Turbulence synthesization methods for hybrid URANS/LES CFD approaches in multi-scale simulation of nuclear cores
Problem description: Fluid-structure interactions in nuclear reactor cores are a result of mechanisms occurring at different space scales. The component scale represents the global flow inside the core and is generally simulated though porous media methods. The local scale represents the fuel assembly: it requires CFD scale-resolving methods to calculate consistent fluid forces on the structures, and it features a certain degree of fluid-structure coupling. With the goal of performing multi-scale simulations of a core, the local scale requires the generation of boundary conditions from the component scale. This can be achieved only by a synthetic generation of turbulence, based on the flow results at the component scale. However, the porous media approach used at the component scale does not contain details on the turbulent quantities: the development of new numerical methods is required for generating consistent synthetic turbulence in this configuration.
Objectives:
1. Identify proper hybrid URANS/LES approaches for fuel assembly vibration related issues
2. Identify available turbulence parameters in porous media methods and explore bottom-up scaling approaches
3. Develop a turbulence synthesization method applicable to any fuel array inside a core
Expected results:
1. A novel approach for fluid-induced vibration analysis based on a multi-scale method
2. Clarify the key parameters to generate proper turbulence-resolved boundary conditions in the specific configuration studied
3. Validate the new methods on available experimental configurations
Generation of Cesium silicate micro-particles from Fukushima
Microscopic in size, but large in environmental impact, cesium microparticles hold one of the keys to understanding the Fukushima nuclear accident. Following the Fukushima Daiichi accident, these cesium-rich silicate glass microparticles (CSMP) were discovered in the environment, carrying a significant portion of the radioactivity. Very poorly soluble in water, they differ from those observed at Chernobyl. A previous thesis demonstrated that these CSMPs could be the result of the interaction between corium and concrete during a severe accident, via small-scale experiments. The study made it possible to reproduce similar particles, made of amorphous silica with crystalline nano-inclusions. However, the results need to be refined, particularly with regard to the presence of zinc and calcium. The proposed thesis aims to explore the physicochemical mechanisms leading to the synthesis of these CSMPs. Laboratory experiments will recreate the corium-concrete interaction conditions, representative of Fukushima, in order to optimize the compositions and improve the modeling of the releases of these particles in current severe accident assessment tools.
Electrolyte ceramics for oxygen potentiometric sensors in aggressive media of advanced nuclear reactor
The solid electrolytes are thought to play major role in future energetic systems (SOFC, SOEC). Among them, oxide ceramics with fluorite structure are particularly important. Correctly doped, their ionic conductivity is high and they are suitable for applications in aggressive media or at high temperatures. However, these properties are closely related to their microstructure, thus to their fabrication route. At CEA IRESNE, we develop fluorite based-potentiometric sensors for oxygen monitoring of advanced reactors coolants.
This thesis proposed to study the relation between the microstructure of two fluorite materials, doped hafnium or thorium oxides, and their behavior in liquid sodium or molten chlorides. The influence of grain size, density and impurity contents on the corrosion kinetic in sodium would provide insights on the corrosion mechanisms. The ultimate aim is to optimize the service life of these ceramics in oxygen sensors for sodium based energetics systems and to test them. The electrolyte will be used in sensors to characterize the behavior of oxygen in these complex media.
The student should be graduated in materials science. The thesis work will take place at the CEA/IRESNE Institute on the Cadarache site (France, Provence) in collaboration with the Institute of separative chemistry of Marcoule (France, Occitanie).
Study and characterization of nucleate boiling in reactor conditions
In the context of the energy transition and the place of nuclear power in the energy mix, controlling safety and optimizing reactor performance represent imperative research areas with high added value. In this context, boiling at high pressure and temperature is a key issue for water reactors widely deployed in France and around the world.
The many works on this subject carried out in the past show their limitation in terms of representativeness and present certain gaps (e.g. the evolution of the topology of the flow at high pressure). The proposed subject therefore concerns the characterization of nucleate boiling for a wide range of pressure and temperature conditions, and more particularly the study of the coupling between the thermal properties of the wall and the flow (bubble sizes, detachment frequency, local void ratio, etc.). This work will also provide data relating to boiling models that can be used in CFD-type numerical calculation tools. Direct visualization of the flow using portholes (a process successfully implemented in the past), coupled with the use of stereological tools (in collaboration with the LRVE at CEA Marcoule) and associated with a measurement of the wall temperature, should make it possible to achieve the set objectives. These measurements carried out under representative reactor conditions (thermohydraulic conditions, real fluid, representative heating surface) make this study original compared to existing work.
After an initial critical literature review, the PhD student will design and test the experimental devices before implementing them through test campaigns on a dedicated installation. The results collected will be analyzed, interpreted, compared with existing models and may, if necessary, lead to the construction of new models. This thesis will take place on the POSEIDON experimental platform, dedicated to flows studies, and will allow the doctoral student to approach all phases of a research project, from the design of experimental devices to the interpretation of the results obtained.
Purification of chloride salts for safe use in energy production systems: development of methods, understanding and optimization.
Chloride molten salts are of major interest as coolants of high temperature energy production systems (solar, nuclear). However, they suffer from the high corrosion rates on structural materials, which is mainly related to their chemical purity. The control of oxygen activity is of prime interest to limit the dissolution of a large number of elements. However, some salts of interest for the nuclear industry (ternary NaCl-MgCl2-PuCl3 and its surrogate NaCl-MgCl2-CeCl3) are particularly difficult to purify, due to their high affinity with water.
Therefore, the understanding of the nature and stability of species formed in non-purified system (chlorides, oxides, oxi-chlorides, hydroxi-chlorides) is mandatory to propose appropriate purification methods for industrial systems. The Ph D will have to purify and characterize different salt mixtures (from binary to quaternary systems) from available methods in the laboratory:
• For purification: electrolysis, precipitation, filtration, chlorinating gas bubbling
• For characterization: electrochemical technics, potentiometric O sensors, Raman spectroscopy, analytical chemistry, materials characterization…
The thesis will take place at the institute of Energy (IRESNE) of the CEA Cadarache (Provence, France). The main laboratory (LMCT) has a large experience of advanced coolants chemistry (in particular sodium). Some collaborations are engaged with other labs of the CEA (Marcoule) and with the LGC Toulouse, both having long experience in molten salt chemistry.
The student should be graduated in electrochemistry or materials science.
Impact forces under flow : water gap effect on the dynamics of a nuclear component
In the framework of the contribution of nuclear power to a decarbonized energy mix, reactors safety is of paramount importance. In the event of an earthquake, dynamic loads experienced by a reactor core could lead to collisions between fuel assemblies. The presence of turbulent flow inside the core has a significant effect on the dynamic behaviour of the assemblies. Recent tests have revealed an additional effect of the flow on impact forces between structures, possibly caused by a high-speed fluid sheet phenomenon.
The objective of this thesis, divided into three parts, is to understand and characterise this high-speed fluid sheet phenomenon in the specific case of a fuel assembly geometry.
A first part will be dedicated to CFD simulations taking into account the deformation of the fluid domain mesh using the Arbitrary Lagrange-Euler (ALE) method [1]. In addition, ambitious experimental campaigns will allow measuring, as close as possible to the impact, the effect of structures displacement on flow velocity field (using optical methods such as Particle Image Velocimetry [2]) and the resulting impact forces. The findings will be translated into an analytical modelling of the phenomenon.
The candidate will be hosted by the laboratory leading work on fluid-structure interactions within CEA Cadarache research centre. He/she will be integrated into a research environment with international outreach (collaboration with George Washington University - USA), will publish his/her research outcomes in leading journals in the field, and will participate in international conferences.
[1] A computationally efficient dynamic grid motion approach for Arbitrary Lagrange-Euler simulations, A. Leprevost, V. Faucher, and M. A. Puscas, Fluids, 8(5), 2023.
[2] Longo, L., Capanna, R., Ricciardi, G., & Bardet, P. (2024). Threshold of Keulegan-Carpenter instability within a 6 × 6 rod bundle, Experimental Thermal and Fluid Science
Elementary characterization by neutron activation for the circular economy
As part of the circular economy, a major objective is to facilitate the recycling of strategic materials needed by industry. This requires, first of all, the ability to accurately locate them in industrial components that are no longer in use. Non-destructive nuclear measurement meets this objective, based on prompt gamma neutron activation analysis (PGNAA). This approach involves interrogating the samples to be analyzed with an electrical generator emitting pulses of fast neutrons that thermalize in a polyethylene and graphite cell: between the pulses, radiative capture gamma rays are measured. The advantage of such an approach lies in the fact that high-value elements such as dysprosium or neodymium have a high radiative capture cross-section by thermal neutrons, and that the latter can probe deep into large volumes of matter (several liters).
A previous thesis demonstrated the feasibility of this technique and opened up promising avenues of research, with two complementary strands to make concrete progress towards practical recycling objectives. The first involves experimental and simulation studies of the performance of gamma cascade measurement on cases representative of industrial needs (size and composition of objects, measurement speed). The second will enrich and improve the exploitation of the vast amount of information available from gamma-ray cascade measurements.
In practice, the work will be carried out as part of a collaboration between CEA and the FZJ (ForschungsZentrum Jülich) institute in Germany. The first half of the thesis will be carried out at CEA IRESNE Nuclear Measurement Laboratory. The second half of the thesis will be carried out at the FZJ (Jülich Centre for Neutron Science, JCNS). The German part of the thesis will involve experiments with the FaNGaS device at the Heinz-Maier-Leibnitz Zentrum (MLZ) in Garching.
Flow rate measurement in a pipeline using thermal noise detection
Flow measurement is a key factor in process management, particularly in the nuclear and industrial sectors. However, current measurement methods require complex installations, especially in environments with strict regulations, such as in the nuclear sector. To address these challenges, the CEA has developed an innovative method for measuring flow in non-isothermal fluids, based on the analysis of thermal fluctuations. This technique, which uses two temperature sensors installed upstream and downstream on the pipeline, is simple to implement and involves minimal constraints. The temperature variations are carried by the flow from one sensor to the other, and by comparing the signals recorded by these sensors, it is possible to calculate the thermal transit time between them, which allows the flow velocity, and consequently, the flow rate, to be determined. The goal of this thesis is to optimize this method by enhancing its reliability. To achieve this, the propagation of thermal noise within the flow will be studied, and both the type and placement of the sensors will be optimized. This work will be carried out within the Core and Circuit Thermohydraulics Laboratory in collaboration with the Instrumentation, System and Method Laboratory, which has state-of-the-art experimental equipment. Numerical simulations will complement the experimental work to validate the obtained results. In parallel, artificial intelligence approaches will be explored to improve the processing of thermal signals. By the end of the thesis, the doctoral candidate will have acquired extensive skills in experimental and numerical work and will be able to leverage these in future endeavors.
Experimental study of the two-phase natural convection and vaporization regimes in the cooling pool of a nuclear facility
Nuclear energy, with low CO2 emissions, is one of the major players in France's energy transition. In this context, the management of the cooling of irradiated fuel elements is a matter of utmost importance. This thesis focuses on two-phase natural convection flows and vaporization phenomena that can develop in the cooling pools of various nuclear facilities, particularly those having a significant vertical variation in the saturation temperature of the coolant due to their great depth. These pools are used to dissipate the residual heat from irradiated fuels in many types of nuclear reactors, both existing and planned. In an accident scenario with a significant heat release from the fuels, the water in these pools can vaporize, eventually limiting their cooling capability. Among the possible phase change mechanisms in deep pools is the gravity-driven flashing, a phenomenon found in various natural or industrial systems analogous to vertical channels heated from below. However, this phenomenon has been little studied in the specific configuration of a pool and was only recently observed in this context. Therefore, the objective of this thesis is to better understand the phenomenon, as well as the turbulence induced within the coolant by the bubbles it generates, in order to improve state-of-the-art thermal-hydraulic models for simulating such pools. The proposed research, of an experimental nature, will be conducted in collaboration with the Catholic University of Louvain (UCLouvain, Belgium) and the LEGI laboratory of CNRS Grenoble, with a significant portion of the research carried out at UCLouvain. The candidate will be affiliated to the Core and Circuit Thermal-hydraulics Laboratory (LTHC) of CEA IRESNE, specialized in the study of two-phase flows in nuclear facilities. During the thesis, finely resolved experimental data in both space and time will be acquired and interpreted, contributing to a better understanding of the phenomenon. To achieve this, advanced techniques such as stereo particle image velocimetry (3D PIV) in two-phase media, thermometry and shadowgraphy will be employed. During this thesis project, the PhD student will be able to develop skills in the field of experimental thermal-hydraulics through the definition, execution, and interpretation of tests, as well as the use of advanced two-phase flow measurement techniques.
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.