Impact of pollution on the dynamics of bubbly flows
In accident conditions, if the core of a nuclear reactor boils, the pollution of the water can have an important role in heat exchanges. The challenge of this thesis is to understand this impact and learn to simulate it, the aim being ultimately to provide reference data for boiling in reactor conditions. To achieve this, this thesis will focus on simulating the transport of a pollutant concentration within bubbly flow. The student will simulate the pollution of interfaces by surfactant molecules, a particular case of pollutant found in most hydraulic systems. This study will be carried out using Direct Numerical Simulations carried out with the TRUST/TrioCFD open-source code. The student will be hosted at the Laboratory of Modeling and Simulation in Fluid Mechanics (LMSF) within a group of researchers and numerous PhD students. In collaboration with the academic world, the student will publish his work and participate in international conferences. We are therefore looking for a student who has completed his studies in computational fluid mechanics (M2 or equivalent). Knowledge of modern C++ language would be a notable advantage. Carrying out an internship prior to the thesis is possible.
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).
Characterisation of the gaseous leak at the contact interface between rough surfaces during loading and unloading - application to the case of metal seals
In various industrial applications, fully metallic seals are employed to guarantee a high level of sealing of mechanical assemblies under severe thermodynamics conditions. Their performance is entirely controlled by the mechanical behaviour of the contacting interface between the facing rough surfaces of the seal and the flange, similar to a fracture, anisotropic and multi-scale by nature. The objective of the thesis is to improve our comprehension and predicting capabilities of the sealing mechanisms of gases in a rough fracture using a numerical approach coupled with experiments.
The work takes place in the continuity of previous studies performed at the laboratory. It will focus first on the conception of an experimental apparatus that will be used to press two metallic rough surfaces against each other with a given force, having the possibility to measure the corresponding leakage rate as well. The experiments will be performed during loading and unloading of the contact to characterise the hysteresis phenomenon brought by the permanent deformation of the sealing material at first loading. The results obtained will be compared to numerical ones in various configurations using models developed at the laboratory, in order to validate these latter. By experience, it is known that the flow simulation gives satisfactory results, but discrepancies persist in the contact mechanics model. Thus, it should be improved regarding the plastic effects specifically encountered in contact, considering the finite thickness of the sealing liner and optimizing the computational cost. Afterwards, the preceding results will be transposed to the industrial case of the HELICOFLEX metal seal, using a two-scale modelling strategy, coupling the macroscopic information at the seal scale to the microscopic one at the roughness scale.
Stabilization of secondary phases in nanoreinforced ferritic steels: High-throughput screening approach of chemical compositions
Ferritic steels reinforced by oxide dispersion strengthening (ODS) are considered for use in 4th Generation and fusion nuclear reactors due to their excellent thermomechanical properties and stability under irradiation. However, these steels are weakened by secondary phases resulting from complex interactions between alloying elements and interstitials (C, N, O) introduced during their processing. Some alloying elements (such as Nb, V, Zr, Hf) could potentially stabilize these undesirable phases and mitigate their detrimental effects on the mechanical behavior of ODS steels. This thesis aims to develop a high-throughput screening method to identify optimal alloy compositions by combining rapid fabrication and characterization techniques. The PhD student will synthesize various compositions of ODS steels through powder metallurgy and carry out chemical, microstructural, and mechanical characterizations. This work will enhance the understanding of interstitial stabilization mechanisms and propose effective methodologies for characterizing new materials. The PhD student will gain in-depth knowledge in metallurgy and data processing, providing opportunities in industry, nuclear start-ups, and research.
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.
Kinetics of segregation and precipitation in Fe-Cr-C alloys under irradiation : coupling magnetic, chemical and elastic effects
Ferritic steels are being considered as structural materials in future fission and fusion nuclear reactors. These alloys have highly original properties, due to the coupling between chemical, magnetic and elastic interactions that affect their thermodynamic properties, the diffusion of chemical species and the diffusion of point defects in the crystal. The aim of the thesis will be to model all of these effects at the atomic scale and to integrate them into Monte Carlo simulations in order to model the segregation and precipitation kinetics under irradiation, phenomena that can degrade their properties in use. The atomic approach is essential for these materials, which are subjected to permanent irradiation and for which the laws of equilibrium thermodynamics no longer apply.
The candidate should have a good background in statistical physics or materials science, and be interested in numerical simulations and computer programming. The thesis will be carried out at CEA Saclay's physical metallurgy laboratory (SRMP), in a research environment with recognised experience in multi-scale modelling of materials, with around fifteen theses and post-doctoral contracts in progress on these topics.
A Master 2 internship on the same subject is proposed for spring 2025 and is highly recommended.
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
Sub-Grid modelling of interfacial heat and mass transfers applied to condensation of bubble swarms
To assess the safety of nuclear power plants, the CEA develops and uses multi-scale thermohydraulic simulation tools. The application of CFD to two-phase flows is limited because it requires many models that are difficult to determine. Among our other tools, direct numerical simulations (DNS) with resolved interfaces provide reference data inaccessible by experimental means. This is for example the case of bubble swarms, where heat and mass transfers are influenced by complex collective effects.
In order to reduce the cost of these DNS simulations, we recently developed an approach [1] which shows promising results: it consists of coupling a fine resolution of thermal transfers at the liquid-vapor interfaces to a far field calculated on a less resolved mesh. To broaden the application of this method to more industrial cases, it is necessary to take into account collisions between bubbles and to adapt the model to the phase change.
During this thesis, we propose to start with this physical modeling work and its implementation in C++ in our open-source simulation code TRUST/TrioCFD [2]. Next, we will use this new capacity to carry out a parametric study and an in-depth physical analysis of the phenomena which would ultimately lead to an improvement in heat transfer models in industrial codes.
[1] M. Grosso, G. Bois, A. Toutant, Thermal boundary layer modelling for heat flux prediction of bubbles at saturation: A priori analysis based on fully-resolved simulations, International Journal of Heat and Mass Transfer, Vol 222, 2024, https://doi.org/10.1016/j.ijheatmasstransfer.2023.124980
[2] Trio_CFD webpage : http://triocfd.cea.fr/recherche/modelisation-physique/two-phase-flows
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.