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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]

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]

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]

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]

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]

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]

• The effects of boiling on molten jet and coolant interactions were studied experimentally. • The process of energetic and mild FCIs were clearly shown by visualization and other characteristic parameters. • The hydrodynamic and thermodynamic fragmentation mechanism were analysed under different boiling modes. In nuclear power plants, core melt accidents are often accompanied with fuel–coolant interactions (FCI), which may escalate to steam explosion destroying the integrity of structural components and even the containment under certain conditions. The vapor film and two-phase region formed by boiling around the molten jet at high temperature have a great influence on the fragmentation of the jet and the possibility of steam explosion. In this study, by changing the initial temperature of coolant, a series of experiments on the interactions of molten tin at high temperature with water have been done and several distinct interactions under different boiling conditions including stable film boiling, unstable film boiling and nucleate or transition boiling were observed. These interactions were distinguished by the high-speed photography, liquid level swell, dynamic pressure, water temperature variation and jet breakup length during interactions as well as the morphology and size of debris after interactions synthetically. It was found that energetic jet-water interactions were only possible under unstable film boiling, and nucleate or transition boiling conditions. While under the stable film boiling, the jet was surrounded by a very stable vapor film, so only mild interactions occur. With the decrease of coolant temperature, the boiling mode transferred from stable film boiling to nucleate or transition boiling. Significant differences in jet breakup length as well as size and morphology of the debris were observed. Because less steam was generated, the melt was more likely to action with water than steam, and water was more conducive to the fragmentation caused by interfacial instability due to higher density. In addition, the thermodynamic fragmentation caused by violent boiling gradually played a more important role. Furthermore, the results were compared with existing theories which ensured the effects of boiling during FCI. [ABSTRACT FROM AUTHOR]