaller sizes if a steam explosion occurred. Small corium droplets of less than ~3 mm did not seem to contribute to a steam explosion owing to solidification at an early stage before the explosion, but large droplets contributed due to their liquid state. Zirconia, with the largest fusion heat, has almost always exploded, and the explosions have been energetic, while the eutectic composition (UO2/ZrO2 = 70/30 at weight percentage) frequently exploded. On the other hand, noneutectic compositions rarely exploded, even though the heat of the fusion was very similar to the eutectic composition that frequently exploded. The main reason why noneutectic corium compositions do not explode seemed to be that they undergo solidification by forming a "mushy zone" with a small freezing temperature range. To determine whether noneutectic corium melts cooled down through the mushy zone, particles of this composition were analyzed from the surface inward using a scanning electron microscope, an electron probe microanalyzer, and X-ray diffraction. However, all particles were found to have a homogeneous solid solution. The large particles showed the typical solidification shapes of a general molten material. The small particles generally had only a few small pores and small cracks. The morphologies of the large and small particles were found to be similar. [ABSTRACT FROM AUTHOR]

Melt droplet solidification is one major phenomenon acting on the mitigation of the strength of vapour explosions in situations related to nuclear safety. The modelling of solidification effects is then an important challenge for the evaluation of fuel–coolant interaction. We present an attempt for modelling the crust that might develop around melt droplets and its effect on the fine fragmentation within an Eulerian formulation of the flow equations, with application to the MC3D code. A physically-based modelling of the melt droplets temperature profile and a fragmentation criterion for partly solidified droplets are presented. The implementation of these models in the specific case of Eulerian modelling of the melt droplet phase is described. Two additional transported model parameters based on the most important droplets features regarding the fuel–coolant interaction behaviour are derived. The first property to transport is the crust stiffness because it enables the correct prediction of the amount of droplets participating in the droplet fragmentation process. The second is related to the energy transfer from the droplet interior to the surface. This enables to improve the surface temperature determination and reflects the history of the droplet cooling. Also the potential effect of solidification on the vapour explosion strength is shown. [ABSTRACT FROM AUTHOR]

A multi-phase hydrodynamic code using volume of fluid method (VOF) is used to simulate the molten droplet deformation, and the surface waves at the stages of pressure propagation and expansion in steam explosion (SE). Both melt-liquid system and melt-gas system are simulated using the code. A benchmark case of Joseph et al.’s experimental data (1999) is performed to validate the code. In this paper, effects of vapor film and surface tension on the droplet behavior under high pressure pulse are investigated. In addition, effects of material density and pressure pulse magnitude on the droplet deformation, and the growth of surface waves are also analyzed. Results of simulation analysis suggested that vapor film can be neglected and the effect of surface tension is not significant for hydrodynamic process under high pressure pulse. Results of simulation also demonstrate that material density and pressure pulse play a significant role in the process of droplet deformation and surface wave growth. The research shows that the effect of hydrodynamic deformation is significant for droplet fragmentation, and droplet penetration can more likely be achieved by surface wave instability at the stages of propagation and expansion in steam explosion. [ABSTRACT FROM AUTHOR]

The article is focused on the hydrogen release by chemical reaction of Fuel and Coolant (FCR) during the molten Fuel–Coolant Interaction (FCI) and its impact on the steam explosion (SE) efficiency. Samples of debris coming from several FCI tests have been characterized by solid-state analytical techniques. Two kinds of tests have been analyzed: (i) Experiments using simulant materials (MISTEE, PREMIX, ECO FCI experiments) and tests using prototypic corium i.e., in the U–Zr–O system (KROTOS experiments). The nonradioactive melts produce during FCI from negligible amounts of hydrogen up to 3.6 g per kg of melt. It was found that UO 2 –ZrO 2 melt solidifies during FCI into face-centered (Fm3m) crystal cell forming a U 1− x Zr x O 2+ y solid solution. The oxygen over-stoichiometry of the solid solution is the evidence of FCR and release of hydrogen. A method combining chemical analysis by Energy Dispersive X-ray Spectrometry (SEM/EDS) and X-ray Powder Diffraction (XRD) allowed the determination of the oxygen over-stoichiometry y . Therefore, it has been indirectly estimated how much hydrogen was formed during the chemical reaction between hot corium and water. These data are provided for KROTOS KS2, KS4 and KS5 tests performed in the frame of the OECD/NEA Serena phase-2 project. It has been shown that up to i.e., 4.7 g of H 2 per kg of delivered corium melt can be formed by the reaction stoichiometric or sub-oxidized UO 2 –ZrO 2 melts with water. The oxygen over-stoichiometry of the U 1− x Zr x O 2+ y debris (initially x = 0.48 for KS2; x = 0.35 for KS4; x = 0.38 and y = −0.37 for KS5) varied between 0.107 ⩽ y ⩽ 0.167. The hydrogen formation during FCI is primarily linked to the melt composition and secondary to the size of the fragmented melt droplets. [ABSTRACT FROM AUTHOR]

The main results of the major international activities related to fuel–coolant interactions (FCI) of the last 4-year period are presented and a summary of the knowledge gained regarding understanding and the improvements of modelling is provided. At first, the major outcomes of the OECD SERENA-2 program are presented and discussed. Important clarifications were obtained on the so-called material effect and on FCI code capabilities. We then summarise complementary analytical analyses and experimental programs performed in the frame of the SARNET community. The focus was put on the role of melt fragmentation and solidification, the impact of void on the intensity of an explosion and the triggering mechanisms. As a conclusion, tables summarising the improvements are proposed as well as research priorities. [ABSTRACT FROM AUTHOR]

• Numerical simulation of jet breakup using high-order MPS method. • Applicability and reliability of the method are validated by the comparison between the simulation results and reference experiments. • Molten corium jet breakup in sodium is studied, some eddies produced by RT instability and KH instability lead to jet breakup. During a core disruptive accident of Sodium-cooled Fast Reactors (SFR), discharged corium may contact with coolant in form of jet, which can be an imminent threat to the integrity of reactor vessel if the jet does not sufficiently break up and cooldown. It is significant to ascertain the morphologies, penetration depth and breakup mechanism of discharged corium jet to evaluate its potential impact on reactor vessel. In this study, numerical simulations are carried out to investigate the characteristics of jet breakup. An advanced moving particle semi-implicit method (MPS), the least square MPS (LSMPS) is applied to study the two-dimensional hydrodynamic corium jet breakup in sodium. Firstly, the applicability and reliability of the method are validated by the comparison between the simulation results and reference experiments. Then, the method is applied to simulate molten corium jet breakup in sodium. It is found that the breakup mechanism is related to Rayleigh–Taylor instability (RT), Kelvin–Helmholtz instability (KH) and Plateau–Rayleigh instability (PR). Under the influence of RT and KH, eddies are formed around the jet and thus the downward velocity of the jet vibrates in a relatively regular pattern. The local velocity difference of the jet narrows the jet and the jet eventually breaks up under the influence of PR. The parameters including diameter and initial velocity of the jet were scrutinized for their influences on jet penetration depth and jet breakup length, which can be useful for the improved design of the SFR reactor vessel. [ABSTRACT FROM AUTHOR]

In light water reactor core melt accidents, the molten fuel can be brought into contact with coolant water in the course of the melt relocation in-vessel and ex-vessel as well as in an accident mitigation action of water addition. For the last several decades, the potential risk of energetic molten fuel coolant interactions (FCIs, steam explosions) has drawn substantial attention in the safety analysis of reactor severe accidents. In this paper, an improved melt jet breakup model is presented and analyses of an energetic fuel-coolant interaction in a PWR cavity (1) partially filled (4 m deep) and (2) completely filled (7 m deep) with water are presented. The TRACER-II code was used in the analyses. For jet breakup model, the full dispersion equation of Kelvin-Helmholtz instability for the melt jet-vapor film-water was solved numerically and the solutions were correlated for use in the TRACER-II code. The new jet breakup model was benchmarked using FARO L28 test data. In reactor calculations the mixing calculations showed that the average melt drop size was much smaller in 4 m deep pool with 3 m free-fall than in 7 m deep pool. The explosion calculations showed that the peak pressure at the center of mixture was ~90 MPa in 4 m deep pool, ~25 MPa in 7 m deep pool. It also showed that the maximum impulse at the cavity wall was found at the lower wall in both cases and it was 50 kPa s in 4 m deep pool and 150 kPa s in 7 m deep pool. [ABSTRACT FROM AUTHOR]

During a severe accident sequence in a pressurized light water reactor, the hot (~3000 K) molten materials (corium) coming from the degraded reactor core may generate a violent interaction if they come in contact with water. This melt-water interaction, called fuel-coolant interaction (FCI), may damage the structures and threaten the reactor integrity if there is a steam explosion. FCI occurs generally in two phases: a premixing phase, during which the molten corium jet is fragmented into large droplets and mixed with the water, and the explosion phase, during which the vapor film that has developed around the fuel droplets is destabilized and the droplets are finely fragmented. The presented work covers a set of experimental studies describing the morphology and nature of the solidified materials after interaction with water. Debris from experiments performed in the KROTOS (Commissariat à l'Énergie Atomique, Cadarache, France); PREMIX, ECO (Karlsruhe Institute of Technology, Karlsruhe, Germany); and MISTEE (Royal Institute of Technology, Stockholm, Sweden) facilities have been characterized by metallographic, analytical, and microscopic techniques. These post-test analyses are able to provide important information on the solidification path and other main phenomena involved during FCI. It was found that the behavior of metallic and oxide melts differs significantly from the standpoint of debris morphology. Oxide melts that underwent simple coarse fragmentation showed spherical or angular rocklike shape, unlike metallic melts. A statistical analysis was performed on the debris from the KROTOS tests; a data set of particles was described by the circularity, solidity, and porosity. The mechanism of water ingression (Kim and Corradini) inside the melt droplet was observed to be the key mechanism of fine (secondary) fragmentation. The particles participating in fine "thermal" fragmentation have significantly higher porosity, up to ~30% for prototypic corium in the KROTOS facility. It was calculated that only a part of the premixed melt participates in fine fragmentation, i.e., about one-third of the melt mass for the KROTOS tests using UO2-ZrO2 mixture. [ABSTRACT FROM AUTHOR]

Hydraulic fragmentation of melt jet is an important phenomenon during fuel-coolant interaction (FCI) of nuclear reactor severe accident. If the melt jet has not broken up completely before contacting the lower head, it may ablate the vessel wall and a compact debris bed may form. The jet breakup length and melt-coolant contact area are important parameters describing melt fragmentation. In this study, therefore, a numerical model for analyzing melt jet hydraulic fragmentation was developed and validated against the experiment. Through the analysis of UO 2 melt jet fragmentation in coolant under various conditions, the relationship between the morphological structures and axial velocities of melt jet was clarified, and an accurate criterion for judging jet breakup length was proposed. The effects of initial melt jet velocity and diameter on the dimensionless breakup length were investigated, and an empirical equation for calculating breakup length was established with a maximum deviation about 10%. The dimensionless contact area of melt and coolant increased as the melt was injected into the coolant, but it finally reached a stable value for each case. It increased as the increase of initial injecting velocity, but was not affected by the initial melt jet diameter. The empirical equation for calculating the dimensionless contact area was established as well, and the prediction deviation was less than 10%. • Hydraulic fragmentation of melt jet in coolant was investigated under different Weber numbers. • The relationship between morphological structures and axial velocities of the melt jet was clarified. • Melt jet breakup length was at the point where the wave peak value of dimensionless velocity less than 1.0. • An empirical equation for calculating breakup length was established with the maximum deviation about 10%. • An empirical equation for calculating the dimensionless contact area was established with the prediction deviation less than 10%. [ABSTRACT FROM AUTHOR]

Sudden transitions from effusive or mildly explosive eruptions to violent phreatomagmatic explosive activity can occur at high-elevation vents on oceanic island volcanoes. Here, we describe near-vent deposits around the large Cova de Paul crater on Santo Antao, Cape Verde Islands, which provide insights into the transition from Strombolian magmatic to violently explosive phreatomagmatic activity during one such eruption. The Strombolian deposits change from welded spatter to bedded clinkery scoria to angular blocky scoria, and contain a distinct population of strongly banded, low-vesicularity angular sub-glassy clasts with strongly prolate vesicles. These sub-glassy clasts also form a discrete layer of larger blocks at the transition to the overlying phreatomagmatic deposits. They are compositionally identical to the Strombolian scoria and are interpreted to be fragments of flow-banded chilled margins from the walls of the eruptive conduit. The layer of large sub-glassy blocks records disintegration and expulsion of the chilled margins at the transition to phreatomagmatic activity. We interpret the sequence from spatter through clinkery scoria to blocky scoria as recording an increase in groundwater inflow into the conduit from pressurised aquifers, but at coolant input ratios below those required for explosions. The shattering of the chilled margins to produce the bed of large sub-glassy blocks appears to have allowed a much greater water flow into the conduit, leading to the phreatomagmatic explosive phase of the eruption. [ABSTRACT FROM AUTHOR]

Abstract: Molten Fuel–Coolant Interaction (FCI) is considered an important phenomenon in the frame of nuclear reactor severe accident research and development. Recently, the so-called “material effect” in FCI was observed in several experimental research programs. It was found that chemically different melts yield to different explosion energetics or trigger-ability during FCI tests. The presented work is focused on the thermodynamic modeling of high temperature chemical reactions during the melt–coolant interaction and consequent comparison with results obtained by material analyses of debris coming from KROTOS (CEA, France), MISTEE (KTH, Sweden) and PREMIX (FZK, Germany) experimental research programs. Calculations at thermodynamic equilibrium were obtained using the Gemini 2 (Thermodata) Gibbs energy minimizer. Information about debris composition, morphology and structure were collected using solid-state characterization techniques such as scanning electron microscopy, X-ray powder diffraction and thermogravimetry. Chemical reactions of melt and water vapor were theoretically predicted and experimentally confirmed, and therefore, features like melt solidification path, hydrogen production and melt oxidation are discussed. [Copyright &y& Elsevier]

Abstract: The moving particle semi-implicit (MPS) method does not require grids for simulating fluid motions. Therefore, the MPS method can easily handle a large deformation of fluid. However, the MPS method has some difficulties in simulating transfer of momentum caused by a physical collision between different fluids because fluid particles have no mass or volume and only have weights for interacting with other particles. To overcome this inherent defect of the MPS method, rigid body dynamics is explicitly coupled with the MPS method in this study. In the first step, the MPS calculation is performed with particles which are considered to have no mass or volume. In the second step, rigid body dynamics comes into the calculation and considers the particles to have a slightly lesser diameter than the initial distance between particles. Then, physical contacts between particles are simulated with the dynamic energy conserved while the incompressibility of fluids is effectively maintained. In the single fluid region, the coupled method deals with the behavior of the particles. For the interface of the different fluids, only rigid body dynamics is used to simulate the transfer of the momentum caused by physical collisions of fluids. Through this coupling of rigid body dynamics and the MPS method, the overall stability related with the incompressibility of a fluid is comparatively increased in the single-phase fluid simulation. For the calculation of the multi-phase fluids behavior, fluids interactions can be easily treated with improving stability of the MPS calculation. In this study, collapse of water column and the isothermal fuel–coolant interaction (FCI), in which a water jet is directed into a denser fluid pool, were simulated to validate the coupling method of the MPS method and rigid body dynamics. [ABSTRACT FROM AUTHOR]

Abstract: The rapid phase change and heat transfer obtained by direct contact heat exchange between a cryogen and water can generate high rates of pressurisation, which is of interest to a number of applications. A visualization study of liquid nitrogen injection into water is conducted in this work, with synchronized pressure and temperature measurement, to obtain insight into this complex phenomenon. High speed imaging reveals a four-stage evolution of liquid nitrogen jet structure upon injection into water, with a thick vapour blanket forming around a liquid nitrogen core and break-up brought on predominantly through impact with the vessel wall. Maximum pressurisation rate occurs in the third stage of injection due to a combination of heat and mass transfer. Pressurisation rates in excess of 350bar/s are recorded and found to vary proportionally with injection pressure. The scenario of gaseous nitrogen injection is also investigated, and compared with liquid nitrogen injection. A clear advantage of liquid nitrogen injection is elucidated from the point of heat transfer and pressurisation, and implications for use in a cryogenic engine are discussed. [ABSTRACT FROM AUTHOR]

An ex-vessel steam explosion in a nuclear power plant may develop when the reactor vessel fails and the molten core interacts with the coolant water in the reactor vessel. At the fuel coolant interaction a part of the corium energy is intensively transferred to water in a very short time scale. The water vaporizes at high pressure and expands, inducing potentially severe dynamic loadings on surrounding systems, structures and components that may lead to an early release of radioactive material into the environment. To get a better understanding of the ex-vessel steam explosion phenomenon, various scenarios analyses for a typical pressurized water reactor cavity were made. A detailed analysis was performed varying the melt release location, the cavity water temperature, the primary system over-pressure at vessel failure and the triggering time for explosion calculations. The main purpose of the analysis was to determine the most challenging ex-vessel steam explosion cases and to estimate the expected pressure loadings on the cavity walls. The performed analysis shows that for some ex-vessel steam explosion scenarios significantly higher pressure loads are predicted than were obtained in the OECD programme SERENA. The detailed analysis of the most challenging central melt pour scenario revealed that the calculated high pressure loads are also a consequence of the axial symmetric geometry of the cavity model and the probably over-predicted melt droplets amount involved in the steam explosion. [ABSTRACT FROM AUTHOR]

Abstract: A physical model of formation of thin-wall hollow solid particles in melt–coolant interaction is developed. Such bubble-like solid particles have been observed in the laboratory experiments with some oxide melts and water as a coolant. The considered range of the problem parameters corresponds to interaction of the core melt with water pool in hypothetical severe accident in some industrial nuclear reactors. The calculations showed that only very small corium droplets of diameter less than 1–2mm can produce “solid bubbles” and it depends on initial overheating of the melt. The results obtained are in qualitative agreement with recently reported laboratory observations. [Copyright &y& Elsevier]

• The models of the melt evolution and the main factors affecting the jet-breakup length were introduced. • The experimental and simulation research on the morphology of debris bed were extensively summarized. • The models that can be applied to the code and the future development direction were prospected. When a severe accident occurs in a nuclear reactor, fuel–coolant interaction (FCI) may occur and cause steam explosion. Energetic FCI or steam explosion will threaten the integrity of the containment and even cause radioactive leakage. Therefore, the research and evaluation of the morphology and development of the melt in the FCI process is very important. In recent decades, many scholars have conducted theoretical and experimental research on this phenomenon, and formed a large number of mathematical and physical models for the degradation, evolution and relocation of the melt. This study retrospects and summarizes the experimental and theoretical research on the FCI phenomenon, and describes the behavior of the melt in the entire evolution process from the melt jet to debris bed. In addition, on the basis of literature review, the models of each stage are classified according to the formation mechanisms and applicable conditions. Finally, in the present study, the future development direction is prospected on the ground of the previous research results. [ABSTRACT FROM AUTHOR]

Abstract: Fast cooling and solidification of high-temperature droplets of opaque melt is considered. The problem parameters correspond to interaction of core melt with ambient water in hypothetical severe accident in some industrial nuclear reactors. A recently suggested approximation for transient temperature profile in the particle during solidification is employed. This approach is combined with an analytical solution for quasi-steady stress–strain state of growing solid crust layer. A computational analysis showed that the resulting tensile stress on the particle surface is maximal at a certain position of solidification front. The latter is considered to be a reason of mechanical breakage of corium particles at time preceding this stress maximum. The results obtained are in qualitative agreement with recently reported observations of some fragments of thick-wall hollow spherical particles in laboratory experiments. [Copyright &y& Elsevier]

Abstract: An initial period of surface solidification of large opaque particles of molten metal oxides is considered. It is shown that low overheating and very high melting temperature of corium lead to very fast formation of solid crust on the particle surface. The transient problem solution showed that maximal tensile stress in this crust due to pressure drop in expanding steam bubble around the particle is much less than the stress in relatively thin crust layer on the surface of alumina particle. The solidifying corium particle seems to be more stable as compared with the alumina particle of the same size. The latter is treated as one of the reason of the experimental results on relatively low explosivity of corium. [Copyright &y& Elsevier]

Abstract: The visible and near-infrared radiative properties of water containing polydisperse steam bubbles and core melt particles are analyzed. The alternative approximate models of radiation heat transfer in the range of water semi-transparency are considered. A new approach based on radiation balance equations for a large computational cell of size about several centimeters is suggested. This approach called large-cell model takes into account not only water heating but also radiation heat transfer between the particles of different temperatures. The latter is important for problems of fuel–coolant interaction in possible severe accident of light water nuclear reactors. The model problems considered in the paper give estimates of nonlocal effects of thermal radiation and confirm the applicability of the large-cell model at parameters typical for fuel–coolant interaction. [Copyright &y& Elsevier]