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The ablation consecutive to hot jet impingement is a safety issue for future Sodium Fast Reactors (SFR) core-catcher that can occur during relocation of molten fuel (corium) under severe nuclear accident situation. Much is still unknown on the ablation phenomenon especially explanations on cavity shape are lacking. However, these data are required to specify the core-catchers which will be used in future nuclear reactors’ vessel to prevent the corium from drilling through the confinement vessel during severe accident. To tackle this subject, data from experiments are obtained and analyzed to identify first order physical mechanisms at stake, and their links to geometry of the cavity. Two ablation mechanisms are noticed, the film ablation regime, for which liquid exits the cavity as a liquid film followed by the pool effect for which the cavity is filled with liquid. The analysis of results shows that the cavity shape is fixed during the film ablation regime and translates as ablation proceeds. Modes of liquid exit from the cavity are analyzed as well as the shapes the cavity assumes. Liquid/air interface temperature is also determined, in the film ablation regime. An explanation of cavity shape is presented. Conditions based on dimensionless numbers are put forward to differentiate between different cavity shapes and liquid exit modes. A first model for transition between film and pool effect ablation regimes is presented. It is the first time to the best of our knowledge that such analyses are undertaken. These give new tools for ablation risk assessment.

Heat conduction problems are usually solved either with analytical or numerical simulations, or with a reduced model using system identification. The use of polynomial models, often used in automation theory, gained the attention of the thermal community in the last decades to elaborate these reduced models because of their simplicity and performance to characterize an invariable system. They allow, for example, estimating a local temperature with a known input heat source. However, using polynomial models (or identified systems) in inverse conduction problems is not straightforward, usually requiring either a second inversion step. In this paper, we present a novel inverse technique based on the polynomial model ARX (autoregressive with exogenous input) that allows the estimation of an unknown input (like an imposed heat flux on a boundary) using a known output (temperature measurement). This new method, named inverse ARX or IARX, only requires a calibration step as a regular polynomial model and, then, it can estimate the input by a direct calculation with the identified parameters. The difference between IARX and ARX is the presence of future exogenous parameters, which were deduced using the initial discrete form of the ARX model. We present herein a numerical example using IARX of a 1D heat conduction simulation and IARX succeeded to estimate the input heat flux, even with high discontinuities and high measurement noises. Finally, we compare the proposed method with the classical Beck’s function specification method. IARX presented advantages like having no restriction for the number of future terms used in the model and performing the calculation 45% faster and with much less memory space consumption than with Beck’s method.

When the water inventory evaporates during a hypothetical loss of coolant accident in the core of a pressurized water reactor, the fuel rods temperature increases substantially, resulting in the clad ballooning and the formation of blocked sub-channels. During the reflooding phase, where water is injected into the core and a steam-droplets flow is created above the water level, the presence of ballooned zone forces the steam flow to redistribute towards intact sub-channels, which degrades the cooling of ballooned clads within blocked sub-channels. Looking to better understand this flow redistribution process, this study presents experimental results of magnetic resonance velocimetry measurements of three-component velocity fields in several 49-element ballooned bundles to evaluate geometric effects (blockage ratio, length and coplanarity) on the flow dynamics, as well as the flow rate effect (Reynolds number from 1,936 to 9,599). The flow redistribution occurred in the transition zone upstream of the balloon. Moreover, the transverse velocities at this location are higher for higher blockage ratios and they can reach nearly the same magnitude of the axial velocity. The amount of deviated flow is approximately equal to the sub-channel’s blockage ratio and is virtually insensitive to the blockage length or the flow rate. Furthermore, the flow’s axial velocity reduces up- and downstream of blocked sub-channels and this reduction is more substantial as the blockage ratio increases. Results with non-coplanar balloons showed that a less intense flow redistribution takes place; however, a downstream balloon can affect the flow redistribution dynamics caused by an upstream ballooned zone. Finally, for all the tested bundles and tested flow rate ranges, the flow was remarkably homogenized by the downstream mixing spacer grid, restricting the clad ballooning effects only to the portion between the two spacer grids where the ballooned zone is located regardless of the balloon geometry and the flow rate. The present results may be useful as well for other research areas involving flow blockages in heat exchangers.

This paper deals with ablation of a solid by a high temperature liquid jet. This phenomenon is a key issue to maintain the vessel integrity during the course of a nuclear reactor severe accident with melting of the core. Depending on the course of such an accident, high temperature corium jets might impinge and ablate the vessel material leading to its potential failure. Since Fukushima Daiichi accident, new mitigation measures are under study. As a designed safety feature of a future European SFR, bearing the purpose of quickly draining of the corium out of the core and protecting the reactor vessel against the attack of molten melt, the in-core corium is relocated via discharge tubes to an in-vessel core-catcher has been planned. The core-catcher design to withstand corium jet impingement demands the knowledge of very complex phenomena such as the dynamics of cavity formation and associated heat transfers. Even studied in the past, no complete data are available concerning the variation of jet parameters and solid structure materials. For a deep understanding of this phenomenon, new tests have been performed using both simulant and prototypical jet and core catcher materials. Part of these tests have been done at University of Lorraine using hot liquid water impinging on transparent ice block allowing for the visualizations of the cavity formation. Other tests have been performed in Karlsruhe Institute of Technology using liquid steel impinging on steel block.

In the scope of safety studies for future sodium cooled fast reactors (SFR), the use of discharge tubes and in-vessel core-catcher are foreseen for severe accident mitigation. During relocation of corium onto the core-catcher, complex thermal hydraulics phenomena can occur. This work focuses on the ablation of the core-catcher by the jet of corium. For that purpose, as experiments using corium and Na are difficult to achieve, we performed experiments using simulant materials: water jet on transparent ice. Transparent ice allows for ablation cavity real-time recording. Two main regimes were identified: a film flowing regime and a pool regime. We also studied the influence of temperature (30, 50, 70 °C) and velocity ( 1 , 2.5 , 5 , 7.5 , 10 m / s ) of the jet. Comparison with existing data have been done and finally, a scaling law in the first regime, for which the ablation rate is constant, has been found.

This paper focuses on the experimental investigation of breakup regimes of a molten fusible metallic droplet in water, at intermediate Weber numbers with emphasis on mass and energy balance. The experiment consists in impacting perpendicularly a molten drop onto the interface of a deep water pool, at a controlled temperature. Using a drop-on-demand device and high-speed shadowgraph, a single drop can be visualized during its evolution. There is a noticeable velocity jump when the droplet crosses the interface that can be modeled using an unsteady Bernoulli equation. As observed for liquid–gas systems, the drop experiences different regimes of fragmentation, depending on its Weber number: oscillations, bag oscillations, prolate drop stretching breakup, and then bowl-shaped bag breakup. However, opposite to the gas–liquid case, a Rayleigh–Taylor instability mechanism seems to be absent and this seems related to the bowl-shaped bag breakup mechanism when compared to the dome-shaped gas–liquid case. Statistics of the daughter droplets are then given, using either image analysis for large droplets size distribution or sieving and weighting of the solidified fragments for measurement of the Sauter mean diameter and surface energy creation. Finally, a simple relation between the Sauter mean diameter and the Weber number is presented based on the energy and mass balances. When comparing with previous higher Weber number results, a viscous transition corresponding to a strong increase in the energy loss is also shown to occur for the higher Weber number.

International audience; In the course of severe accident in a nuclear power plant, the corium mixture may flow down in the reactor pit and start a thermal attack of the basemat concrete. A simple way to terminate the melt progression is to add water on top of it. Among the various physical processes that may participate in the quenching, water may penetrate into the solidified corium through cracks generated by the thermal stress during the solidification. The present paper aims at providing a clarification of the process though an analysis of the few experimental data available using proto-typical corium, namely the SSWICS 1-7 experiments. At first, a general reminder and analysis of the data is given. Then, the thermal-hydraulic aspects are investigated through the construction of a dedicated heat flux correlation and its use via the experimentally measured post-test permeabilities. The analysis is supported by 1D and 2D evaluations with the multi-phase flow code MC3D-PREMIX, slightly modified for the purpose. It is concluded that the heat flux drastically decreases with the amount of added concrete material. Furthermore, 2D border effects are investigated and their importance highlighted in view of the experimental results analysed. These effects should explain the absence of observation of water ingression heat flux in the cases with large concrete amounts.Following, the paper proposes a model for the created permeability. Due to the complexity of the process and to the large uncertainties of the needed material properties in the considered situation with very high temperatures, a semi-empirical model is derived and reproduces the available data to a good approximation.Lastly, the model is adapted to the situation with internal decay heat, although no open experimental data is available to precisely support any model. A complete modeling is out of the scope of the paper, hence the focus is to provide hints for a first analysis of the impact of residual power on the melt progression and its coolability.

International audience; Reflooding experiments with rod bundles at Loss Of Coolant Accident (LOCA) conditions usually use intrusive methods with limited access and, consequently, their data are not always adequate and comprehensive to validate simulation codes. For this reason, sub-channel scale experiments are useful to obtain detailed data in a more controlled environment for an accurate thermal–hydraulics investigation. In this study, we present experiments of the cooling phase with an internal steam-droplets flow in a vertical pipe simulating an undamaged sub-channel in a nuclear reactor. Simultaneous measurements were performed of the wall temperature and droplets characteristics (velocity, diameter and temperature) using only optical techniques. An analysis is made on how the steam flow rate and maintained heating power during the cooling phase affect the wall heat dissipation, wall rewetting and droplets dynamics. Results show that wall rewetting normally occurs from bottom to top, and the temperature at minimum heat flux is highly affected by the droplets dynamics. Furthermore, droplets are accelerated when passing through the heated tube, especially at higher wall temperatures, and their temperature is nearly the same up- and downstream of the test section. Results with heating during the cooling phase show that wall rewetting takes place at higher wall temperatures and advances slower with the increase in the maintained heating power. Moreover, for the time period and flow conditions used in this work, wall rewetting does not occur for maintained powers higher than 1.5 kW/m.

Much effort has been devoted by the nuclear community to develop mechanistic models to predict the heat dissipation of fuel rods during a loss of coolant accident (LOCA). However, there is still the challenge to find correlations that accurately predict the rates of heat transfer corresponding to each involved phenomena and gather them all in a single physical model. Looking for improving thermal-hydraulics calculations in LOCA conditions, this work introduces a novel mechanistic model implemented in a new code named NECTAR to calculate heat and mass transfer phenomena and droplets dynamics in a polydispersed flow film boiling. The simulation results are compared with experimental measurements using three different geometries that represent the cladding ballooning at sub-channel scale. NECTAR can predict with great accuracy the heat dissipation by the internal steam-droplets flow with less than 8% mean deviation for all the test cases. The droplets characteristics downstream of the heated tube are also well predicted by NECTAR. In a detailed analysis of each heat transfer process, we found that wall-to-steam convection plays the major role in the internal heat dissipation, followed by the heat removed by the droplets impact onto the wall in spite of the very low volume fraction of droplets. Moreover, droplets impact and steam-to-droplets convection are also the main responsible for the droplets evaporation. Finally, considering the droplets diameter polydispersed distribution into thermal-hydraulics calculations (instead of a single mean diameter) has an effect only for higher volume fractions of droplets.

During a loss of coolant accident (LOCA), blocked sub-channels may appear due to the swelling of the fuel rods’ cladding, which results in flow redistribution during the reflooding phase. For this reason, special attention has been paid to the effect of fuel rods ballooning on the thermal-hydraulics in LOCA conditions. Due to the practically impossible physical or optical access to blocked sub-channels, no experiment so far has performed precise three-component velocity field measurements in the presence of ballooned regions. In this study, we used magnetic resonance velocimetry (MRV) to obtain three-component velocity fields of water flow within two 7 × 7 fuel rods bundles built mainly in plastic, one regular and one containing sixteen ballooned fuel rods with 90% blockage ratio and 240 mm blockage length. We present herein results with 50 lpm water flow rate, which corresponds to a Reynolds number of 1936. With the regular bundle, the performance of spacer grids’ mixing vanes to homogenize the flow was notable. With the ballooned bundle, we observed transverse velocities upstream of the ballooned zone that are as intense as the bulk mean velocity. Furthermore, there are substantial decreases in the axial velocity within blocked sub-channels up- and downstream of the ballooned zone, reaching near-zero and even negative values downstream, indicating flow recirculation. Although the flow is highly affected by the ballooned zone, the mixing spacer grid placed downstream remarkably homogenized the flow and effects of the flow redistribution disappeared. Finally, with the present ballooned bundle configuration, about 90% of the flow that should pass through blocked sub-channel deviates towards less resistant regions, which suggests a predominant geometric effect on the flow redistribution.

International audience; This paper presents experimental results on the fragmentation of a low meting point liquid metallic alloy jet into water. The liquid is Field's metal whose melting point is 62 °C. Data are obtained using high-speed camera acquisition and the solidified particles are sieved, a size Probability Distribution Function (PDF) is obtained from their mass distribution. These results are compared to separate data acquisitions obtained using a phase Doppler anemometer (PDA used in reflexion regime). Injection diameter range from 1 mm to 5 mm and injection velocity from 2.28 m/s to 4.97 m/s resulting in a Weber number ranging from 26 to 309 and a Reynolds number ranging from 4577 to 24875. The conclusion is that for these intermediate Weber and Reynolds numbers, the size of the droplets can mainly be related to Kelvin-Helmholtz instabilities. However, there exists also a long tail of small sized droplet whose distribution can be attributed to turbulent re-agglomeration of ligaments. This part of the distribution is very close to a log-Lévy law thanks to a model developed by [Rimbert & Castanet, PhysRevE, 84, 016318, 2011] By estimating the different turbulent scales, it is even possible to construct the small size distribution of droplet without using any fitting parameter.

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International audience; In the design of every new nuclear reactor, the possibility of a core meltdown is studied. In the case of ASTRID fast breeder a core-catcher will be placed beneath the core. To ensure that it performs as planed its ablation by a liquid jet of corium is investigated. To do so, an experimental set up was designed where the ablation of a solid by a liquid jet is studied using transparent ice as solid and water as liquid. The aim is to visualize the evolution of the cavity shape, and the ablation rate, during the ablation process. We observed that the cavity first assumes a conical shape, and the ablation rate stays constant. Then the cavity shape changes, and the ablation rate decreases with time. The aims to clarify the influence of the temperature, diameter and speed of the jet are studied as well as the inclination of the solid block on the ablation rate and mechanisms. A correlation giving the Nusselt number as a function of the Reynolds and the Prandtl number was obtained for our system. This will be used as a base to identify and model the mechanisms at stake. Introduction

International audience; During a Loss of Coolant Accident (LOCA) in a Pressurized Water Reactor, partial or complete drying of fuel assemblies may take place. In these conditions, the fuel temperature increases and may lead to substantial deformation of the fuel rod cladding and a partial blockage of the fluid sub-channels. These regions could significantly affect the cooling capacity of the nuclear core during the reflooding phase by the emergency core cooling systems. Understanding the cooling process and the thermal-hydraulic characteristics of the flow in these deformed regions is decisive to guarantee nuclear safety. Looking to provide valuable experimental data to validate existing and new models, this work is an experimental study on the thermal hydraulics during the cooling of a vertical tube with an internal steam-droplets flow, representing the LOCA conditions at sub-channel scale. The effect of the blockage ratio on the tube temperature, heat dissipation, wall rewetting, and droplets characteristics is evaluated by testing three configurations (0%, 61% and 90%). Optical techniques were used for a comprehensive characterization of the process, being them infrared thermography, phase-Doppler analyzer and three-color laser induced fluorescence thermometry. In general, wall rewetting in the test section occurs from bottom to top, although there is a discontinuity in the rewetting front with the 90% blockage ratio configuration. The droplets diameter reduces downstream of the test section because of evaporation. Droplets breakup was specifically observed with 90% blockage ratio. In all the cases, the droplets temperature was approximately the same up-and downstream of the test section, which indicates they are in nearly thermal equilibrium state and, therefore, representative of a LOCA situation.

The aim of the chapter is to give physical insights regarding solid/liquid interfaces and solid/liquid nanocomposites via atomistic simulations and improved macroscopic analytical approaches. Particularly, the use of molecular dynamics for the study of a nanodroplet located on a solid substrate will be discussed. The impact of line tension, Tolman length and adsorption layer on the wetting characteristic will be considered in details.

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International audience; Large panels of studies have been performed in order to investigate the dynamic behavior of a droplet during the impingement onto a rigid wall. Most of them are dealing with single droplets and only fewer are devoted to sprays, for which several diagnostics were implemented. Moreover, when sprays are involved, the liquid mass flux is generally quite low (on the order of several kg/m²/s) and the case of a cold surface was mainly considered. Therefore, the present experimental work aims at investigating the impingement of sprays onto hot surfaces (up to 800°C) with liquid mass flux up to 13 kg/m²/s. Specifically, the main objective is to characterize the secondary droplets as a function of the surface temperature, starting from the Leidenfrost regime. To that purpose, a 2D Phase Doppler Analyzer (PDA) is synchronized with an infrared camera in order to measure simultaneously the time evolution of the local droplets size and velocity distributions with the surface temperature. Six full cone sprays were used in order to obtain a wide range of impingement conditions (normal incident Weber and liquid mass flux). A comparison of the impingement characteristics is also performed when the surface remains at room temperature. Another important point concerns also the effect on the spray flow of the presence of the surface. Results show clearly that the incident droplet trajectories are modified by the presence of the solid wall itself and depend strongly on the temperature surface. The in-depth investigation of the impingement characteristics of the spray is mainly focused on the description of the statistical mean diameter and the mass flux after the impingement. The main results highlight that the mean diameter value of the secondary droplets does not change during the Leidenfrost regime and increases for lower surface temperature. This behavior is strongly correlated to the liquid film formation for temperatures lower than the Leidenfrost point. The expelled mass flux exhibits a similar behavior but a decrease is observed after a given temperature, which can be attributed to the deepening of the liquid, leading to a reduction of the number of expelled droplets.

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International audience; An experimental test-rig was designed to study the cooling of hot surfaces under semi-industrials conditions. Two cooling methods were compared: liquid jet and full cone sprays. The wall is a 175 mm diameter nickel disk heated by electromagnetic induction up to 800 °C. The heat removed during the cooling phase was investigated with the use of infrared thermography while the droplet properties, in the case of sprays, were characterized simultaneously with a Phase Doppler system. In the case of spray cooling, the Phase Doppler technique was mainly applied to study the statiscal properties of the droplets in the vicinity of the heated surface. The influence of the liquid mass flux distribution as well as the droplets properties on the cooling are studied by using five different spray nozzles. The liquid mass flux distribution is also measured with the help of patternator. The main results show that even if higher heat flux can be removed with the liquid jet the efficiency cooling is poor because of its non uniformity. When spray cooling is considered, the results are in qualitative agreement with previous studies performed in similar conditions.

International audience; This experimental work aims at investigating the cooling of hot surfaces by using full cone sprays; comparison with the use of a liquid jet is also considered. The wall is a 175 mm diameter nickel disk and 5 mm thickness heated by electromagnetic induction up to about 800 °C. In the case of the sprays, the goal is to link the spray properties with the heat flux removed from the heated surface. For the spray, the influence of the mass flux distribution as well as the droplets properties on the cooling are studied by using three different spray nozzles. The mass flux distribution for each of the sprays is measured with the help of a patternator. The heat removed during the cooling phase is investigated with the use of infrared thermography while the droplet properties are characterized simultaneously by a Phase Doppler system. The Phase Doppler technique is mainly applied to study the statistical properties of the outcom-ing droplets in the vicinity of the heated surface. A decrease of the outcoming droplets size compared to the incoming one is well observed. Moreover, Phase Doppler device has also highlighted that the presence of the surface may have a significant influence on the upstream spray flow. Compared to the liquid jet, heat flux measurements have clearly demonstrated that the sprays lead a more spatial uniformity of the extracted heat flux and a better cooling efficiency. When spray cooling is considered, the influence of the mass flux on the heat flux and the cooling efficiency is in qualitative agreement with previous studies performed in similar conditions. In addition, a comparative study of performances, in term of liquid consumption and cooling duration, is performed by considering a similar surface temperature decrease for the three sprays and the liquid jet. Among the four experiments, the liquid jet has the longest cooling time as well as the highest liquid consumption. Furthermore, the best performances are reached for the spray having the highest mass flux value.

Combustion, spray and flow dynamics for aerospace propulsionCombustion, spray and flow dynamics for aerospace propulsion; International audience; In aero-engines, droplet/wall interaction phenomena have a considerable influence on the mixture formation process and on wall heat fluxes. Impinging droplets may rebound, splash into secondary droplets or form a liquid film onto the solid surface. Droplet rebound and splashing is also a mechanism for the back penetration of the fuel vapor in the central region of the combustion chamber where the gas temperature is high enough for ignition. This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80ֱ80 ֭) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the two-color Laser-Induced Fluorescence Thermometry which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the liquid sensible heat. Measurements are performed at surface conditions well above the Leidenfrost temperature. A wide range of Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparison between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.

International audience; When droplets impinge onto a hot wall, different regimes can be observed depending on the wall temperature and Weber number. In the case of interest, the temperature of the wall is more than the Leidenfrost temperature and the Weber number based on normal velocity of the droplet is less than the threshold value leading to the splashing regime (We < 80). We particularly focused on the perfect bouncing regime (We < 30) for which an impinging droplet levitates on a thin layer of its own vapour. This vapour is instantaneously created between the base of the deforming droplet and the heated surface so that direct contact with the hot solid is avoided. For these low Weber numbers, the droplet surface energy is high enough compared to its kinetic energy to permit the rebound, so that the droplet recovers its initial shape without breaking up after the bounce. Although the generated vapour insulates the droplet, some heat is exchanged with the wall during the interaction (i.e. during the resident time). In this paper, we report on experimental measurements of heat transfer due to droplet impact in the Leidenfrost regime. The energy released by the wall and measured using an inverse conduction method leads to an estimation of the heat transfer coefficient during impact in the Leidenfrost regime. For that, the time evolution of the droplet base surface is estimated using a simple modelling validated on experimental data. Finally, the energy measured is compared to existing models.

International audience; In this paper, we consider a two-dimensional inverse heat conduction problem (IHCP) in a jet cooling experiment. A nickel disk is heated by an induction device up to 600 °C before being cooled by a water jet impinging on its front side and, at the same time, the temperature on the rear side is measured by a high resolution infrared thermography. The transient heat flux at the cooled face is determined from the IR measurements. To achieve this goal, an inverse heat conduction problem (IHCP) is formulated using an analytical solution of the direct problem expressed in the Hankel space. Then, results of simulated experiments are given in order to illustrate the performance of this method but also its limitations. Finally, an example of heat flux estimation from real temperature measurements is made.

Nous avons mene des travaux de recherches sur l’hydrodynamique des suspensions dans le but de bien apprehender les problemes lies a l’influence de la phase dispersee des ecoulements charges en spheres dures dans l’industrie agroalimentaire. Dans ce papier nous presentons les resultats de ces travaux. Cette etude experimentale a permis de verifier si les modeles rheologiques etablis pour les suspensions de spheres dures de structure microscopique peuvent s’appliquer a des suspensions de « grosses » spheres dures de diametre d tel que d = 4,4 mm et d’expliquer les problemes lies au mecanisme de transport des ecoulements charges en particule d’aspect d/D = 0,15 dans une conduite horizontale de diametre D tel que D = 30 mm. Ces resultats ont permis de mettre en evidence l’influence de la phase des particules solides en suspension dans une solution de carboxymethylcellulose (C.M.C), fluide au comportement non newtonien. Ils ont valide l’hypothese du milieu effectif pour l’etude la perte de charge des suspensions de grosses particules a l’aide du facteur de correction de Quemada. Mots cles : suspension, solide liquide, rheologie, spheres, hydrodynamique, non-newtonien.

International audience; In hot rolling, the mechanical properties of steel alloys are conditioned by the rolling process but a great part is ensured by the cooling of the hot strip mill. Well controlling this cooling rate and its homogeneity is thus of primary importance for obtaining steels with desired mechanical properties. As the water used in the cooling stage of the rolling process can be polluted by oil (in hot mill strip, some oil is used to lubricate the rolls and a part of it can pollute the water), it is important to know how much varies the cooling rates when water is polluted. In this study, transient cooling has been investigated during quenching of a hot metal disk with various subcooled oil-in-water emulsion jets. The aim of this work is to compare the cooling efficiency of oil-in-water emulsion jet with a pure water jet. Experimental investigations of axi-symmetric jet impingements on a preheated hot metal disk (500–600 °C) have been performed with various oil-in-water emulsions. The transient cooling heat fluxes on the quenched side are estimated by coupling the measurement of the temperature field of the other side (rear face) with a semi-analytical inverse heat conduction model.

Thèse confidentielle jusqu'au 10 mars 2030.; In the case of a hypothetical nuclear severe accident with partial or extensive core meltdown, the superheated magma made of molten steel and fuel, called corium (T > 2500K), may threaten the integrity of the reactor pressure vessel and subsequently the reactor containment building, if long-term corium coolability is not assured. The coolability by water injection and successive water penetration into the corium through the upper surface is analyzed for two expected configurations: particle bed, and corium pool overlaying the concrete. The second configuration is linked to the situation of Molten Corium-Concrete Interaction (MCCI), where a crust is formed in the upper corium surface when it comes into contact with water and is later subjected to thermal stresses that lead to its fracturing. The challenge is to characterize the effectiveness of extracting heat by the possible water penetration into the crust. The first configuration can be expected in two different situations: melt fragmentation coming from the rupture of the reactor pressure vessel and expulsion of the corium, or during melt eruption episodes through the corium crust during MCCI via corium entrainment by the concrete decomposition gases. The phenomena linked to the water penetration into the corium for these two configurations are examined through an in-depth analysis of the available experimental results, by the development of an analytical model and finally through the modification and use of the Computational Multi-Fluid Dynamics (CMFD) code MC3D. One dimensional analysis conducts to a better understanding of the minutia of the two-phase countercurrent flow through the porous media and leads the proposal of a simplified heat flux model for the water penetration with corresponding relations applicable for both configurations of interest. Furthermore, the development and the impact of penetrating front instability are studied with the help of 2D MC3D simulations, which show important effects of the initial temperature and the permeability of the corium configuration on the penetration front velocity and appearance of the instabilities. The analytical model is extended to a pseudo-two-dimensional two-zone configuration (with one zone subjected to a two-phase countercurrent flow and another through which monophasic superheated vapor flows) to analyze in greater detail the impact of the penetrating front heterogeneity over the extracted heat flux. The mechanism of water penetration through a fractured crust is revisited. The analysis indicates strong border effects in the SSWICS tests (Argonne National Laboratories) dedicated to the study of this phenomenon. The conclusions of precedent studies on the efficiency of the phenomena could not, therefore, be confirmed due to important uncertainties over the process of fracturing, overly sensitive to the mechanical properties of corium, which in turn are not properly characterized. Finally, the models, and simulations, are applied to real accidental scenarios, including the presence of residual power. For the debris bed, the extracted heat flux, and the cooling capabilities are less than those found using the simplified dry-out heat flux criteria.; En cas d'accident grave avec fusion du cœur, le magma surchauffé constitué d'acier et de combustible fondu, appelé corium (T> 2 500 K), peut menacer l'intégrité de la cuve du réacteur, puis du bâtiment de confinement, si le refroidissement du corium n’est pas assuré. La capacité de refroidissement en situation hors-cuve, par l’injection d’eau et pénétration de celle-ci dans le corium en surface supérieure, est étudiée pour deux configurations attendues : le lit de particules et bain de corium. La seconde configuration est liée à la situation d’interaction corium-béton (ICB) où une croûte se forme en face supérieure en contact avec l’eau, puis est soumise à une fracturation à cause des effets thermiques dans cette croûte. L’enjeu est de caractériser l’efficacité d’une éventuelle pénétration de l’eau dans la croûte. La première configuration peut intervenir en particulier dans deux situations suite à une fragmentation du corium dans l’eau : lors de l’éjection hors de la cuve, ou suite à des périodes d’éjection à travers la croute en phase d’ICB par entraînement du corium par des gaz issus de l’ablation du béton. Les phénomènes de pénétration de l’eau dans le corium sont examinés par une analyse approfondie des résultats des expériences disponibles, par la mise au point d’un modèle analytique 1D et par la modification et l’utilisation du code de thermohydraulique multiphasique multi-fluides (CMFD) MC3D. L’analyse 1D permet de mieux comprendre les détails de l’écoulement diphasique dans la matrice poreuse et conduit à proposer un modèle simplifié de pénétration de l’eau, avec des relations correspondantes applicables pour les deux configurations d'intérêt. Par ailleurs, le développement et l’impact d’instabilités au front de pénétration sont étudiés avec des simulations 2D avec MC3D, illustrant le rôle important de la température initiale du lit et sa perméabilité sur la vitesse de pénétration du front, et sur l’apparition des instabilités. Le modèle analytique est alors étendu à une configuration à deux zones (une zone soumise à un écoulement diphasique en contre-courant et une zone monophasique dans laquelle la vapeur surchauffée traverse) pour analyser plus en détail l’impact des hétérogénéités de progression du front sur les flux thermiques extraits. Le mécanisme de pénétration de l’eau dans les croûtes est discuté. L’analyse indique de forts effets de bords sur les processus de fracturation dans les essais SSWICS (Argonne Nat. Lab.), dédiés à ce phénomène. Les conclusions des travaux précédents sur l’efficacité du phénomène ne peuvent dès lors être confirmées du fait des fortes incertitudes sur les processus de fracturation, très sensibles aux propriétés mécaniques du corium, très mal caractérisées. Finalement, les modèles sont appliqués aux situations réelles impliquant la présence de la puissance résiduelle. Pour les lits de débris, les flux extraits et les capacités de refroidissement sont moindres qu’avec l’utilisation du critère simplifié de « flux d’assèchement ».