Core Melt Research Articles

Study on the effect of Lorentz force on the nuclear reactor core melt stratification in electromagnetic cold crucible

To understand the effect of the Lorentz force on core melt stratification during experiments, a multi-physical field coupling model of electromagnetic induction, non-isothermal flow, and two-phase flow is established in this paper. The evolution of the layered core melt in the electromagnetic cold crucible is studied for different relative densities and phase volume fraction ratios between metal and oxide. The calculation results show that the coupling model can accurately describe the structural evolution of core melt. The Lorentz force cannot change the relative positions of the metal and oxide, but it can significantly affect the morphology of the core melt. If the density of the metal is less than that of the oxide, the Lorentz force causes the light metal with a small phase volume fraction to form a hemispherical shape in the top center of the crucible. As the phase volume fraction increases, the Lorentz force become weaker than those of gravity and buoyancy. The light metal is then flat. When the metal density is greater than the oxide density, the Lorentz force causes the heavy metal with a small phase volume fraction to appear as a hemisphere, but causes the heavy metal with a large phase volume fraction to appear as an ellipsoid in the lower part of the crucible.

Uncertainty and sensitivity analysis of the fission product behaviour in the Phébus FPT1 test with the system code AC2

The Phèbus FP (Fission Product) research program, conducted by IRSN and CEA, aimed to improve the understanding of the phenomena governing core melt down, fission product release and transport, as well as the fission product behaviour in the containment. The test facility represents a 900 MWe PWR at a 1/5000 scale. The second test, FPT1, was carried out in 1996 and utilised an Ag-In-Cd alloy control rod. There is a growing interest in Best Estimate Plus Uncertainty analysis both in the research, as well as in the licensing. It has a long-standing history in case of thermal–hydraulic system codes focusing on design basis accidents, but its application is limited on the severe accident domain. In recent times however an increasing attention has been paid to evaluating severe accident codes’ uncertainties. This study presents an uncertainty and sensitivity analysis (UaSA) of the FPT1 test, focusing on fission product behavior modeling within the severe accident code system AC2. The detailed application of the coupled codes ATHLET-CD and COCOSYS in this context is novelty. Detailed description of the methodological and practical approach is provided. The uncertain input parameters chosen and quantified are directly related to the phenomena describing the fission product transport and retention phenomena both in the primary circuit, as well as in the containment, while thermal–hydraulic conditions within the primary circuit were kept as much as possible constant. Finally, the study has been complemented by a sensitivity analysis deriving the Spearman's rank correlation coefficient for each uncertain input parameter.

A scoping investigation on debris bed formation with high-temperature melt simulant Fe-Sn

In hypothetical severe accidents of Nordic boiling water reactors (BWRs), the core melt (corium) has the potential to relocate to the lower head, leading to the failure of the reactor pressure vessel (RPV). Consequently, the corium, a molten mixture of either UO2-ZrO2 or Fe-Zr, is anticipated to undergo fuel coolant interactions (FCI) in a deep-water pool within the reactor cavity which is supposed to be flooded during severe accidents. The characteristics of the resulting debris bed play an important role in corium coolability that is paramount to accident stabilization and termination. This study is dedicated to characterizing the debris bed resulting from the FCI using Fe-Sn as the simulant of Fe-Zr. Compared with other simulants in our previous studies, the Fe-Sn alloy has a a higher melting point of 1130 °C and the same element of Fe as in prototypical Fe-Zr. The Fe-Sn is melted in a clay graphite crucible with glass coating layer to minimize contamination of molten materials by graphite. Argon containing 1.26 % hydrogen is employed as cover gas to avoid the oxidation of Fe-Sn at high temperature. The process of debris bed formation, including melt jet breakup, debris fragmentation, and fragments’ sedimentation on the pool bottom, is meticulously recorded using high-speed cameras. The porosity of the debris bed is determined in the post-test analyses using a three-dimensional laser scanner and a graduated water tank. The laser scanner data are also used to reconstruct the geometric appearance of the final debris bed. The debris particulates are sieved to obtain their size distribution. The chemical interaction between water and melt is examined by the SEM analysis. The experimental results show that the debris bed of Fe-Sn has significant differences from those of low melting point simulants (Sn-Bi, Sn, Zn) in our previous studies.

Optimal formulation and performance evaluation of functional ultra-high performance concrete with barium ferrite

The employment of a core catcher is instrumental in enhancing the safety of nuclear power facilities in the event of severe accidents, with sacrificial materials serving as fundamental constituents. Currently, cement-based sacrificial materials are prevalently utilized due to their straightforward construction methodologies and economical production costs. These materials are designed to withstand significant impact loads arising from the descent of core melt during severe accidents. To augment the impact resilience of sacrificial materials, a novel functional ultra-high performance concrete (FUHPC) incorporating barium ferrite has been developed. Using the Modified Andreasen and Andersen particle packing model to determine the initial mixture of FUHPC, and the impact of barium ferrite on the mechanical and thermal properties of FUHPC was systematically evaluated. Furthermore, the influence of barium ferrite on the FUHPC's microstructure was examined with mercury intrusion porosity and scanning electron microscopy techniques. The findings indicate: (1) Optimal barium ferrite incorporation led to a decrease in the porosity and the threshold pore diameter of FUHPC; (2) The flexural and compressive strengths, as well as the elastic modulus of FUHPC, were found to increase within the ranges of 4.41%–17.50 %, 2.88%–22.91 %, and 5.80%–15.06 %, respectively, as a result of suitable barium ferrite additions; (3) At a barium ferrite concentration of 2 vol%, the chloride migration coefficient of FUHPC was reduced by 21.43 %; (4) The presence of barium ferrite enhanced the impact performance of FUHPC, with a 21.38 % increase in impact resistance noted at a barium ferrite content of 2 vol%; (5) In high-temperature scenarios, the formation of channels from melted polypropylene fibers served to mitigate the spalling of FUHPC; (6) Upon considering the effects of barium ferrite on both room temperature and elevated temperature performance, as well as on microstructural characteristics, the optimal barium ferrite content was determined to be 2 vol%.

Application of moving particle semi-implicit method on simulating melt spreading within OECD/ROSAU project

In the context of severe accidents, considerable research efforts throughout the world are currently directed towards ex-vessel corium behavior. The NEA Reduction of Severe Accident Uncertainties (ROSAU) project aims to reduce knowledge gaps and uncertainties associated with two areas: the spreading of core melt in the containment cavity as well as ex-vessel core melt and debris coolability. One pre-test and five large underwater melt spread tests (MST) with molten prototypic material in a newly designed facility are conducted at the Argonne National Laboratory (ANL) in the United States, under the co-ordination with the US Nuclear Regulatory Commission. Part of KTH contributions is to provide numerical results with the developed Moving Particle Semi-implicit (MPS) method code, with specific focus on the temperature distribution and leading-edge progression. The MST-0 and MST-2, conducted in a dry and wet spreading channel, are simulated in the present study. The predicted temperature by MPS code indicates a noticeable decrease at the melt leading edge and a slow decrease in bulk melt in both simulations. Additionally, it is found that the MPS code underestimates the melt average thickness in both simulations due to the absence of a debris porosity model. Overall, the simulation results suggest that the MPS code predicts the melt leading-edge progression and immobilization for all the tests.

Design, fabrication and experiment of soft fingers with double-chamber based on layer jamming

PurposeThe purpose of this paper is to propose a casting process for the production of double-chamber soft fingers, which avoids the problems of air leakage and fracture caused by multistep casting. This proposed method facilitates the simultaneous casting of the inflation chamber and the jamming chamber.Design/methodology/approachAn integrated molding technology based on the lost wax casting method is proposed for the manufacture of double-chamber soft fingers. The solid wax core is assembled with the mold, and then liquid silicone rubber is injected into it. After cooling and solidification, the mold is stripped off and heated in boiling water, so that the solid wax core melts and precipitates, and the integrated soft finger is obtained.FindingsThe performance and fatigue tests of the soft fingers produced by the proposed method have been carried out. The results show that the manufacturing method can significantly improve the fatigue resistance and stability of the soft fingers, while also avoiding the problems such as air leakage and cracking.Originality/valueThe improvement of the previous multistep casting method of soft fingers is proposed, and the integrated molding manufacturing method is proposed to avoid the problems caused by secondary bonding.

From simulation to reality: CFD-ML-driven structural optimization and experimental analysis of thermal plasma reactors

Thermal plasma reactors offer an environment of high temperature, enthalpy, and reactivity, making them highly efficient for solid waste treatment and promising for clean energy production from municipal and industrial waste. Optimal geometrical parameters of the reactor can enhance waste treatment and reactor performance. This study presents a comprehensive analysis of 11 key geometrical parameters of a thermal plasma reactor. Utilizing CFD Fluent software, numerical simulations were conducted to generate a dataset. Subsequently, a predictive model focusing on the average temperature in the core melt zone was trained using six Machine Learning (ML) algorithms. The Particle Swarm Optimisation (PSO) algorithm optimized the hyperparameters of the Gradient Booster Regression (GBR) model, which was combined with a Genetic Algorithm (GA) to identify the reactor's optimal geometrical parameters. A DC arc plasma torch-solid waste thermal plasma reactor treatment system was established on this basis. The study also explored the effects of gasification coefficient, reaction temperature, and thermal plasma jet mode on system performance. Findings indicate that the PSO-GBR model achieved the highest prediction accuracy, with the temperature in the core reaction zone reaching 3621 K. The deviation between numerical simulations and machine learning predictions was a mere 1.3%. Enhancing syngas yield and energy efficiency is achievable by controlling reaction temperature and increasing the gasification coefficient. A laminar plasma jet mode, at equal power, provides a more effective reaction environment. The accuracy and reliability of the machine learning-driven regression model and optimization results are significant in guiding the optimal design of plasma reactors and advancing waste-to-energy conversion processes.

Investigation of the interaction between corium and metal-coolers at the VCG-135 test bench in the conditions of a severe accident

This article presents the results of research after a series of experiments at the VCG-135 test bench to study the interaction between corium and candidate metal-coolers. Metal-coolers can be used to organize a heat removing from the surface of the corium in the core catcher until the cooling water supply. Zinc, antimony and manganese were selected as the investigated candidate metal-coolers.As a result of the conducted research, the patterns and features of the interaction of metals with corium in the conditions of discharging fragments of solid metal into the corium melt have been established. The macro and microstructure of the solidified compositions of the products of the interaction of corium with metal coolers, the elemental composition to determine the presence of the studied metals in their structure were studied. An elemental analysis of material samples in various areas of the experimental assembly and the working chamber of the VCG-135 test bench was conducted to determine the pattern of migration of corium components and interaction products from the melting volume.

Modeling of micron-sized aluminum particle combustion in hot gas flow

This paper presents a model for micron-sized aluminum (Al) particle combustion in hot oxidizing environments, forming hollow spheres. The model comprises four sub-models describing the physical and chemical processes during Al combustion: melting of the solid core, ejection of liquid Al droplets from the breaking solid shell, vaporization of liquid droplets, and ignition and establishment of vapor flame surrounding the solid particles. The model is of critical importance when the ambient gas temperature is higher than the melting point of the Al core, Tc,m, but lower than the alumina (Al2O3) shell’s melting point, Ts,m. In the model, the Al core is assumed to be surrounded by a thin, compact alumina shell that blocks the diffusion of oxidizer into the core and prevents surface reactions. The alumina shell’s cracking and liquid Al’s eruption are triggered by thermal expansion and pressure buildup in the liquid core. The splashed liquid Al droplets vaporize quickly and initiate gas-phase reactions, followed by the vaporization of the liquid Al core as the particle temperature Tp increases. Al vapor combustion heat is redistributed to simulate the gaseous flame near the particle. The model is implemented using the Lagrangian particle tracking method and is validated through simulations of micron-sized Al particle combustion in hot gas and comparison with experiments. The results can explain the formation of the sharp-edged holes on hollow aluminum oxide spheres and the ignition behavior observed in the experiments.

Wetting property of Fe‐S melt in solid core: Implication for the core crystallization process in planetesimals

AbstractIn differentiated planetesimals, the liquid core starts to crystallize during secular cooling, followed by the separation of liquid–solid phases in the core. The wetting property between liquid and solid iron alloys determines whether the core melts are trapped in the solid core or they can separate from the solid core during core crystallization. In this study, we performed high‐pressure experiments under the conditions of the interior of small bodies (0.5–3.0 GPa) to study the wetting property (dihedral angle) between solid Fe and liquid Fe‐S as a function of pressure and duration. The measured dihedral angles are approximately constant after 2 h and decrease with increasing pressure. The dihedral angles range from 30° to 48°, which are below the percolation threshold of 60° at 0.5–3.0 GPa. The oxygen content in the melt decreases with increasing pressure and there are strong positive correlations between the S + O or O content and the dihedral angle. Therefore, the change in the dihedral angle is likely controlled by the O content of the Fe‐S melt, and the dihedral angle tends to decrease with decreasing O content in the Fe‐S melt. Consequently, the Fe‐S melt can form interconnected networks in the solid core. In the obtained range of the dihedral angle, a certain amount of the Fe‐S melt can stably coexist with solid Fe, which would correspond to the “trapped melt” in iron meteorites. Excess amounts of the melt would migrate from the solid core over a long period of core crystallization in planetesimals.

Coupled AC2-CFD simulations for a high-pressure core melt accident scenario

Abstract Even though very unlikely to occur, severe accident scenarios in nuclear power plants have to be analyzed. During high-pressure core meltdown scenarios in a pressurized water reactor the primary circuit should fail first. Previous analyses found that a free convection flow within the vertical steam generator (SG) tubes with a simultaneous stratified gas counterflow in the hot legs could arise. This phenomenon leads to higher thermal loads on individual SG tubes which might then fail leading to a containment bypass and the release of radioactive material into the environment. Lumped parameter system codes used for safety analyses do not provide the models necessary to simulate phenomena like mixing in three-dimensional flows and could not consider local turbulence effects. Computational fluid dynamic (CFD) codes provide such capabilities but are much more computationally expensive. Coupling of a system code with a CFD code can therefore be used to simulate such phenomena. The advantages of both approaches can be maximized by splitting up the simulation domain between the codes, depending on the expected flow conditions. The system code AC2 coupled with the CFD code OpenFOAM was used to simulate part of the severe accident transient. Free convection in the hot leg and the U-tubes of the vertical SG was observed in case of high-pressure severe accident sequences. The thermal load of individual SG tubes has been estimated from the results. These loads can be used as inputs for structural-mechanical analyses to estimate which part of the primary circuit would fail first.

High-temperature Mechanical Analysis of Discharge Pipeline of Pressurizer

The severe accident is the beyond basis design accident due to several initial faults. The discharge pipeline of pressurizer is mainly used for pressure relief and evacuation of hydrogen in the primary circuit in severe accident. It can reduce the possibility of high-pressure core melt and the explosion of hydrogen. But in severe accident, the highest temperature of gas in the pipeline can reach 1200℃, which far exceeds the design temperature of pressurized water reactor. So it may cause the buckling or rupture of the pipeline. The pipeline may loss its discharge function. In this passage, we study the resistance of the discharge pipeline to high temperature in severe accident from the aspects of severe accident situation, high-temperature mechanical properties of pipeline material, the temperature field and stress of the pipeline, the evaluation of the stress and so on. In order to demonstrate the function of a certain nuclear power plant discharge pipeline, RCC-MR which is suitable for the high-temperature evaluation, is choose to evaluate the stress, the creep and the buckling of the pipeline. The results of the evaluation show that the discharge pipeline in this nuclear power plant can meet the functional requirements in the severe accidents.

Study on contact melting process of vertical cylindrical materials in a circular container

In view of the reactor core melting in a severe reactor accident, the contact melting process of vertical cylindrical materials inside a circular container is investigated. The melt analysis model and governing equations are established by the contact melting theory. The dimensionless analytical expressions of melting velocity and melting height were obtained by theoretical solution. The influence of temperature difference, cylinder solid size and filling rate was discussed and analyzed. It is found that the melting velocity decreases gradually during the melting process. Before all the molten liquid submerges the solid, the buoyancy of solid increases gradually and the melting velocity decreases rapidly. After all the molten liquid submerges the solid, the buoyancy and gravity decrease at the same time, and the melting velocity changes gently. Compared with the melting of an individual solid with the same height and volume, the melting velocity of multiple cylindrical solid can be effectively accelerated.

Core atoms escape from the shell: reverse segregation of Pb–Al core–shell nanoclusters via nanoscale melting

Melting is a phase transition that profoundly affects the fabrication and diverse applications of metal nanoclusters. Core–shell clusters offer distinctive properties and thus opportunities compared with other classes of nano-alloys. Molecular dynamics simulations have been employed to investigate the melting behaviour of Pb–Al core–shell clusters containing a fixed Pb147 core and varying shell thickness. Our results show that the core and shell melt separately. Surprisingly, core melting always drives the core Pb atoms to break out the shell and coat the nanoclusters in a reversed segregation process at the nanoscale. The melting point of the core increases with the shell thickness to exceed that of the bare core cluster, but the thinnest shell always supresses the core melting point. These results can be a reference for the future fabrication, manipulation, and exploitation of the core–shell nanoalloys chosen. The system chosen is ideally suited for experimental observations.

Viscosity measurement of molten alumina and zirconia using aerodynamic levitation, laser heating and droplet oscillation techniques

Reliable thermophysical properties of core melt (corium) are essential for the accurate prediction of the severe accident progression in light water reactors. Zirconia is one of the most important materials in corium. Despite the high interest in the viscosity of molten zirconia, few experimental data have been reported due to its high melting temperature and high vapor pressure. In the present study, the viscosity of molten zirconia was measured using aerodynamic levitation, laser heating and droplet oscillation techniques. A material sample was levitated by argon gas flow in a conical nozzle and then melted into a droplet by laser beams. The initial quiescent droplet was forced to oscillate by the excitation of a loudspeaker, and the viscosity was deduced based on the characteristics of the droplet damped oscillation after the loudspeaker was turned off. The viscosity of molten alumina was first measured for verification of the measurement system. Afterwards the viscosity of molten zirconia was measured. The results showed that the viscosity of molten zirconia at melting temperature (2988K) was 12.87 ± 1.03 mPa s and decreased with increasing temperature. The measurement uncertainties are within 21 %.

Modified correlations for the Nusselt numbers at the boundaries of a bottom-heated molten metal layer in a stratified corium melt pool and an assessment of heat transfer conditions during a severe accident

The paper discusses new correlations for the Nusselt numbers at the side and bottom surfaces of a bottom-heated molten steel layer under the conditions of mixed natural convection of the melt at Rayleigh (Ra) numbers from ∼ 106 to 1012. The melt layer under consideration is characterized by radial non-uniformity of the temperature distribution, when the averaged temperatures of the top and side surfaces of the layer show a difference in value. This thermal pattern of the molten metal layer is typical when a stratified corium molten pool forms during a severe accident (SA). Upper metallic melt layer goes above the oxide phase of corium pool, and it is heated along its bottom surface. For an adequate prediction of heat transfer at the side and bottom surfaces of a layer in a SA, we developed special correlations for the Nu numbers. They take into account Rayleigh number and additional factors that determine geometric parameters of the layer and the temperature conditions at the bottom, side and top surfaces of the layer. Parameters in the developed correlations for the Nu numbers were determined via CFD simulation and mathematical processing of the simulation results. CFD simulations by means of the ANES code were completed in accordance with a specially developed experiments matrix. Two-parameter k-ω turbulence model by Wilcox, which includes Menter’s improved near wall functions, has been implemented in the CFD code and applied accordingly. To evaluate the accuracy and predictability of the developed correlations we ran a comparative analysis of the Nu numbers at the sidewall and bottom surfaces of the melt layer, as defined on the basis of the developed correlations for the Nu numbers and as defined in CFD simulation. The comparison showed a strong correspondence of the results within a wide range of Rayleigh number values of ∼ 106 < Ra < 1012. The analysis of the developed correlations allowed shaping a number of critical arguments that define heat transfer conditions at the melt layer boundaries. It was found that the Nu numbers at the side and bottom surfaces are largely dependent on the difference in the temperatures of the top and side surfaces of the layer. As the difference between the these temperatures widens, the Nu numbers increase drastically (by tens of percent or more) in comparison to similar Nu numbers defined on the basis of the well-known correlations. It is specified that when the temperatures at the top and side surfaces of the layer are equal, the heat transfer conditions at the side surface are defined by “Globe and Dropkin” correlation used for the horizontal surfaces of the layer. We also evaluated the significance of the geometric parameters on the Nu values for the sidewall and bottom surfaces of the melt layer. Multiplicative effect of this parameter and of the parameter for temperature conditions at the boundaries of the layer mentioned above may lead to a significant (by times) increase of the Nu values. That must be considered in the thermal analysis of the corium pool and the processes of the SA.

Numerical simulation of melt jet breakup under non solidification conditions

The nuclear reactor core melting accident can cause the leakage of radioactive materials, resulting serious harm to the surrounding environment and the safety of the public. For modern PWR, if the core structure was melted in a severe accident, the generated melt will move downward and interact with the remaining coolant at the lower head, resulting a series of physical phenomena. In this paper, the numerical model based on the VOF to DPM method is presented to analyze physical processes of the interaction between melt jet and coolant. The influence of coolant phase change on the jet breakup length, degree of fragmentation, and particle diameter of melt debris are analyzed under different conditions. This paper analyzes the impact of the incident velocity of melt, the incident diameter, the density ratio of melt and coolant on the jet breakup length based on Saito's empirical formula, and compares the simulation results with the empirical formula. The working condition of Yutaka experiment is simulated in this paper and the diameter distribution of the broken fragments is analyzed, as well as the influence of the incident temperature, incident mass and incident velocity of the melt on the average mass diameter of the generated particles. The numerical results reach a good agreement with the experimental data. This work provides a feasible numerical scheme for the analysis of melt jet entering the lower head coolant under accident conditions.

An experimental study on the effect of chemical additives in coolant on steam explosion

In assessment of severe accident risk in light water reactors (LWRs), steam explosion is a nonnegligible phenomenon following a relocation of core melt (corium) into coolant, and thus various research efforts have been paid to steam explosion. There had been numerous studies showing that the occurrence of steam explosions is influenced by several factors such as melt and coolant temperatures, melt materials, non-condensable gasses, etc. However, most of the existing experiments used deionized (DI) water or tap water as coolant, with little consideration of the effect of chemicals (e.g. boric acid, sodium hydroxide, sodium phosphate) commonly applied in reactor coolant. To examine the effect of the chemical additives in coolant on steam explosion, the present study performs a series of molten Tin droplet-coolant interaction tests using DI water and different chemical solutions, including H3BO3 solutions, NaOH + H3BO3 neutral solutions, and Na3PO4 + H3BO3 neutral solutions. The experimental results show that adding NaOH and Na3PO4 in boric acid solution significantly affects the occurrence probability of spontaneous steam explosion, because of the presence of PO43− and H+ ions. When different solutions have equivalent concentrations of H3BO3, the peak pressure values of the spontaneous steam explosion of Sn droplets are similar among various solutions. Compared with those in DI water, steam explosion in the chemical solutions occurs predominantly within a narrow range of depth from 28 mm to 40 mm and produces a much higher peak pressure. This implies that more energetic steam explosions may occur in the chemical solutions.

Relevance of the Wigner–Seitz Cell Approximation for the Coulomb Clusters

A molecular dynamics simulation of a system of massive charged particles on a compensating homogeneous background confined by a spherical surface has been carried out. A crystallized cluster is a set of nested spherical shells of almost the same structure and a core. It is shown that cluster melting is a combination of shell and core melting. It is found that the values of the Coulomb coupling parameter Γ corresponding to these two types of melting do not depend on the cluster size. Methods for determining Γ based on the Wigner–Seitz cell model are discussed. It is shown that the estimate based on the root-mean-square deviation of a particle from the center of its cell is unreliable due to the self-diffusion of particles. A relation is proposed that defines Γ in terms of the root-mean-square velocity and acceleration of the particle and does not include the root-mean-square deviation of the particle from its average position. It is shown that this relation is satisfied with high accuracy not only for the crystallized, but also for the liquid state. Thus, it has been demonstrated that the Wigner–Seitz cell model is applicable to the strongly inhomogeneous system under consideration.

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.