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.

CFD development and modeling applied to thermal-hydraulics of hydrogen storage in salt caverns

A PhD thesis is available at LMSF lab of CEA in collaboration with Storengy, a world specialist in natural gas storage in salt caverns. Measurements carried out in the cavity showed that gas is in convective motion in the upper part of the cavity and is not necessarily in thermodynamic equilibrium with the brine at the bottom of the cavity, leading to gas stratification phenomena. The different flow regimes (convective or not) will strongly influence, on the one hand, mass exchanges between the gas and the brine and therefore the evolution of the gas composition (in moisture and other components) at the cavity exit and, on the other hand, thermal exchanges between the gas and the rock mass surrounding the cavity. In this context, CFD-based prediction tools are highly beneficial for understanding these phenomena and will contribute to a better interpretation of the physical measurements made in the cavity, to the design improvment of surface installations and to monitoring storage facilities, particularly for hydrogen storage. In this doctoral project, the aim is to develop a thermal-hydraulics model based on TrioCFD software for gas storage in realistically-shaped cavities and under cavity operating conditions (injection and withdrawal phases). To this end, the operation of storage salt cavities will be modeled, initially for a real geometry and in single-phase flow, then in two-phase flow, taking into account mass exchanges between the brine and the gas in the cavity.

New condensation model in stratified flow at CFD and macroscopic scale by two-phase upscaling

In the context of safety of Pressurized Water Reactor (PWR), the Primary Coolant Loss Accident (LOCA) is of great importance. The LOCA is a hypothetical accident caused by a breach in the primary circuit. This leads to a pressure decrease in the primary circuit and a loss of water inventory in this circuit. Its resulting in heating of the fuel rods, which must remain limited so that damage to the fuel does not reduce cooling of the reactor core and prevents meltdown.

To remedy this situation, safety injection is activated to inject cold water, in the form of a jet, into the horizontal cold branch, which is totally or partially dewatered by the presence of pressurized steam. A stratified flow appears in the cold branch, with significant condensation phenomena in the vicinity of the jet and at the free surface in stratified flow zones. Numerous experimental and numerical works have been carried out on interfacial transfers at the free surface on rectangular and cylindrical cross-sections. CFD simulations of condensation at the free surface are carried out with the Neptune_CFD code, used by FRAMATOME, EDF and CEA. Currently, three models for heat transfer at the free surface are available in Neptune_CFD. These models have been established from a reduced number of simulations (DNS, LES and RANS) on rectangular configurations that remain far from the configuration of interest. Flows in a rectangular section tend to be parallel, whereas flows in a cylindrical section are three-dimensional.

The aim of this thesis is to improve the modeling of free surface condensation in a cylindrical cross-section configuration. Initially, a bibliographic study will be carried out on the free surface flow map, as well as on experimental works devoted to characterizing of interfacial area, mean interfacial velocity, turbulence terms in the vicinity of the free surface and heat transfer. In parallel, a new model will be developed in relation to the various improvement elements identified, and the associated validation carried out. Work is also planned to upscale two-phase CFD simulations to a macroscopic CATHARE approach. This up-scaling method will be based on Tanguy Herry's thesis work.

Numerical simulation with Front Tracking of a Taylor bubble

Two-phase flows can develop in industrial systems ranging from nuclear reactors to chemical processes. The presence of two phases (a liquid and its vapour, two liquids, a liquid and a gas) can be sought or constrained depending on the case. In a steam generator, bubbles are bound to form and grow before the liquid is completely vaporised. In the primary circuit of a reactor, steam slugs can appear in incidental or accidental situations. We want to study these steam slugs using numerical simulation to model them. The effect of the slug is primarily the reduction of the mass flow in the pipe. It can also have an effect on the mixing of the liquid and therefore on the temperature.
Our simulation software (TrioCFD) allows to perforum two-phase simulations with an interface tracking method. We want to simulate the dynamics of a vapour slug (sometimes called a Taylor bubble) in order to gain a better understanding of the flow dynamics. The first application of the simulations will be to evaluate the experimental correlations of pressure losses with our results and possibly enrich them. We would also like to observe the effects of geometry (cylindrical tube or rectangular channel). The work will involve setting up simulations based on existing functional cases, analysing the results and proposing modelling approaches.

Taylor bubbles: experiments and modeling

This doctoral position focuses on the microscale phenomena that occur in the near-wall region at bubble motion in a capillary tube (also known as Taylor bubble). These are elongated bubbles of vapor having bullet-like shapes that are formed in compact heat exchangers used in a variety of industrial applications such as cooling of electronics and steam generators in nuclear reactors, for instance. The phenomena associated include dewetting dynamics, formation of micrometric thick liquid layers and heat transfer. The PhD candidate will conduct an experimental study at STMF/CEA and SPEC/CEA (Paris-Saclay, France) using advanced non-intrusive optical diagnostics to evaluate these phenomena, in particular the film profile. The student will use the experimental data to validate a numerical approach in the open-source software OpenFOAM that will be developed in partnership with the University of Nottingham (United Kingdom).

Modeling of droplets entrainment in horizontal an inclined pipes

The study of flows with a free surface between a liquid and a gas (vapor, air,...) is important in many fields. Under specific conditions, gas can carry liquid droplets generated in pipes. This thesis focuses on the investigation of evaporation in the horizontal and inclined pipes of Pressurized Light-Water Reactors (PWR) prior to the entry of vapor generators during an accident. The prediction of the equilibrium between deposit and detachment of liquid droplets by gas, on - or from - a liquid layer evaporating in close proximity to the wall of these types of pipes, is critical in these configurations. The thesis will begin with a bibliographic review of transition models, as well as empirical or mechanistic models of entrainment, deposit, and droplets diameter in horizontal and inclined pipes. Some experimental campaigns performed at CEA such as REGARD and MHYRESA tests will support the models analysis. In parallel, some digital experiences obtained by local calculations, DNS or CFD scale are planned to simulate the deformation of liquid/gas interfaces. These models will be implemented in the CATHARE 3 ( ) code system developed by the CEA in collaboration with EDF, Framatome, and the IRSN, with validation on selected experimental tests.

Experimental study and physical modelling for the characterization of the flow in an inclined pipe and of the exiting jet trajectory

The thermohydraulic circuits found in the nuclear industry consist of a complex network of horizontal, vertical, or inclined pipes. In particular, the Emergency Core Cooling (ECC) pipes are connected to the primary circuit and are intended to inject cold water into it in accidental situations. Different configurations and inclinations can be found depending on the reactor type. The flow in the ECC pipe, as well as the efficiency of the reactor cooling, are influenced by these different configurations. Therefore, characterizing the flow in these pipes is crucial.
The objective of this thesis is to acquire new experimental data to characterize the water-air flow in an inclined pipe and the trajectory of the exiting jet at atmospheric pressure. The experimental data obtained will be used to develop and/or improve the modeling of flows in inclined pipes.
This thesis should provide the following contributions:
• Set up the experimental facility and the measurement methodologies ;
• Acquisition of the experimental data for different test section geometries (round vs. square), pipe diameter, roughness, inclination, fluid density/viscosity, and flow rate;
• Development of physical models to characterize the flow in an inclined pipe, including liquid hold-up and detachment length;
• Development of a mathematical formulation to predict the trajectory of the exiting jet;
• Study of the impact of stratification in the ECC pipe on the Cocci et al. jet condensation model;
• Optionally, simulation of the experiment using a CFD code (e.g., NEPTUNE-CFD) to extend code validation and identify potential improvements.

Numerical methods for the simulation of two-phase flows taking into account singularities

For nuclear safety calculations as well as for the design of new reactors, it is necessary to accurately simulate two-phase flows. The TrioCFD software, developped at CEA and based on the TRUST digital platform, offers the Front-Tracking module to study a two-phase flow by precisely following the interfaces between the phases. This method is functional and is validated in many application cases. However, it could benefit from recent mathematical developments to improve its robustness and to take into account the singularities in the neighbourhood of the interface. These can appear when two interfaces come together, as it is the case during the coalescence of two bubbles, at the tip of a vortex or at the level of a triple line (water, gas and solid wall). To faithfully resolve the phenomena in these regions, it is currently necessary to significantly refine the mesh around these zones, implying significant computational costs. The aim of this thesis is to propose and implement numerical methods to improve the Front-Tracking of TrioCFD. Firstly, we will seek to improve the current method in terms of robustness. Next, we will look at alternative methods to front tracking. The second part of the thesis work will aim to develop the treatment of interface singularities using a semi-analytic approach, called the singular complement method. This approach would greatly reduce the need for mesh refinement around the singularities of the interface.

Modelling of spacer grid effect during reactor-core reflooding

During a loss of coolant accident in a nuclear power plant, the primary circuit runs out of water. That leads to a total or partial core uncovery. The core consists of bundles of fuel rods protected by metal claddings. During such accidents, the clad cooling is no more ensured regarding the fact that less and less water is available to extract the heat produced in the fuel. Safety systems automatically activate in order to refill the primary circuit during the so-called “reflooding” phasis. When water enters the core, fuel rods are still very hot and prevent the liquid from rewetting the clad. Many droplets appear in the core while the quench front progresses. Theses droplets exchange heat and momentum with vapor and structures in the core: fuels rods and spacer grids. These grids maintain fuel rods and increase the mixing in the core with the help of mixing vanes. Exchanges are very dependent to the droplet size that is quite difficult to estimate, especially as grids have a high influence on it. A dry grid can split a droplet into many little ones, making the mean diameter decrease downstream the grid. In the opposite, it has been experimentally observed that a wet grid (with a stable liquid film on it) is responsible of an increase of the mean diameter.
The objectives of this thesis are then to take into account effect of grids, phase change and most influent phenomena on the mean droplet diameter in the core during the reflooding. As the droplet flow is polydispersed and is not accurately described by a unique diameter, the effect of this polydispersion should be studied.
In order to validate the models developed in the thesis, the work will lean on many experiments especially RBHT (Rod Bundle Heat Transfer) that simulate the reflooding in a core and give many informations on the droplet diameter and wall heat exchanges. The main objectives are then to develop physical models (grids effect, break-up, coalescence, droplet entrainement and deposition) and validate them after implementation in a calculation tool with the help of experimental data available in literature. This work is of a great interest in an academic and industrial point of view. The student will learn to build a physical model, use and develop scientific calculation tools, what will be an investment for his career.

Radial transfers modeling in PWR cores

The goal is to model the radial transfer terms for 3D flows within fuel bundles of PWR cores. The industrial context consists in the thermal-hydraulics safety code CATHARE 3 that has to reproduce 3D effects when strong assymmetries occur in specific accidental configurations while computing the whole core geometry. The main term to be modeled is the rods drag, that depends on the flow regime and its incidence angle. We will first consider only single-phase situations.
The modelling work will rely upon both the experimental data from the METERO-V rig and LES computations obtained with the TrioCFD code.
Advection momentum radial transfers will have to be properly identified to not be confused with diffusion phenomena and to allow for proper modelling. Finally this work will be extended to a 2-fluid configuration (either dispersed bubbles or droplets, to be defined). This will shift the focus onto modelling the 3D interfacial friction, two-phase friction multplier and void dispersion terms. At this point the goal will be to extend further the CATHARE 3 prediction capabilities for 3D flows in fuel bundles, for use in specific two-fluid conditions.