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
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.
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.
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