Proceedings of the 9th International and 49th National Conference on Fluid Mechanics and Fluid Power (FMFP) December 14-16, 2022, IIT Roorkee, Roorkee-247667, Uttarakhand, India FMFP2022–8305 Towards analysis of Corium hydraulics in liquid sodium Ram Kumar Maity1, T. Sundararajan2, M. Rajendrakumar1, K. Natesan1 1 Thermal Hydraulics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India 2 Department of Mechanical Engineering, IIT Palakkad, Palakkad-678557, India a mixture of reactor fuel, fission products and structural material. An expected post-accident scenario with respect to movement of molten corium and its relocation is depicted in Fig. 1 [2, 3]. Moreover, during such an accident christened as a severe accident in a fast reactor in addition to molten corium, large volumes of fission gases are released into reactor pool as well. Accident progression in the fast reactor is expected to be distinct from that in the thermal reactor. Accidents, which can lead to degradation and melting of the whole core, are classified as severe accidents and specifically called a core disruptive accident (CDA) in the context of fast reactors. Due to the extreme low probability of occurrence of CDA, it is often referred to as a hypothetical accident [4]. Despite such low frequency, an accident like CDA needs to be analyzed in view of its possible serious impact on the reactor systems and its surroundings. It is to be demonstrated that any radioactivity release to environment stays within specified limits [4]. ABSTRACT During a hypothetical severe accident in a sodium cooled fast reactor, core melt-down is expected to occur. As a result, molten corium would fall through sodium as a jet. The dispersion of the fissile material within the molten jet is important to avoid re-criticality. This occurs in several stages with different sequence of complex controlling physical phenomena. The present work is in the direction of understanding the initial jet behavior influenced primarily by flow instabilities. An in-house force balanced two phase flow solver namely THYC-MP has been used to study the behavior of two dimensional jets of corium and simulant material namely wood’s metal within liquid sodium. The expected physics and jet breakup behavior is studied and described. Keywords: Corium jet, two phase flow, Volume of Fluid, severe accident, fast reactor 1. INTRODUCTION Fast reactors are a type of nuclear reactors in which the nuclear fission reaction that produces thermal energy is sustained by fast (energetic) neutrons. In order to limit neutron flux and fissile inventory, fast reactors work with compact cores with enriched fuel. Because of low core volume, the power density and linear power rating of fuel becomes high for such reactors (typical power density is 400 to 500 kW/litre and linear power rating is 400 to 500 W/cm of pin length). Such high power densities warrant use of a highly efficient coolant medium. Many operational fast reactors around the world therefore use liquid metal in the form of liquid sodium as the coolant. A pool type reactor is characterized by a large pool of sodium which holds the primary heat transport system. A large capacity fast reactor is preferentially designed as a pool type reactor due to the large thermal inertia of reactor pool and associated enhancements in reactor safety. Details of design of Indian fast reactors can be found in ref. [1]. Accidents leading to core meltdown that can release molten corium into reactor pool are rare events with very low probability and is considered as beyond design basis event [2]. However, in the eventuality of an incident that leads to core meltdown and release of molten corium into reactor pool the transport of the same becomes important and needs to be characterized. Corium is the material created in a nuclear reactor during core meltdown accident and would primarily be Figure 1: Schematic showing transport of molten corium after a severe accident (top) [3]. It is necessary to develop tools that allow characterization of transport of various species after a severe accident in a fast reactor. Such characterization would allow to improve and optimize accident management strategies. For example, one of the major tasks after release of corium in sodium pool is to avoid clumping (aggregation of fuel fragments) that may result in re-criticality and further large 1 energy release. Development of such tools becomes even more pertinent in view of the planned expansion of the Indian fast reactor program [2]. The understanding of severe accident related phenomena need to be improved substantially as a part future advanced reactor designs. The focus of the present study is on the movement of corium in liquid state within liquid sodium with resolution of flow structures resulting from two phase instabilities. Phase change is beyond the scope of the present study. The studies are done using an in-house force balanced two phase flow solver based on the volume of fluid (VOF) method called THYC-MP. movement of simulant material (wood’s metal) in water and corium in sodium. Preliminary results from these studies are presented. 3. MATERIALS AND METHODS Out of the different fixed grid two phase flow analysis methods, the VOF method sees widespread use. This method is capable of accurate conservation of mass for each phase. Volume conservation is a very important aspect in two phase flows and must be ensured in order to capture the right flow physics. In two-phase problems, methods to track the physics of an interface between the two phases present within the domain need to be implemented in addition to the solving the flow. As a results, two-phase flow problems are in general more challenging than single phase flow problems. The VOF method is a fixed grid ‘one-fluid’ approach that makes use of a marker function to separate the two phases within the domain. Integration of the marker function within a control volume or computational cell gives the value of volume fraction ‘ ’. If the presence of phase designated ‘k’ is specified by the marker function ( , , , ), then the volume occupied by this phase and subsequently the volume fraction ( ) within a cell can be calculated using the following expressions: 2. LITERATURE REVIEW AND OBJECTIVE A brief description of numerical studies on movement of corium in coolant using two phase analysis methods with capabilities similar to VOF method is presented here. The motion of a jet of corium in coolant takes place primarily after a severe accident has occurred. A majority of the studies till now have been experimental in nature. This is due to the complex physics involved and the lack of adequate computational power along with suitable development on numerical algorithms. The aim of these experiments includes determination of jet instabilities and jet breakup length and characterization of corium spread on structures like core catcher. A comprehensive review of experimental works on jet break-up and fragmentation phenomena can be found in ref. [5]. Oh et al. [6] described a study on the break-up behavior of a molten metal jet in still gas when in the presence of sinusoidal oscillations. The studies that included amplitude-modulated waveform were done using a VOF based method. Thakre et al. [7] presented a two dimensional VOF analysis of melt jet fragmentation using commercial CFD code FLUENT. The study included identification of jet fragmentation pattern, influence of Weber number and physical properties. Lin et al. [8] described studies using a VOF based method when a melt droplet has a vapor film and is exposed to a pressure pulse. The effect of surface tension is studied and found to be insignificant. The results also indicated that the vapor film can be neglected as well with material density and pressure pulse playing the most important role in droplet deformation and surface wave growth. Zhou et al [9] presented a paper on metal jet breakup that also analyzed cooling and solidification behavior. Solidification was modelled using an enthalpy based approach. The results were found to be in good agreement with experimental observations at different Weber numbers. More recently, using the color-gradient Lattice Boltzmann method, a very detailed study on corium jet break-up and fragmentation in sodium has been studied by Cheng et al. [10]. Several recent advances and application of two phase flow algorithms to study the interactions of two immiscible liquid metals or equivalent materials have been described in literature. They also prove the viability of using two phase flow analysis tools for characterization of corium behavior in liquid sodium. Corium interaction with liquid sodium involves several intricate interfacial and phase change phenomena. For the present study a force balanced two phase solver based on the VOF method is developed and described in more detail in the next section. The code developed in applied to analyze = ( , , , ) , = / (1) The value of volume fraction can take any value from 0 to 1. The implications of the values taken by the variable are listed: = 1 Cell completely filled with primary phase 0< <1 Mixed cell containing both phases = 0 Cell completely filled with secondary phase The primary fluid is also referred to as the dark fluid. The first step of the overall method, involves reconstruction of the interface between the primary and secondary fluids. This is done using the volume fraction values of mixed cells. The interface would pass through mixed cells and is a zone of discontinuity across which a large change in fluid properties occurs. Either with the help of the reconstructed interface or directly operating upon the values of volume fractions, the next step involves calculation of curvature. The calculated curvature of interface is required for calculation of surface tension forces. These forces arise due to the existence of the discontinuity in medium at interface and need to be imbibed into the momentum equations as part of the flow solver using special force balanced schemes. After the flow field advances, the interface is advected based on the prevalent flow velocities. Thus, a single time step or iteration of calculations for two-phase flow is completed. The present implementation is on a fully staggered grid that stores quantities like pressure, volume fraction, temperature, fluid properties and curvature at center of continuity cells. Two separate cells viz. u-momentum and v-momentum cells are used for integration of u, v momentum equations and for storage of u and v velocities. Specifically, u-momentum cells are centered around centers of left and right faces and v-momentum cells are centered around centers of top and bottom faces of a continuity cell. Further details of the main steps of the VOF method along with their implementation as part of the present work are described in the succeeding sections. 2 These relations are relevant for right face (marked as R) of the computational cell and are used to determine coordinates of points 3 ( 3, 3) and 4 ( 4, 4) with respect to coordinates of points 1 ( 1, 1) and 2 ( 2, 2). Flux polygons are constructed for all other sides of the cell sussing similar relations. The method is an unsplit advection method that is mass conserving and uses edge matched flux polygons with no overlap or uncovered region. Geometric integration requires the construction of flux polygons from the prevailing velocity field and subsequently, the area of intersection between the flux polygon and the dark polygon (region covered by primary/dark fluid) derived from the interface reconstruction step is calculated that gives the flux of primary fluid through respective face. This is depicted in a schematic as given in Fig. 4. The area of flux polygon represents the total fluid flux passing through each edge of cell. The sum of these areas over must be zero (to satisfy overall continuity). 1 (6) + + + ) , = ( 4 3.1 Interface Reconstruction The first step is reconstruction of interface using the prevailing volume fraction field. An interface will lie within mixed cells alone and is defined using piecewise linear segments. Such methods are called Piecewise Linear Interface Construction (PLIC) methods. The equation of a line segment can be defined in terms of components of normal and a constant as: + + = 0. Thus, defining a linear segment representative of the interface within a mixed cell requires values of interface normals ( , ) and a line constant ( ). The method used for the present work for determination of interface normals is the E-PLIC method. This has been discussed in detail in ref. [11]. The method uses a c0 correction template (shown in Fig.3) that is imposed over a primary PLIC interface. Averaged interface Averaged interface A2 Original interface A1 C2 B1 C1 Cell 1 Cell 0 B2 Cell 2 B0 Primary Fluid A0 C3 Original interface A3 Cell 3 B3 Figure 3: c0 correction template for PLIC interface For the present study this primary PLIC interface is the Linear least squares (LLS) method [12]. The LLS method is known to be one of the most accurate PLIC methods reported in literature. The application of the c0 correction template has been shown to further improve the accuracy of PLIC reconstruction [11]. 3.2 Interface Advection A major step in two phase flow simulation is the computation of two phase interface kinematics. This requires obtaining a solution for the equation of conservation of volume fraction shown below: (4) + ⃗⋅∇ =0 Where, ⃗ is the velocity vector, is the volume fraction. The equation is integrated and discretised geometrically to allow a diffusion free advection of the interface. This maintains sharpness of interface through a two phase flow transient being studied. For the present work the edge matched flux polygon algorithm (EMFPA) proposed by Lopez et. al. [13] is implemented. The method requires the following constraints are respected during construction of the flux polygons (referring to Fig. 4): / = = = / ( ( − − − − Figure 4: Flux polygons constructed using EMFPA (top) and method used to calculate geometric advection using flux polygon and dark polygon. / , = )−( / )−( / Similar relations are used to derive , , , , , with the following condition to be satisfied (L-left, T-top, B-Bottom): (7) , + , + , + , =0 In the present work a set of analytical relations are developed to derive the flux polygons [14]. This makes the implementation of the method highly efficient. The integration / − ⁄ ) − ⁄ ) (5) 3 of Eq. 4 is done over a continuity cell. Using Gauss Divergence theorem and integrating Eq. 4, we get: 1 (8) = + , The value of capillary pressure ( ) is determined such that the surface tension force is equal to the derivative of . The divergence of the difference between surface tension force and gradient of capillary pressure is made zero as shown in the following equation: (12) ∇⋅ ⃗ − =0⇒ . ⃗ = . The above equation is integrated and discretised over the continuity cell. The equation is discretised as an implicit (in time) equation and is stored by using a matrix solver. The velocity and pressure fields are coupled using a projection method described in detail in ref. [17]. The discretisation scheme for volume fraction (f) and capillary pressure (P ) follows the same exact discretisation in order to maintain perfect force balance. The time integration of all conservation equations including the energy equation is done using fully implicit schemes. After discretisation these are solved using matrix solvers. For the present study, a BiConjugate Gradient stabilised (Bi-CGSTAB) linear solver is used. The flow solver has been validated extensively against complex two phase flow problems not described as part of the present study. In the next section results from application to the problem of movement of corium in sodium and simulant wood’s metal in water is presented. The standard energy equation is solved along with flow equations. The buoyancy forces due to temperature differences are not turned on due to the large buoyancy forces imparted by the density differences between the primary and secondary fluids. The code is named as Thermal Hydraulics Code – Multi-Phase viz. THYC-MP. 3.5. Calculation domain and properties , , , Here, represents area of cell (i,j). The second term of Eq. 5 represents the area of intersection of flux and dark polygons as shown in Fig. 4. 3.3. Calculation of curvature Calculation of curvature required for determination of surface tension force is done using the Height Functions method. The height functions used here is the variable template version of Hernandez et. al. [15]. The basic principle of the method involves the following three steps: 1. The dominant component of interface normal is found in order to orient the direction of height functions. For the present study components of normal from Young’s PLIC are used. 2. Next the volume fractions are summed up along the dominant direction of the interface normal from step 1. This gives the local values of height functions X(y) or Y(x). 3. Differentiation of height functions gives the value of Curvature. This step is valid only when the height function lies within the cell dimensions. Thus, if < , Three height functions viz. Yi-1,j , Yi1,j , Yi+1,j can be defined: , =∑ , ( − ) (9) Curvature can be calculated from height functions as: = ( ) 1+ (10) Inlet =− , 500 mm The original height functions [16] uses a stencil of size 7×3 cells. The version used in this work is a more accurate version that uses a variable stencil size depending on the volume fraction field. The steps for these can be found in ref. [15]. The surface tension force for u-momentum equation (for u-momentum cell ( , ) on the left face of continuity cell ( , )) is given by: (11) , ` Local curvatures are calculated and stored for continuity cells and are to be interpolated to u-momentum cells using and v-momentum cells. The surface tension forces are calculated using these curvature values. 3.4. Flow solver The inclusion of surface tension forces need to be carefully implemented using a ‘well balanced’ or a ‘force balanced’ method. This means the discretised form of the momentum equation(s) should be able to describe the Laplace equilibrium correctly. In the present work the scheme (PROST) proposed by Renardy and Renardy [17] is implemented for a staggered grid system. Components of velocity viz. u-velocity is stored on the left and right faces of a continuity cell with the v-velocity being stored at the top and bottom faces of the same. In this method, the pressure is divided into two components ( = + ) where component represents the capillary pressure component arising due to presence of surface tension. Outlet Inlet Outlet 75 mm Walls (Not to scale) Figure 5: Details of domain and boundary conditions Two systems of fluids viz. Wood’s metal-water and Corium-sodium are studied. The former system is a popular choice as simulant system to study hydraulics of the later. It must be noted that no phase change is considered as part of this work. Phase change is otherwise expected to occur and is the subject of future extension. The flow domain, boundary conditions and fluid properties are similar to those used in ref. [7]. The domain selected is a 75 mm width ×500 mm depth column of fluid (secondary fluid – water and sodium in study 1 4 and 2 respectively) as shown in Fig. 5 kept at an initial temperature of . A slot jet of width 5 mm is injected at 0.5 m/s and temperature of . The top boundary otherwise (excluding faces through which jet is injected) is an outlet boundary. All the remaining walls (side and bottom) are adiabatic walls. Flow and thermal properties for the present study are listed in Table 1. and are sourced from ref. [18]. A uniform grid of 150×1000 cells is used along with a time step of 1e-4 s for the numerical studies. The Woods metal-water system corresponds to an Ambient Weber number ( ) of 1.25 while that for Corium Sodium study is 2.2. Table 1: Properties for the present study Woods Metal Property – water 9700 Density (kg/m3) 998.16 Dynamic 1.94e-3 Viscosity 1.002e-3 (Ns/m2) 168 Specific Heat (J/kgK) 4180 Thermal 18.8 Conductivity 0.59953 (W/mK) 200°C 50°C Surface Tension 1 Coefficient significantly higher than that for the wood metal studies. It can also be seen that jet breakup is seen later and at longer jet lengths in the case of corium studies. Moreover, there is similarity between the behaviour of the two primary fluids. Corium in Sodium CoriumSodium 8756 857 6.49e-3 0.281e-3 460 1278.5 2.5 0.01 s 0.1 s 0.2 s 0.3 s 0.4 s 0.5 s 0.01 s 0.1 s 0.3 s Evolution of Temperature Evolution of Volume Fraction Woods metal in water 71.2 3000°C 460°C 0.484 4. RESULTS AND DISCUSSION The evolution of volume fractions of corium (in sodium) and woods metal (in water) as predicted by THYC-MP code is shown in Fig. 6. The computational elements used for the present study are of size 0.5 mm. The total number of elements used is 1.5 lakhs. The studies reported here are preliminary in nature designed to be more like demonstration studies. From the contours of volume fraction in Fig. 6 it can be seen that both large scale and smaller scale instabilities are present. Due to the deceleration of the jet as it enters further into the secondary fluid, the whole jet tends to bend and eventually breakup. Smaller structures continuously break and separate from the main jet. These structure mainly arise from the trailing edge of the mushroom cloud like structure that arises in both the cases studied. The mushroom like structure at the leading edge of the jet of primary fluid can be ascribed to the presence of Rayleigh Taylor instability modes. As the jet travels further blobs of primary fluid break out from the trailing edge as can be seen clearly from the plots of Fig. 6. It must be remembered that the smallest structures predicted as part of this study is limited by the mesh size used. The mesh size is limited by the computational resources available and the efficiency of the code. At present steps are being taken to improve the efficiency of the code in addition to parallelisation that would allow studies on finer grids. The jets of corium and woods metal are injected into respective secondary fluids at the same velocity. This makes the Weber number for the corium studies 0.01 s 0.1 s 0.2 s 0.3 s 0.4 s 0.5 s 0.01 s 0.1 s 0.3 s Evolution of Volume Fraction Evolution of Temperature Figure 6: Evolution of volume fraction and temperature for the two fluid systems studied. In addition, the shedding of blobs of primary fluid from the trailing edge of the mushroom like structure is much more pronounced in the case of corium-sodium combination. This can be explained by the significantly higher Weber number for corium flow in sodium. Even though the velocity of the jet injection is same, the movement of corium is significantly faster into the column of secondary fluid. From the contours of temperature, the effect of high thermal conductivity of sodium with respect to water can be clearly seen. This can be concluded from the significantly cooler corium jet w.r.t the woods metal jet after 0.3 s. The range for the two plots is decided by the extreme domain temperatures ( to ). Both the jet 5 and ambient temperatures in the case of corium-sodium study is significantly higher. In addition, phase change is not modelled. Thus, these results are indicative in nature. Thus, as seen in both the case studies the jet of corium (or woods metal) becomes completely fragmented after penetrating into the column of secondary fluid for a distance of about half of the column depth. This is desirable as it proves that corium jet would be completely fragmented before reaching tank bottom and would be well distributed without any clumping. [5] [6] [7] [8] [9] [10] Figure 7: Jet Break-up length with respect to time for the two fluid systems studied. The jet break-up length extracted from plots of jet interface are presented in Fig. 7. It is seen that jet break-up length increases i.e. the jet grows in time, eventually breaking at an intermediate length. This leads to sudden shortening of the jet. The corium and woods metal jets grow at similar rates even though the length of corium jet is slightly greater. [11] [12] 5. CONCLUSION The development of a VOF based code for simulation of two phase flow and heat transfer (THYC-MP) is described. The code developed uses advanced algorithms for interface reconstruction, advection and curvature calculations. The flow solver is force balanced and is capable of reproducing Laplace equilibrium conditions exactly. 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