Assimilation of transient data and calibration of simulation codes using time series

In the context of scientific simulation, some computational tools (codes) are built as an assembly of (physical) models coupled in a numerical framework. These models and their coupling use data sets fitted on results given by experiments or fine computations of “Direct Numerical Simulation” (DNS) type in an up-scaling approach. The observables of these codes, as well as the results of the experiments or the fine computations are mostly time dependent (time series). The objective of this thesis is then to set up a methodology to improve the reliability of these codes by adjusting their parameters through data assimilation from these time series.
Work on parameter fitting has already been performed in our laboratory in a previous thesis, but using scalars derived from the temporal results of the codes. The methodology developed during this thesis has integrated screening, surrogate models and sensitivity analysis that can be extended and adapted to the new data format. A preliminary step of transformation of the time series will be developed, in order to reduce the data while limiting the loss of information. Machine learning /deep learning tools could be considered.
The application of this method will be performed within the framework of the nuclear reactor severe accident simulation. During these accidents, the core loses its integrity and corium (fuel and structure elements resulting from the reactor core fusion) is formed and can relocate and interact with its environment (liquid coolant, vessel’s steel, concrete from the basemat…). Some severe accident simulation codes describe each step / interaction individually while others describe the whole accident sequence. They have in common that they are multiphysic and have a large number of models and parameters. They describe transient physical phenomena in which the temporal aspect is important.
The thesis will be hosted by the Severe Accident Modeling Laboratory (LMAG) of the IRESNE institute at CEA Cadarache, in a team that is at the top of the national and international level for the numerical study of corium-related phenomena, from its generation to its propagation and interaction with the environment. The techniques implemented for data assimilation also have an important generic potential which ensures important opportunities for the proposed work, in the nuclear world and elsewhere.

Multi-block and non-conformal domain decomposition, applied to the 'exact' boundary coupling of the SIMMER-V thermohydraulics code

This thesis is part of the research required for the sustainable use of nuclear energy in a decarbonized, climate-friendly energy mix. Sodium-cooled 4th generation reactors are therefore candidates of great interest for saving uranium resources and minimizing the volume of final waste.

In the context of the safety of such reactors, it is important to be able to precisely describe the consequences of possible core degradation. A collaboration with its Japanese counterpart JAEA allows the CEA to develop the SIMMER-V code dedicated to simulating core degradation. The code calculates sodium thermohydraulics, structural degradation and core neutronics during the accident phase. The objective is to be able to represent not only the core but also its direct environment (primary circuit) with precision. Taking this topology into account requires partitioning the domain and using a boundary coupling method. The limitation of this approach generally lies in the quality and robustness of the coupling method, particularly during fast transients during which pressure and density waves cross boundaries.

A coupling method was initiated (Annals of Nuclear Energy 2022, Implementation of multi-domains in SIMMER-V thermohydraulic code at LMAG, which consists of merging the different decompositions of each of the domains, with the aim of constituting a unique decomposition of the overall calculation. This method was developed in a simplified framework where the (Cartesian) meshes connect in a conformal manner at the boundary level. The opportunity that opens up is to extend this method to non-conform meshes by using the MEDCoupling library. This first step, the feasibility of which has been established, will make it possible to assemble components to constitute a 'loop' type system. The second step will consist of extending the method so that one computational domain can be completely nested within another. This nesting will then make it possible to constitute a domain by juxtaposition or by nesting with non-conforming domain meshes and decompositions. After verifying the numerical qualities of the method, the last application step will consist of building a simulation of the degradation of a core immersed in its primary tank ('pool' configuration) allowing the method followed to be validated.

This job will enable the student to develop knowledge in numerical techniques and modeling for complex physical systems with flows. He or she will apply techniques ranging from method design to validation, as part of a dynamic, multidisciplinary team at CEA Cadarache.

Modeling and numerical simulation of compressible multi-material and multi-phase hydrodynamic flows for the simulation of Corium-Sodium interaction

This thesis is part of the drive to revive the nuclear industry, and in particular the safety studies associated with Severe Accidents (SA) in fourth-generation sodium-cooled fast breeder reactors (SFR). During such a hypothetical event, a jet of Corium (a mixture of molten fuel and core structural elements) interacts with the reactor coolant. This phenomenon, known as FCI (Fuel Coolant Interaction), generates an energy transfer to the coolant that can lead to steam explosions. The aim of this thesis is to improve FCI simulation tools for SFRs.
The development of the SCONE CFI code is based on TRUST, CEA's open source numerical platform. TRUST is a mature tool from an HPC and numerical robustness point of view, but suffers from certain numerical limitations for the simulation of GAs: transport terms to order 1, schemes ill-suited to the simulation of highly compressible flows and therefore steam explosion, numerical diffusion.
This thesis proposes to overcome these limitations by following a three-part research approach. The first part consists in extending the existing SCONE schemes to make them compatible with the simulation of strong shocks. A derivation via a principle of least action will be used for thermodynamic consistency. In a second part, the schemes developed will be extended to multiphase using the two classic models in the literature (pressure equilibrium and Baer-Nunziato). Finally, the last part will consist in the reduction of numerical diffusion of phase indicators, thanks to the rise in order (with multidimensional limitation) of advection terms and an interface reconstruction (of the VOF-PLIC type). As these algorithms are classically numerically expensive, Machine Learning techniques will be used to accelerate the identification of interface configurations.
The proposed work opens up professional prospects in the field of simulation and numerical methods, in particular towards research centers and R&D departments in industry.
Un stage de master 2 est proposé par l’équipe en complément de la thèse.

Modelisation and simulation of boiling under high flux with sodium

This thesis is part of the framework of safety studies associated with Severe Accidents in Sodium-cooled Fast Neutron Reactors, and more particularly when the melted core relocates toward the core catcher at the bottom of the tank by gravity. A corium jet (mixture of combustible and structural elements of the melted core) then interacts violently with the coolant fluid of the reactor. This phenomenon is called FCI for Fuel Coolant Interaction. The interaction involves among others a fragmentation of the corium jet (dispersed particles) coupled to film boiling of the coolant. Characteristics of steam films are determining to study and model the fragmentation step of fuel leading potentially to sodium steam explosion.
DNS (Direct Numerical Simulation) of film boiling in these conditions is very costly especially, because of the small thickness of films, mass transfer to take into account at the smallest scales and constraints of the compressibility.
The goal of the thesis is then to simulate film boiling with an out of equilibrium compressible model able to relax these constraints with a great generality. Indeed, the knowledge of phases disequilibrium allow to evaluate locally the exchanges, for instance of heat thanks to semi-empirical correlations, while conserving a proper solving of the main flow scales. The proposed model will have to take into account the evaluation of the interfacial area in order to evaluate accurately mass and heat transfers at the liquid-vapour interface.
On this basis, the thesis work will be broken down into 3 parts, in addition to the initial bibliographical study. The first part will concern the choice or proposal of envelope macroscopic models, on the one hand, and high-precision models on the other, enabling the simulation of film boiling. The second part of the project will then involve the implementation, in the Computational Fluid Dynamics (CFD) code SCONE based on TRUST (an Open Source code developed at CEA), of models and numerical methods enabling reliable resolution of the problems considered. Finally, the last part of the work will be dedicated to prospective work and sensitivity studies of the numerical models, particularly in terms of geometry and thermal conditions (undercooling of the coolant, temperature field in the solid), in order to determine the validity range of the proposed work and potential avenues for improvement.
In a general manner, the thesis will allow characterizing numerically steam film boiling. Improvement of the modelling of film boiling goes far away the context of FCI phenomena and will be therefore applicable to a large variety of industrial and academic problematics. This work thus opens up career prospects particularly in research centers and R&D departments in industry.
A master internship is proposed by the team in addition to the thesis.

experimental an numerical contribution to heat transfer enhancement in TPMS exchangers

The proposed work is dedicated to the technological challenges associated with the highly innovative Molten Salt Reactor (MSR) family of nuclear reactors.
In a loop type MSR, the fuel salt carries with it delayed neutrons precursors, which generate fissions outside the core. Under these conditions, to facilitate reactor control, the volume of out-of-core fuel salt must be minimized. This constraint imposes an exchanged power density greater than the core power density at the intermediate heat exchangers, which extract power from the core.
The ISAC project is developing a fast-neutron MSR with a core power density of 250 MW/m3. This power density, combined with the specific characteristics of the fuel salt, represents an ambitious target for conventional heat exchanger technologies. One of the solutions proposed to maximize the power density of the intermediate exchanger is to adopt new exchange patterns. TPMS geometries, assembled to form 3D exchange channels, are interesting candidates. The construction of such exchangers is made possible by additive manufacturing processes.
The subject of this thesis is the experimental validation of heat exchange and friction coefficient correlations specific to TPMS geometries. CFD calculations will be used to develop the experimental program. The development of simulating fluid tests will be based on Reynolds number and Prandtl number equivalence. The main challenges of the proposed research relate to the 3D nature of the channels and understanding the influence of roughness in additive manufacturing channels. Finally, the experimental results obtained will be used to develop the CFD models.
The compactness of heat exchangers is a recurring issue in the development of any energy conversion system. The correlations validated during the course of this PhD will be used to dimension other TPMS exchangers for various applications. Morever, the work on offer opens up career prospects, particularly in research centers, industrial R&D departments and innovative systems design units.
Un stage de master 2 est proposé par l’équipe en complément de la thèse.

Characterization of vortex on the surface of a representative model of a 4th generation reactor

4th generation sodium-cooled fast neutron reactors are a lasting solution to the energy crisis and global warming. In the framework of the ASTRID program, the CEA has been involved for about 10 years in the design of an industrial demonstrator. Despite the end of the project in 2019, research on SFR technology continues through innovative reactor concepts studies (Anais, Attrium, etc.).
In the design of this type of reactor, the vessel has a free surface covered with argon to reduce heat transfer to the cover. Under certain conditions, this gas can be entrained through vortices in the lower part of the reactor due to the suction of the pumps The release of a pocket of gas in the fuel assemblies is a source of increased neutron reactivity and raises safety questions. It is then essential to be able to characterize the operating conditions for which vortices can appear on the surface. This task being very difficult to apprehend by computer codes, experiments have been carried out to study the occurrence of gas entrainment in various configurations. However, the tools implemented during these test campaigns do not allow a sufficiently in-depth analysis to understand the phenomenology of vortex formation.
The purpose of this thesis is studying the vortices on the surface of the representative MICAS mock-up at a 1/6 scale of the ASTRID reactor project. Since sodium is complex to handle for experiments, a simulating fluid, water, will be used. The objective of this study is to characterize vortices in terms of occurrence, diameter, depth and velocity in order to provide elements necessary to their modeling and prediction in computer codes. This thesis work will be divided, except the bibliographical part, into 3 parts, approximatively matching the three years od the thesis: development of the characterization system based on imagery, characterization of the vortices according to different configurations (geometric and operating), implementation into a model. A priori, the vortex characterization system will be based on a network of wide-field cameras placed at different angles, but other solutions could be also considered.
The host laboratory already has the mock-up and the means of measurement (fast cameras, velocity measurement system, etc.) in order to carry out the experiments as soon as the PHD student arrives. They may be supplemented by other tools depending on the characterization system developed. Finally, an international opening could be envisaged through a collaboration with the Japanese agency JAEA.
This work opens up career prospects particularly in research centers and R&D departments in industry.

Numerical simulation of particles driven by natural convection. Application to severe accidents of Generation 4 nuclear reactors.

This thesis is part of the general framework of safety studies associated with Severe Accidents in Sodium-cooled Fast Neutron Reactors. More specifically, it focuses on the modelling of corium debris beds and associated phenomena, which are specific to this reactor technology. In particular, it aims to produce physical models describing the self-levelling of these beds by following an upscaling approach based on the so-called volume averaging method. Based on the results of previous work, and on a simulation tool coupling LBM (Lattice Boltzmann Method) and DEM (Discrete Element Method), the proposed research will follow a two-scale approach. First, the phenomena of interest will be simulated at the pore scale, selecting the most significant closure problems. It will be followed by the proper upscaling to propose new physical laws for the most influential effective properties of the debris bed.
The work carried out here in the context of the safety of sodium fast reactors is generic and the establishment of the methodology for determining closure relationships (in this case, effective properties of the medium) for a larger-scale numerical tool is innovative and will find applications in many other contexts dealing with problems related to fluid/grain interactions.
This work participates in the revival of nuclear power with a flourishing of start-ups based on sodium reactors and collaboration with Japan on the safety of this type of reactor.
A master internship is proposed by the team in addition to the thesis.

Transverse dispersion of pollutants in a river with riffle-pool systems : example of the middle Durance River

When releasing a pollutant into a river, it is important to know the downstream distance from which the pollutant will be homogeneously distributed throughout the cross-section, in order to be able to delimit the mixing zone. To estimate this, the usual method is to apply an advection-diffusion model based on the estimation of a transverse mixing coefficient. While numerous formulas exist for estimating this coefficient, most of them have only been validated in certain river configurations.
In a previous study, Lorris Gond [2022] divided a part of the Durance into successive reaches, according to the hydromorphological facies encountered, and determined the transverse mixing coefficient in each section by releasing dye. These results confirm the hypothesis of a transverse mixing process specific to the riffle-pool sequences encountered in the Middle Durance. The aim of the proposed thesis is to verify this hypothesis in rivers with riffle-pool type structures. The aim is then to determine a methodology for computing a global mixing coefficient for the structure based on a priori knowledge of the facies, so as to avoid the need for new in situ measurements each time the bed undergoes morphogenic changes. To this end, the thesis will involve field characterization of the geometry of a section of the middle Durance, dye release to quantify the transverse mixing coefficient in the river, and laboratory experiments on a small-scale riffle-pool structure.
A master internship is proposed by the team in addition to the thesis.

Study of an innovative cleaning process dedicated to the treatment of residual sodium in facilities using liquid sodium as a coolant

Sodium is used as a heat transfer fluid in fast neutron nuclear reactors. Given the operating temperatures of these facilities, all surfaces in contact with liquid sodium remain wetted with residual sodium once the circuits have been drained. The treatment of this residual sodium is required to ensure the safety of interventions on components and structures in a dismantling process. The reference method for this action is cleaning with water in a dedicated cleaning pit. This process involves a reaction of sodium with water in different forms, by controlling the reaction kinetics, which is instantaneous and highly exothermic without controlling the contacting of the reagents.
An exploratory study was carried out at CEA (PhD thesis defended in 2014) on the use of salts to mitigate reaction kinetics. The Sodium and advanced coolant technology laboratory (DES/IRESNE/DTN/STCP/LESC) thus has R&D facilities, instrumented and dedicated to the study of sodium cleaning processes and equipped with the functionalities of an industrial cleaning pit , such as spray nozzles, atomizing nozzles and an immersion device.
The main scientific objective of this new PhD is now to identify, understand and model the physicochemical mechanisms involved in the sodium-water reaction kinetics involving salts. This work will make it possible to limit or avoid pressure wave phenomena or of explosion during the treatment of residual sodium from fast neutron nuclear reactor circuits during their decommissioning and dismantling. The PhD student's mission will be to define the experimental design, to actively participate in carrying out the test campaigns, to analyse the results and to propose an interpretation of the observed phenomena (kinetics, pressure peak, local temperature rise, etc.). The aim of the experimental campaign will be to acquire reliable thermodynamic and reaction kinetic data, such as reaction times, variation of dynamic pressure, temperature rise, composition of the gas and liquid phases, speciation in liquid phase and visualization of the phenomenology via high-speed camera. Modelling tools will be used to establish and simulate a reaction kinetic model. Ultimately, the proposed work will make it possible to qualify the process for industrial application in the field of decommissioning/dismantling, which is a major challenge for the French nuclear industry.
In addition to the experience acquired in the field of nuclear systems dismantling, the proposed work opens up professional prospects, particularly towards research centers and R&D departments in industry.
A master internship is proposed by the team in addition to the thesis.

Computations and experiments on liquid metal MHD flows : application to electromagnetic pumps for the sodium industry.

Electromagnetic (EMP) pumps move an electrically conductive liquid metal without contact. As a result, they provide an excellent seal for coolant in fast neutron or fusion reactors while minimizing waste inventory. In induction EMPs, the pumping Lorentz force results from the interaction between the exciting magnetic field and the current it induces in the conductive liquid moving at a relative velocity. This coupling is typical of magnetohydrodynamics (MHD).
When MHD flows become turbulent, the scientific challenge is to describe the turbulent boundary layers. Direct numerical simulation (DNS) makes it possible to dispense with sub-mesh models to describe the boundary layers. The trade-off is computational time, which is prohibitive for engineers who want to design a PEM in real geometry. The goal of this work is to calculate MHD quantities (velocity, current, and electric potential) using DNS in a simplified geometry that is sufficiently representative of an EMP. Calculations can be performed in parallel using models with closure laws that are more accessible to the engineer. The goal is to establish domains of validity for these closure laws, if they exist.
An MHD flow in a channel will be modeled, either laminar or slightly turbulent. The magnetic field can be imposed as uniform, non-uniform, sliding and/or oscillating. The numerical simulations will be validated on an experimental device to be completed, which will allow Galinstan flow (metal alloy which is liquid at room temperature) and ultrasonic or electric potential velocimetry.
The aim of this thesis is to gain a better understanding of turbulent MHD flows in channels, to implement into future work on modeling electromagnetic pumps for representative Reynolds and Hartmann numbers. This work opens up career prospects particularly in research centers and R&D departments in industry.