Molten Fuel-coolant Interaction Research Articles

Investigation of molten fuel coolant interaction with simulated corium in sodium

In case of a hypothetical core meltdown accident in Sodium cooled Fast Reactor (SFR), corium interacts with cold pool sodium and results in Molten Fuel Coolant Interaction (MFCI). During MFCI, the corium fragments and resultant debris settle on the core catcher as a bed and continue to generate decay heat. The size and state of fragmented debris play a crucial role in post-accident heat transfer. An experimental study has been undertaken to investigate the fragmentation of simulated corium in sodium using real-time X-ray imaging. Experiments were conducted by releasing ∼400 g of molten mixture of alumina and iron at ∼2400 °C into ∼4 kg of sodium at various temperatures. The fragmentation phenomenon was acquired using a digital flat panel detector and the captured images were subjected to image analysis to extract information on melt fragmentation. Post-experiment, fragmented debris was retrieved and subjected to sieve analysis to obtain characteristics towards assessing the dominant fragmentation mechanism. Experiments were also conducted with water for identifying key differences in the fragmentation and debris characteristics. Non-energetic fragmentation, generation of fine debris, and relatively minor sodium vaporization observed in the small-scale experiments present a reduced potential for energetic interaction in sodium than that of water.

A numerical investigation of thermal stress induced cracking pattern in the solidifying nuclear fuel droplet during MFCI

During Molten Fuel Coolant Interaction (MFCI) occurring in a Sodium-cooled Fast Reactor (SFR) following a severe accident, the corium jet breaks up into coarse droplets due to hydrodynamic instabilities and fine flaked debris are generated due to the thermal stress cracking occurring on the solidifying droplet. In particular mixed oxide core debris quenched by liquid sodium are more prone to thermal stress cracking because of the excellent heat transport properties of liquid sodium, which leads to a large temperature gradient across the crust of the ceramic fuel. In this paper, thermal stress initiated cracking of solid crust of the nuclear fuel droplet is studied using Fracture Bifurcation Theory (FBT) and Minimum Potential Energy Principle (MPEP). FBT is able to predict the minimum crack length needed for crack propagation or catastrophic failure depending on the computed KI values. On the other hand, MPEP can predict the periodic and hierarchical cracking pattern that can develop on the core debris. This helps in the prediction of number of cracks and the crack length that can develop on the solid crust. Parametric studies have been carried out by varying MOX droplet sizes, crust thickness and cooling conditions and calculating KI by semi-analytical means and comparing it with KIC. The analysis revealed that solidifying droplets with radius equal to or more than 2 mm are susceptible to thermal stress initiated crack growth and catastrophic breakage even if a few tens of micron size initial crack develops on its surface. Then a particular case of 2 mm radius core debris is considered and its cracking pattern is studied by applying MPEP. This optimal crack configuration is obtained by minimizing potential energy for a given range of crack length and spacing. The analysis has been carried out with ANSYS APDL code which calculates the strain energy for a given initial crack length and spacing. The 2D computational model is validated with a similar study carried out for Alumina disc quenched in water and the results are in good agreement. The minimum fine fragment size predicted by the analysis for mixed oxide debris is a flake like debris with equivalent radius of 41 µm when approximated to be a sphere. The particle size correlates well with the fine debris sizes reported in literature of MFCI experiments.

Characteristics of debris resulting from simulated molten fuel coolant interactions in SFRS

Sodium cooled Fast Reactors (SFR) are built with several engineered safety features and hence a severe accident such as a core melt accident is hypothetical with a probability of <10−6/ry. However, in case of such accidents, the mixture of the molten fuel and structural materials interacts with sodium. This phenomenon is known as Molten Fuel Coolant Interaction (MFCI) and results in fragmentation of the melt due to various instabilities. The fragmented particles settle as a debris bed on the core catcher at the bottom of the reactor vessel, and continue to generate decay heat. Characteristics of the debris particles play a vital role in heat transfer from the bed and need thorough investigation. The size, shape, and physical state of the debris depend on the associated fragmentation mechanism, superheating of the melt, and sodium temperature. Experiments have been conducted by releasing simulated corium, a molten mixture of alumina and iron generated by the aluminothermy process at ∼2400 °C into liquid sodium, to study the fragmentation phenomena. After the experiment, the fragmented debris was retrieved and the particle size distribution was determined by sieve analysis. The debris was subjected to microscopic investigation for obtaining morphological characteristics. Based on the characteristics of debris, an attempt has been made to assess of fragmentation mechanism of simulated corium in sodium.

Complex styles of phreatomagmatic explosions at Kīlauea Volcano, Hawaii, controlled by magma structure

Explosive eruptions at basaltic volcanoes remain poorly understood. Kīlauea Volcano is a type locality for basaltic eruptions and is well-known for effusive activity. However, more than 7 m of phreatomagmatic Keanakākoʻi Tephra unit D deposits from explosive eruptions crown the southern rim of the summit caldera and provide a stark reminder of Kīlauea’s explosive past and future potential. We used detailed field observations as well as granulometric and morphological analysis of 100 samples from two proximal sections to assess the eruption style and fragmentation mechanism. The deposits can be divided into four subunits, six different lithofacies, and contain three juvenile tephra components. Each juvenile component shows distinct shape variability resulting from molten fuel-coolant interaction (MFCI) explosions of magma of variable vesicularity. Fragmentation of dense glass generates olive-green ash, fragmentation of low to moderately vesicular magma generates a dark gray ash-lapilli component, and fragmentation of highly vesicular magma generates light-yellow pumice. Our work shows that magma structure impacts MFCI explosion efficiency. Small-scale planar bedding throughout most of the deposit points to a general eruption style of small, frequent explosions generating low plumes. Thicker beds of accretionary lapilli of fine-extremely fine ash are related to very efficient magma-water mixing. Pyroclastic density current (PDC) deposits in the upper part of the stratigraphy contain at least three flows but show no significant dune or cross-bedding structures. We suggest that this is a function of the vent being situated in a caldera that was then ∼600 m deep, where the caldera wall acted as a barrier and changed the flow dynamics to very dilute overspills and co-PDC plume falls over the wall. Deconvolution modeling of the polymodal grain size distributions is used to assess grain size changes of each juvenile component for this deposit, which greatly improves interpretation of lithofacies generation and eruption dynamics. Size-correlated shape parameters show that shape data across a wide size range are needed to accurately track grain shapes. This study demonstrates how careful examination of grain size and shape of juvenile tephra clasts can help volcanologists understand how effusive basaltic volcanoes can become violently explosive.

Evaluation of jet breakup length with a CFD code under steam generation condition in a pre-flooded cavity

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.

Effect of Internal Decay Power and Surface Cooling on Nuclear Fuel Droplet Solidification During MFCI in a SFR—A Numerical Study

Melting of the nuclear core is one of the severe accident scenarios in a Sodium-cooled Fast Reactor (SFR). During such an event, molten corium may come into contact with the coolant sodium. This interaction of the molten fuel and the coolant is commonly termed molten fuel–coolant interaction (MFCI) in the nuclear industry. In this study, a numerical analysis is carried out to study the solidification of a molten fuel droplet in the liquid sodium pool. In the first part of the study, the effect of constant internal heat generation on the solidification of the droplet is evaluated with convective heat dissipation prescribed at the droplet surface. The internal heat generation (decay power) and the heat transfer coefficient are varied as parameters, and the time required for complete solidification of the molten droplet is obtained. Based on the results, the freezing of the droplet is categorized into three regimes: conduction limited, transition, and internal heat generation dominated regimes. It is observed that the solidification process of nuclear fuel droplets generated during MFCI is not influenced by internal heat generation and lies in a conduction-limited regime for decay power level prevailing in a medium-sized SFR. Hence, in the next part of the study, the numerical analysis is carried out by incorporating the time-dependent decay power and the temperature-dependent heat transfer coefficient in the computational model by developing user-defined subroutines depicting a realistic scenario of an accident. The results of the analysis show that because of the high subcooling of sodium, film boiling is ruled out; nucleate boiling with a maximum heat transfer rate occurs briefly. The heat transfer coefficient then declines as the interface temperature between the droplet and the sodium decreases rapidly until the natural convective regime is reached. A parametric study on the droplet diameter is also carried out by varying the diameter from 0.5 to 10 mm, spanning the typical particle size spectrum expected during MFCI.

Magmatic and phreatomagmatic contributions on the ash-dominated basaltic eruptions: Insights from the April and November–December 2005 paroxysmal events at Karthala volcano, Comoros

Basaltic eruptions are commonly associated with lava emissions and relatively weak explosive activities, but they can sometimes produce strong explosive eruptive phases. In April and November 2005, two paroxysmal eruptive events occurred within the summit crater of Karthala basaltic shield volcano (Grande Comore Island, Comoros), which hosted a water lake before each of these events. Both 2005 ash plumes spread across the Comoros Archipelago and heavily impacted the whole Grande Comore Island. Associated deposits on the volcano summit are extremely fine-grained (up to 50 wt% of fine ash <63 μm for some analyzed layers) and rich in millimeter-sized rounded accretionary lapilli aggregates. Field observations, as well as textural and chemical analyses performed on both coarse- and fine-grained pyroclasts permit to identify juvenile and non-juvenile components and quantify their peculiar characteristics. Coarse ash (710–1000 μm) mainly consists of juvenile pumice particles (vesicle number density NV = 4.5 104 mm−3 and gas to melt ratio VG/VL = 1.5, on average), characterized by glassy groundmasses and representative of magma portions ascending quickly within the eruptive conduits (up to 10 m s−1). A relatively low amount of juvenile scoria particles are also observed in the coarse ash fractions, which are characterized by magma degassing (NV = 4.7 104 mm−3 and VG/VL = 0.5 on average) and associated crystallization (occurrence of dendritic microlites). Non-juvenile fragments (from blocks to coarse ash) are dense lava or intrusive fragments. Their amount decreases exponentially towards the fine ash fractions, which are mainly composed of juvenile, blocky, dense and glassy particles that are characterized by unambiguous textural signs of brittle fragmentation (hackle lines, stepped features and cracks). We support that Molten Fuel-Coolant Interactions between highly porous fast ascending basaltic magmas and external waters occurred during the paroxysmal phases of the studied eruptions, leading to a brittle-dominant and efficient regime of magma fragmentation. Variable but large amount of fine ash grains through the stratigraphic depth of the deposits can be ascribed to the brittle failure of the vesicle walls of the initial porous magma. Concurrently, thermohydraulic explosions caused the host rock fragmentation at shallow level, generating the relatively coarse non-juvenile particles. A short-lived episode of intense lava fountaining associated with steam explosions eventually occurred at the end of the November 2005 paroxysm, forming the last and relatively coarse tephra layer at the top of the studied eruptive sequence. Each paroxysmal phase lasted about a day as each associated water lake and shallow water table progressively vaporized and dried away. Both eruptions ended with lava pond and weak lava fountaining activities confined within the summit crater. We conclude that the contributions of both magmatic processes and phreatomagmatic interaction mechanisms ultimately generated the grain size, grain component and grain texture variabilities observed within the paroxysmal deposits. This work contributes to a better understanding of the generation of unusual fine ash from basaltic explosions as well as their eruptive dynamics and associated mechanisms, from magma ascent in the conduit to the fragmentation level and the interaction with intra-crateric lake waters.

Fluid‐driven Hydrovolcanic Activity along Fracture Zones and near Seamounts: Evidence from Deep‐sea Fe‐rich Spherules, Central Indian Ocean Basin

AbstractAn insight on occurrence of Fe‐rich spherules from the Central Indian Ocean Basin (CIOB) provides an understanding of their distribution at a water depth of &gt;5,000 m. In the present study, Fe‐rich spherules are identified to occur in two different sediment types (i.e., siliceous and pelagic) and tectonic settings (i.e. near seamounts and fracture zones). These are single spheres or aggregates, of different sizes (63 to 390 μm) and show textural variability (smooth/quenched, brickwork, corkscrew, interlocking and dendritic). A comparative study based on physical morphology and chemical composition suggests a common mechanism of formation. The association of spherules with fracture zones (FZ) and seamounts signifies that morpho‐tectonic features play an important role in fluid‐driven hydrovolcanism. Based on the evidence and geologic conditions existing in the basin, we conclude that molten fuel‐coolant interaction (MFCI) coupled with submarine hydrothermal exhalations could be an ideal mechanism for the formation of spherules and Fe‐particles. The accretion of the spherules on the surface sediments could be a result of recent volcanic phenomena, while those occurring at different depths (280–355, and 460–475 cm‐bsf) within the sediment core indicate two different episodes. The study provides a global implication in understanding fluid‐driven magmatism in a deep‐sea intraplate environment.

Apparently ‘dry’ littoral rootless cones in Hawaiʻi formed by sustained, “confined” mixing of lava and sea water

Hydrovolcanic rootless cones form through the explosive interaction between lava and external water at locations remote from primary vents. Littoral rootless cones such as those on the coast of Hawaiʻi form at the ocean entries of basaltic lava flows and most consist dominantly of thinly bedded ash and lapilli. In contrast, the littoral rootless cones at ʻAuʻau Point and South Puʻu Kī Cone on the southwestern flank of Mauna Loa, Hawaiʻi consist of alternating layers of clastogenic lavas and thermally oxidised, tack- to moderately welded lapilli and coarse bombs with little to no ash. The pyroclastic layers have characteristics traditionally thought to be indicative of ‘dry’ explosive magmatic activity at primary vents, including the high degree of sorting in fall deposits, the paucity of ash and quench textures, extensive thermal oxidation, a high degree of welding, dominantly fluidal pyroclastic morphologies and association with clastogenic lavas. We propose that littoral rootless activity at both cones was driven by the “confined mixing” of tube-fed lava and sea water. In this configuration, the mingling of water and lava is complicated, resulting in relatively inefficient Molten Fuel Coolant Interactions (MFCIs). Cycles of more vigorous (littoral lava fountains) and less vigorous (lava bubble bursts) activity produced the alternating layers of clastogenic lava and welded bombs and lapilli. These cycles probably reflect the local depletion and replenishment of sea water injected into the lava tube. Similar apparently ‘dry’ facies found in the upper sequences of Icelandic rootless cones formed when the water content of the underlying wet sediment was depleted and MFCIs were relatively inefficient. This study suggests that both “confined mixing” of tube-fed lava and external water, and lava–water interactions with unfavourable lava–water mass ratios, such as during the late stages of formation of Icelandic rootless cones, can produce seemingly ‘dry’ deposits that nevertheless are fundamentally hydrovolcanic.

Using the characteristics of rootless cone deposits to estimate the energetics of explosive lava–water interactions

During volcanic eruptions, the interaction of magma and groundwater can produce thermohydraulic explosions capable of significantly increasing the eruption energy. The most well-known mechanism by which explosive magma–water interactions occur, molten fuel–coolant interaction (MFCI), is a complex series of macro- and microscale processes which have been simulated using laboratory-scale experiments. As a natural analog for MFCI experiments, we utilize rootless cone beds formed by lava–water explosions to estimate explosion energy. The specific mechanical energy of the lava–water explosions studied here occurs over a broader range (4 to 178 kJ/kg) than MFCI experiments and includes estimates for the highest-energy lava–water explosions studied to date. Explosion energy is partitioned similarly over the two systems, with kinetic transport and fragmentation energy making up 25–40% and 42–80% of the mechanical energy, respectively, which overlap the ranges estimated for MFCI experiments. Our study of lava–water explosions therefore provides a field-scale analog of MFCI laboratory experiments for understanding the energetics, and therefore hazards, of MFCI in natural systems.

Effect of Internal Heat Generation on Solidification of Molten Fuel Droplet during its Interaction with Coolant in a Nuclear Reactor

One of the important issues in safety analysis of severe accidents in fast reactors is Molten Fuel Coolant Interaction (MFCI). MFCI is the phenomenon in which fragmentation of the molten fuel takes place when it comes in direct contact with liquid sodium coolant. For complete understanding of MFCI, we need to study the solidification of the molten fuel happening along with fragmentation. Solidification / melting of any material initially held at its melting temperature is the classic Stefan’s problem. In the case of a severe reactor accident the molten fuel might have a significant amount of decay heat, even though nuclear fission reactions have been stopped by reactor shut down systems. The presence of decay heat in this case, makes the transient Stefan problem more difficult to solve analytically. Here, an investigation of this special case of Stefan’s problem with internal heat generation in nuclear materials has been carried out considering a typical droplet size of 4 mm diameter. The numerical heat transfer analysis has been carried out using the commercial software Fluent. The validation of the problem is done with the steady state analytical solution available for the position of phase change front. Parametric studies have been carried out by varying internal heat generation rates and convective heat transfer coefficient at the surface of the droplet. Temperature profiles and liquid fraction are obtained at different instants. Solidification time and final equilibrium temperature are estimated from the results. It is observed that there is significant increase in drop solidification time for heat generation rates (HG) greater than 100 MW/m3 for the drop size of 4 mm. The maximum HG which is tolerated by the droplet without reaching its boiling point is about 6 GW/m3 when the heat transfer coefficient from its surface is fixed at h=56000W/m2K.

The role of external water on rapid cooling and fragmentation of magma

The cooling rate of magma in the presence of external water during phreatomagmatic and submarine eruptions is one of the key parameters governing non-explosive to explosive magma fragmentation and eruption transitions, but remains poorly constrained. Combining results from laboratory experiments with realistic eruptive temperatures of magma cooling in ambient water of variable temperatures, and numerical modeling of transient heat transfer, we find that magma-to-water heat flux can be up to 4×106 Wm−2. The experiments exhibit two distinct water boiling regimes: A film-boiling regime defined by the presence of a coherent water vapor film between magma surface and ambient water, and a nucleate boiling regime below a critical magma surface temperature (known as the Leidenfrost temperature), where the vapor film breaks and numerous bubbles form at the magma-water interface. In general the vapor film was stable in our experiments for time scales of ≤ 5 s, indicating that this might be a limiting factor in pre-explosion magma-water mixing for energetic molten fuel-coolant interaction and explosive volcanic eruptions. The time scale of vapor film stability increases and the Leidenfrost temperature (1223 to 948 K) decreases with increasing water temperature (276 to 366 K). We show that for the empirically obtained large heat flux to external water, the cooling rate of magma can reach up to 106 Ks−1 at length scales of few microns, thus magma may undergo fine fragmentation due to quench-induced large thermal stresses. Our experimental and modeling results demonstrate that the time scales of various water boiling regimes, and erupting magma and ambient water temperatures determine the magma-to-water heat transfer rates, which in turn determine the transition to explosivity under subaqueous eruption conditions.

Review of Explosive Hydrovolcanism

Hydrovolcanism is a type of volcanism where magma and water interact either explosively or non-explosively. The less frequently used term, hydromagmatism, includes all the processes responsible for magma and water interaction in a magmatic system. Hydrovolcanism is commonly used as a synonym for phreatomagmatism. However, in recent years phreatomagmatism appears more in association with volcanic eruptions that occur in shallow subaqueous or terrestrial settings and commonly involves molten fuel-coolant interaction (MFCI) driven processes. Here a revised and reviewed classification scheme is suggested on the basis of the geo-environment in which the magma-water interaction takes place and the explosivity plus mode of energy transfer required to generate kinetic energy to produce pyroclasts. Over the past decade researchers have focused on the role hydrovolcanism/phreatomagmatism plays in the formation of maar craters, the evolution of diatremes and the signatures of magma—water interaction in the geological record. In the past five years, lithofacies-characterization is the most common approach to studying hydrovolcanism. By far mafic monogenetic volcanic fields generated the greatest number of research results. Significant knowledge gaps are identified, especially in developing tools to identify the textural signatures hydrovolcanism leave behind on eruptive products and exploring the role of hydrovolcanism in the growth of intermediate and silicic small volume volcanoes.

Explosive interaction of impact melt and seawater following the Chicxulub impact event

Abstract The impact of asteroids and comets with planetary surfaces is one of the most catastrophic, yet ubiquitous, geological processes in the solar system. The Chicxulub impact event, which has been linked to the Cretaceous-Paleogene (K-Pg) mass extinction marking the beginning of the Cenozoic Era, is arguably the most significant singular geological event in the past 100 million years of Earth’s history. The Chicxulub impact occurred in a marine setting. How quickly the seawater re-entered the newly formed basin after the impact, and its effects of it on the cratering process, remain debated. Here, we show that the explosive interaction of seawater with impact melt led to molten fuel–coolant interaction (MFCI), analogous to what occurs during phreatomagmatic volcanic eruptions. This process fractured and dispersed the melt, which was subsequently deposited subaqueously to form a series of well-sorted deposits. These deposits bear little resemblance to the products of impacts in a continental setting and are not accounted for in current classification schemes for impactites. The similarities between these Chicxulub deposits and the Onaping Formation at the Sudbury impact structure, Canada, are striking, and suggest that MFCI and the production of volcaniclastic-like deposits is to be expected for large impacts in shallow marine settings.

Fragmentation characteristics of molten materials jet dropped into liquid sodium pool

The nuclear reactor is locally melted to form molten corium under instantaneous total blockage accidents in fuel subassembly of Sodium-cooled Fast Reactor (SFR). Molten Fuel-Coolant Interaction (MFCI) will result in violent interphase heat transfer and intensive fragmentation of molten core materials, which significantly affects the formation and cooling of core debris bed. Due to approximate ambient Weber Number (Wea) with molten metallic uranium, kilogram quantities of molten copper are used as fuel simulant to drop into liquid sodium pool to study the fragmentation characteristics during MFCI. Considering the influence of instantaneous solidification at the contact interface on fragmentation, initial temperature of molten copper is decreased to obtain the instantaneous contact interface temperature (Ti) less than the melting point (Tmp) of copper. Furthermore, hydrodynamic fragmentation is independently examined through experiments with molten aluminum jet under different initial temperatures of molten aluminum jet. The Ti between molten aluminum jet and liquid sodium is varied within the range of Tmp of aluminum and boiling point (Tbp) of liquid sodium. According to the present results, nucleate boiling of liquid sodium is proved to occur during molten copper-liquid sodium interaction and thermodynamic effects are much more predominant than hydrodynamic effects to cause finer fragmentation. The fragmentation of same molten material generally increases consistently with Ti. Due to considerable energy release capability of molten copper, molten state can be potentially maintained at the contact interface to cause adequate fragmentation under the same thermal conditions. Instantaneous solidification originating from contact interface greatly increases the criteria of fragmentation. Due to poorer thermal attributes, molten UO2 is provided with smaller variation rate of Ti under the approximate Ti, which suggests longer duration for violent sodium boiling to stimulate fragmentation. As the temperature of liquid sodium increases, interfacial instability is enhanced to fracture the solid crust with reduced thickness, contributing to fragmentation. The fragmentation caused by transition boiling seems to be more significant than that of nucleate boiling. Furthermore, the fragmentation mechanism of molten materials jet discharged into sodium pool is further discussed in the present study.

Experimental research on fragmentation characteristics of molten stainless steel discharged into sodium pool and comparison with molten copper

Molten Fuel-Coolant Interaction (MFCI) is especially focused in Sodium-cooled Fast Reactor (SFR) with the consideration of possible recriticality and predominant effect on resultant consequences. As the nuclear fuel cladding material of SFR, kilogram quantities of molten stainless steel jet are used in the present experiments to study the fragmentation behavior when discharged into sodium pool. During the interaction between molten stainless steel jet and liquid sodium, the great differences in temperature, velocity and thermophysical properties result in violent sodium boiling, hydrodynamic instability and viscous shearing at the contact interface. The molten stainless steel jet suffers from significant fragmentation under multiple interfacial forces, which increases contact interface area to enhance heat transfer. However, the accelerated solidification of molten stainless steel jet at the contact interface greatly raises the criteria of fragmentation, conversely decreasing the heat transfer. In the present study, the sodium pool depth is varied to study the effect of the dropping height of molten stainless steel and inventory of liquid sodium on the fragmentation characteristics. The temperature variations in sodium pool are measured to estimate energy release during molten stainless steel-liquid sodium interaction. The resultant stainless steel debris is recorded and measured against debris mass with respect to size intervals for morphology analysis and size distribution. Furthermore, experiments with kilogram quantities of molten copper jet are conducted in the present study. The results with molten stainless steel jet are compared to those with molten copper jet and those with molten stainless steel droplets to distinguish fragmentation characteristics. According to the present findings, adequate fragmentation of molten material can be caused by increased relative velocity, high superheat and considerable energy release capability, enhancing heat transfer and accelerating solidification of molten material. With few researches on MFCI in SFR, a fragmentation mechanism is proposed based on the growth and collapse of sodium bubbles to further understand the fragmentation behavior of molten material in sodium pool.

Experimental study on fragmentation characteristics of molten aluminum/copper jet penetrating in sodium pool

Under Hypothetical Core Disruptive Accidents (HCDA) in Sodium-cooled Fast Reactor (SFR), Molten Fuel-Coolant Interaction (MFCI) will cause intensive fragmentation of molten fuel materials and violent sodium boiling, possibly resulting in recriticality due to sodium void effect and reactor core compaction. And the formation and sufficient cooling of core debris bed are significantly affected by the fragmentation of molten fuel materials in sodium pool. Due to approximate ambient Weber Number (Wea) with molten metallic fuel, kilogram-level molten copper jet is dropped into sodium pool to study the fragmentation characteristics under varied experimental parameters in the present experiments. The nucleate boiling of liquid sodium is estimated to occur at the instantaneous contact interface between molten copper and liquid sodium. The molten copper jet suffers from intensive fragmentation and solidification to produce many fragments under thermodynamic and hydrodynamic effects. Besides, experiment with molten aluminum jet injected into sodium pool is conducted without occurrence of sodium boiling to independently study the effect of hydrodynamic interaction on interphase penetration and fragmentation. During the experiments, temperature variations and dynamic pressure change in sodium pool are monitored to evaluate energy release. The resultant fragments are recorded and measured for morphology analysis and size distribution to examine the fragmentation characteristics. Based on experimental results, the fragmentation mechanism is discussed against hydraulic and thermal factors. The present findings contribute to further understanding the fragmentation characteristics of molten corium in sodium pool and optimization of mitigation systems against MFCI for SFR.

Analysis of experimental uncertainties in jet breakup length and jet diameter during molten fuel-coolant interaction

The jet breakup length is one of important phenomenological parameters to evaluate the corium debris bed coolability during the progress of severe accident in nuclear power plant, since the parameter is related to the determination of the liquid melt fraction in debris bed. Existing jet breakup length data reveal relatively large scattering and different trends, similar to the Saito or Epstein correlation, depending on cases. The scattering and separation of the data may be due to uncontrolled factors in the melt discharge condition and also inconsistent evaluation methods used for the jet breakup length.Through melt jet breakup experiments with low melting point metal alloy (Bi-Sn alloy), the influence of the estimation methods for the jet diameter (measured or calculated value) and the jet breakup length (position- or velocity-based value) was investigated. According to the comparison between the measured and calculated jet diameters, the experimental uncertainty was reduced by producing well-controlled melt jet. The concept of the jet breakup length was revisited in detail, emphasizing the shorter breakup length by the velocity-based method. The modification of the Saito correlation constant from 2.1 to 1.7 was suggested to fit the velocity-based breakup length. The importance of a well-defined data analysis process for non-dimensional jet breakup length was highlighted for the minimization of the experimental uncertainty.

Investigation of molten fuel coolant interaction phenomena using real time X-ray imaging of simulated woods metal-water system

Abstract In liquid metal fast breeder reactors, postulated failures of the plant protection system may lead to serious unprotected accidental consequences. Unprotected transients are generically categorized as transient overpower accidents and transient under cooling accidents. In both cases, core meltdown may occur and this can lead to a molten fuel coolant interaction (MFCI). The understanding of MFCI phenomena is essential for study of debris coolability and characteristics during post-accident heat removal. Sodium is used as coolant in liquid metal fast breeder reactors. Viewing inside sodium at elevated temperature is impossible because of its opaqueness. In the present study, a methodology to depict MFCI phenomena using a flat panel detector based imaging system (i.e., real time radiography) is brought out using a woods metal-water experimental facility which simulates the UO 2 -Na interaction. The developed imaging system can capture attributes of the MFCI process like jet breakup length, jet front velocity, fragmented particle size, and a profile of the debris bed using digital image processing methods like image filtering, segmentation, and edge detection. This paper describes the MFCI process and developed imaging methodology to capture MFCI attributes which are directly related to the safe aspects of a sodium fast reactor.

The “suevite” conundrum, Part 1: The Ries suevite and Sudbury Onaping Formation compared

AbstractThe term “suevite” has been applied to various impact melt‐bearing breccias found in different stratigraphic settings within terrestrial impact craters. Suevite was coined initially for impact glass‐bearing breccias from the Ries impact structure, Germany, which is the type locality. Various working hypotheses have been proposed to account for the formation of the Ries suevite deposits over the past several decades, with the most recent being molten‐fuel‐coolant interaction (MFCI) between an impact melt pool and water. This mechanism is also the working hypothesis for the origin of the bulk of the Onaping Formation at the Sudbury impact structure, Canada. In this study, the key characteristics of the Ries suevite, the Onaping Formation and MFCI deposits from phreatomagmatic volcanic eruptions are compared. The conclusion is that there are clear and significant lithological, stratigraphic, and petrographic observational differences between the Onaping Formation and the Ries suevite. The Onaping Formation, however, shares many key similarities with MFCI deposits, including the presence of layering, their well‐sorted and fine‐grained nature, and the predominance of vitric particles with similar shapes and lacking included mineral and lithic clasts. These differences argue against the viability of MFCI as a working hypothesis for genesis of the Ries suevite and for a required alternative mechanism for its formation.