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.
A macroscale approach to evaluate the long-term degradation of concrete structures under irradiation
In nuclear power plants, the concrete biological shield (CBS) is designed to be very close of the reactor vessel. It is expected to absorb radiation and acts as a load-bearing structure. It is thus exposed during the lifetime of the plant to high level of radiations that can have consequences on the long term. These radiations may result especially in a decrease of the material and structural mechanical properties. Given its key role, it is thus necessary to develop tools and models, to predict the behaviors of such structures at the macroscopic scale.
Based on the results obtained at a lower scale - mesoscopic simulations, from which a better understanding of the irradiation effect can be achieved and experimental results which are expected to feed the simulation (material properties especially), it is thus proposed to develop a macroscopic methodology to be applied to the concrete biological shield. This approach will include different phenomena, among which radiation-induced volumetric expansion, induced creep, thermal defromations and Mechanical loading.
These physical phenomena will be developed within the frame of continuum damage mechanics to evaluate the mechanical degradation at the macroscopic scale in terms of displacements and damage especially. The main challenges of the numerical developments will be the proposition of adapted evolution laws, and particularly the coupling between microstructural damage and damage at the structural level due to the stresses applied on the structure.
Study of MOx and model compounds leaching in underwater storage conditions
This thesis deals with nuclear fuel recycling in France, with a focus on the multi-recycling of uranium and plutonium from MOX fuels, planned for 2040. Spent fuel is stored underwater in pools, where a cladding defect could lead to water contamination and complicate reprocessing. This thesis proposes to study the leaching of these fuels and the appearance of secondary phases under conditions simulating storage. The work is divided into three parts: preparation of model compounds, study of chemical durability of model and industrial materials, and analysis of secondary phases forming on the surface of irradiated fuels. The aim is to gain a better understanding of the stability of these phases as a function of chemical and irradiation conditions, as well as their transformation mechanisms. The results will enable us to develop models for the behavior of defective rods over several decades, contributing to safer and more efficient management of irradiated fuels.
Investigation of autocatalysis phenomena occurring in nitric acid dissolution through electrochemical methods
The nuclear fuel recycling process, used at the La Hague plant in France, begins with the nitric dissolution of spent fuel, mainly composed of uranium and plutonium oxides. In a context of plant renewal and widespread of MOX fuel recycling, innovative new dissolution equipment are currently studied. The sizing of such devices is currently limited by the absence of a fully comprehensive model for the dissolution of mixed oxides, which is a highly complex reaction (three-phase involved, self-catalytic, heterogeneous attack, etc.). Despite substantial progress made in previous studies, a number of questions remain unanswered, particularly concerning the reaction mechanisms involved and the nature of the catalyst.
Electrochemical methods (cyclic voltammetry, electrochemical impedance spectroscopy, rotating electrode, etc.) have never been used to understand dissolution, yet they should prove relevant as already demonstrated by the studies carried out on this subject by CEA Saclay in the field of corrosion. Therefore, the aim of this thesis is to apply these experimental methods for the first time to the dissolution of nuclear fuels, through a phenomenological approach. To achieve this, the student will be able to rely on the teams and facilities of Saclay and Marcoule centers, specialized respectively in electrochemical methods for the corrosion studies and the physico-chemical modeling of dissolution.
This cross-disciplinary study, involving materials science, electrochemistry and chemical engineering, will follow a stimulating fundamental research approach, but will also take place in a highly dynamic industrial context. Initially, the work will be carried out on inactive model and noble materials (at the Saclay center), then on real materials containing uranium and/or plutonium (at the Marcoule center).
Understanding the mechanisms of oxidative dissolution of (U,Pu)O2 in the presence of platinum group metals
The treatment of MOx fuel, composed of a mixed uranium and plutonium oxide (U,Pu)O2, is aimed at recycling plutonium. Plutonium dioxide (PuO2) is notably difficult to dissolve in concentrated nitric acid. However, by introducing a highly oxidizing agent, such as Ag(II), into the nitric acid, plutonium can be solubilized with fast dissolution kinetics—a process known as oxidative dissolution. The fission products present in irradiated MOx, particularly platinum group metals, can potentially impair the effectiveness of plutonium’s oxidative dissolution through side reactions. For the industrial deployment of this method, it is therefore crucial to understand how platinum group metals influence the dissolution kinetics. Yet, there is currently very limited data on this subject.
This thesis aims to address this knowledge gap. The proposed research involves a parametric experimental study of increasing complexity: initially, the impact of platinum group metals on Ag(II) consumption will be investigated separately, followed by their effect during the dissolution of (U,Pu)O2. These findings will enable the development of a kinetic model for the dissolution process based on the studied parameters.
By the end of this thesis, the candidate, with a strong background in physical or inorganic chemistry, will have gained expertise in a wide range of experimental techniques and advanced modeling methods. This dual competence will open up numerous career opportunities in academic research or industrial R&D, both within and beyond the nuclear sector.
Towards a Method for characterizing the electrokinetic Properties of Particles in water at high Temperatures
In the field of industry and particularly energy, liquid water circuits are omnipresent. Fluids, by interacting with pipes made from metal alloys, inevitably lead to the formation of corrosion products.
Predicting the behavior of small particles (order of magnitude of µm) is therefore of particular interest. Indeed, due to their size, the behavior of the latter is governed by forces of electrical origin responsible for their adhesion to the surfaces. The electrokinetic properties and in particular the surface potential thus control the fate of the particle and can be defined using the zeta potential. This quantity characterizes a solid/solution couple and takes into consideration both the particle and its surface chemical properties as well as the solution where the particle is located.
If the characterization of the zeta potential at room temperature is quite widespread, its determination at high temperature is today confined to a few examples (theses by C. Cherpin 2022 [1] and M. Barale 2006 [2], studies of VTT [3] and EDF with the University of Besançon 2002 [4] and the EPRI patent 1994 [5]). The CEA (LC2R) has developed an innovative measurement method currently being patented to explore poorly developed experimental techniques based on theoretical hypotheses to be confirmed.
Through multi-physics (flow, temperature, chemistry, electrochemistry, etc.) and multi-scale (microscopic particles influencing a macroscopic state) approaches, the objective of the thesis is therefore to carry out measurements of the surface properties of particles in water at high temperature depending on the physicochemical conditions (pH, RedOx and temperature), to adapt existing models or propose new ones then validate them with experimental data.
The data thus obtained is intended to feed the simulation codes in order to better understand and control the aging of the circuits.
[1] C. Cherpin, PhD, 2022, Modelling the behaviour of colloidal corrosion products in the primary circuit of Pressurized Water Reactors
[2] M. Barale, PhD, 2006, Etude du comportement des particules colloïdales dans les conditions physico-chimiques du circuit primaire des réacteurs à eau sous pression
[3] E. Velin, Master’s Thesis, 2013, The effect of Temperature on the Zeta Potential of Magnetite Particles in Ammonia, Morpholine and Ethanolamine Solutions
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.
Prediction of Soiling on PV modules/systems through Real-World Environment Modeling and Data Fusion
Photovoltaic (PV) systems, particularly those installed in regions prone to soiling such as arid areas, coastal sites, and agricultural zones, can experience energy losses of up to 20–30% annually. These losses translate to financial impacts exceeding €10 billion in 2023.
This thesis aims to develop a robust and comprehensive method to predict soiling accumulation on PV modules and systems by combining real-world environmental modeling with operational PV data (electrical, thermal, optical). The research will follow a bottom-up approach in three stages:
1. Component/Module Level: Reproduction and modeling of soiling accumulation in laboratory conditions, followed by experimental validation. This stage will leverage the CEA’s expertise in degradation modeling, including accelerated testing.
2. Module/System Level: Implementation of monitoring campaigns to collect meteorological, operational, and imaging data, combined with field soiling tests on a pilot site. The data will validate and enhance CEA diagnostic tools by introducing innovative features such as AI-driven soiling propagation prediction.
3. System/Operational Level: Validation of the proposed method on commercial PV modules in utility-scale PV plants, aiming to demonstrate scalability and real-world applicability.
The outcomes of this thesis will contribute to the development of an innovative tool/method for comprehensive soiling diagnostics and prognostics in PV installations, enabling the minimization of energy losses while anticipating and optimizing cleaning strategies for PV plants.
Multi-physical characterization of potassium hybrid supercapacitors for performance improvement
The PhD subject focuses on the optimization of potassium hybrid supercapacitors (KIC), which combine the properties of supercapacitors (power, cyclability) and batteries (energy). This system, developed at the CEA, represents a promising technology, low cost and without critical/strategic materials. However, performance optimization still requires overcoming various obstacles observed in previous work, in particular on the intercalation of potassium in graphite and the heating phenomena of cells during operation. In order to explore in depth the operating mechanisms of the KIC system, an essential part of the thesis project will include experiments conducted at the ESRF (European Synchrotron Radiation Facility), where advanced diffraction and imaging techniques will be used to analyze the structure of the materials and their behavior in real operating conditions. The processing of the data collected will also be crucial in order to establish correlations between the physicochemical properties of the materials and the overall performance of the system. This thesis will contribute to the fundamental understanding of the multi-physical mechanisms at stake in KIC to develop innovative design strategies and thus improve their capacity, energy efficiency and lifetime.
Learning Interpretable Models for Stress Corrosion of Stainless Steels Exposed in the Primary Environment of PWRs
Stress corrosion cracking (SCC) of austenitic alloys in water-cooled nuclear reactors is one of the most significant component degradation phenomena. SCC occurs due to the synergistic effects of tensile stresses, environment and material susceptibility. For reactor life extension, understanding this mechanism is essential. The methodology most frequently employed to investigate SCC cracking is an experimental one, requiring lengthy and costly tests of several thousand hours. Furthermore, the considerable number of critical parameters that influence susceptibility to SCC cracking and coupling effects have resulted in test grids increasing in length and complexity. This thesis proposes a novel approach based on the use of interpretable models that are driven by the artificial intelligence of fuzzy logic. The aim is to reduce the length and cost of research activities by focusing on relevant tests and parameters that can improve environmental performance. The key issues here will be to add the performance of artificial intelligence to the experimental approach, with the aim of defining susceptibility domains for the initiation of SCC cracks as a function of the critical parameters identified in the model, and providing data for the development of new materials by additive manufacturing. The thesis will develop a numerical model that can be used as guidance in decision-making regarding the stress corrosion mechanism. The future PhD student will also carry out experimental work to validate this new numerical approach.