Structure and mobility of unterstitial clusters and loops in uranium oxide
Uranium oxide (UO2) is the usual fuel used in nuclear fission power plants. As such, its behaviour under irradiation has been extensively studied. Irradiation creates vacancies or interstitial defects that control the evolution of the material's microstructure, which in turn impacts its physical (e.g. thermal conductivity) and mechanical properties. Interstitial clusters in particular play a major role.
On the one hand, at the smallest sizes, the diffusion of interstitials in UO2 is still relatively poorly understood. Experimentally, we observe the appearance of dislocation loops made up of interstitials as large as ten nanometres. Conversely, no cavities are observed and the vacancy defects remain sub-nanometric in size. This indicates that interstitials diffuse more rapidly than vacancies, with diffusion allowing interstitials to agglomerate and form loops. However, atomic-scale calculations show no major difference between the diffusion coefficients of vacancies and interstitials in UO2. One hypothesis to explain this apparent contradiction is that interstitial clusters diffuse rapidly (Garmon, Liu et al. 2023).
On the other hand, the three-dimensional interstitial clusters are expected to be the seeds of the dislocation loops observed by transmission electron microscopy in irradiated uranium oxide. However, the mechanisms by which the aggregates transform into loops and the nature of the loops changes remain poorly understood in uranium oxide. These mechanisms have very recently been elucidated for face-centred cubic metals (Jourdan, Goryaeva et al. 2024). It is possible that comparable mechanisms are at work in UO2 with the complication induced by the existence of two sub-lattices.
We therefore propose to study interstitial clusters in UO2 using atomic-scale simulations.
We will first study the structure of these three-dimensional subnanometric clusters. To do this, we will use artificial intelligence tools for classifying defect structures developed in the laboratory (Goryaeva, Lapointe et al. 2020). We will study the diffusion of these objects using molecular dynamics and automatic searches for migration saddle points using kinetic-ART type tools (Béland, Brommer et al. 2011). Secondly, we will study the relative stability of 3D clusters and loops of faulted and perfect dislocations and the transformations between these different objects.
This study will be based on interatomic interaction potentials. We will start by using empirical potentials available in the literature before turning to Machine Learning-type potentials (Dubois, Tranchida et al. 2024) under development at the CEA Cadarache Fuel Studies Department.
Béland, L. K., et al. (2011). ‘Kinetic activation-relaxation technique.’ Physical Review E 84(4): 046704.
Chartier, A., et al. (2016). ‘Early stages of irradiation induced dislocations in urania.’ Applied Physics Letters 109(18).
Dubois, E. T., et al. (2024). ‘Atomistic simulations of nuclear fuel UO2 with machine learning interatomic potentials.’ Physical Review Materials 8(2).
Garmon, A., et al. (2023). ‘Diffusion of small anti-Schottky clusters in UO2.’ Journal of Nuclear Materials 585: 154630.
Goryaeva, A. M., et al. (2020). ‘Reinforcing materials modelling by encoding the structures of defects in crystalline solids into distortion scores.’ Nature Communications 11(1).
Jourdan, T., et al. (2024). ‘Preferential Nucleation of Dislocation Loops under Stress Explained by A15 Frank-Kasper Nanophases in Aluminum.’ Physical Review Letters 132(22).
Atomistic modeling of fracture in heterogeneous borosilicate glasses
Heterogeneous borosilicate-based glasses contain crystalline or amorphous precipitates forming secondary phases embedded within the glass matrix. These materials are valued for their high thermal shock resistance and excellent chemical durability, making them ideal for various applications such as cookware and laboratory equipment. In particular, within the nuclear industry, many wasteforms effectively function as glass-ceramics due to the presence of elements that form precipitates.
It is well known that secondary phases can significantly affect mechanical properties, particularly fracture toughness. However, the specific mechanisms by which they influence mechanical properties at the atomic scale remain poorly understood. In particular, whether they are crystalline or amorphous and the structure of their interface with the bulk glass are expected to play a crucial role.
The primary aim of this project is to investigate the specific mechanisms by which precipitates influence mechanical properties at the atomic scale.
Additionally, it seeks to understand how these precipitates affect crack propagation.
For this purpose, numerical modelling tools based on molecular dynamics will be employed.
This technique simulates the behaviour of individual atoms over time under different testing conditions.
Thus, it enables probing the local structure of crack tips and how they interact with precipitates at the atomic level, providing valuable insights into the mechanisms underlying crack resistance in heterogeneous glasses.
Hydrogen and ammonia combustion within porous media: experiments and modelling
- Context
Current energy prospects suggest the use of hydrogen (H2) and ammonia (NH3) as carbon-free energy carriers to achieve neutrality by 2050. NH3 offers advantages like high energy density and safe storage but faces combustion challenges such as narrow flammability and high NOx emissions. Interestingly, some H2 can be obtained by partial cracking of NH3 to create blends of more favourable combustion properties, with open questions regarding pollutant emissions and unburnt NH3 content.
- Challenges
Porous burners show promise for safe and low-pollutant combustion of NH3/H2 blends. However, material durability issues and the complexity of flame stabilization pose significant hurdles. Fortunately, recent advances in additive manufacturing enable the precise tailoring of porous matrices, but the experimental characterization remains difficult due to the opacity of the solid matrix.
- Research objectives
The PhD candidate will operate an experimental bench at CEA Saclay to conduct combustion experiments with NH3/H2/N2+air mixtures in various porous burners. Key tasks will include designing new burner geometries, comparing experimental results with numerical simulations, and advancing the modelling of porous burners using 1D Volume-Averaged Models and asymptotic theory. Experimental measurements will include hotwire anemometry, infrared thermometry, output gas composition analysis, chemiluminescence, and laser diagnostics. The porous burners will be manufactured using 3D printing techniques with materials such as stainless steel, inconel, alumina, zirconia, and silicon carbide.
The research aims to develop more robust and efficient porous burners for NH3/H2 combustion, enhancing their practical application in achieving carbon neutrality. The candidate will contribute to advancing the field through experimental data, innovative designs, and improved modelling techniques.
Modeling of the fall of a drop in a volume, in support of the system code CATHARE
This thesis focuses on the study of droplet fall in free volumes, as part of the continuous improvement of the physical models in the CATHARE code, used for safety studies of Pressurized Water Reactors. The current models are based on the work of Ishii and Zuber, who model the fall velocity of droplets in a two-phase fluid. The objective of the thesis is to refine the precision of this model by incorporating additional parameters and validating it through experiments such as those of Dampierre and CARAYDAS. The PhD candidate will be required to develop a more representative mechanistic model, based on experimental data or CFD simulations if necessary. The innovation lies in developing a more accurate model of droplet fall processes, paving the way for specific applications such as spray modeling, and thus contributing to the validation of the CATHARE code in additional fields.
Study of of the thermodynamic of K2CO3-CO2-H2O for the development of NET and SAF technologies
.Bioenergy with Carbon Capture and Storage (BECCS) uses biomass energy while capturing the carbon dioxide released by the process, resulting in negative emissions into the atmosphere. The reference process in Europe uses potassium carbonate but at atmospheric pressure [1], whereas its sequestration or hydrogenation into sustainable molecules requires high pressures.
The thesis consists in acquiring new thermodynamic and thermo-chemical data at high temperature/pressure [2] required for the energy optimization of such a process, and integrating them into a thermodynamic model.
The overall process will then be reassembled in order to quantify the expected energy gain.
The thesis will be carried out at the Thermodynamic Modeling and Thermochemistry Laboratory (LM2T), in collaboration with LC2R (DRMP/SPC) for the experimental part.
References :
[1]K. Gustafsson, R. Sadegh-Vaziri, S. Grönkvist, F. Levihn et C. Sundberg, «BECCS with combined heat and power: assessing the energy penalty,» Int. J. Greenhouse Gas Control, vol. 110, p. 103434, 2021.
[2] S. Zhang, X. Ye et Y. Lu, «Development of a Potassium Carbonate-based Absorption Process with Crystallization-enabled High-pressure Stripping for CO2 Capture: Vapor–liquid Equilibrium Behavior and CO2 Stripping Performance of Carbonate/Bicarbonate,» Energy Procedia, 2014
Effect of microstructure and irradiation on susceptibility to intergranular cracking of alloy 718 in PWR environment.
Alloy 718, a nickel-based alloy, is used in fuel assemblies for pressurized water reactors (PWRs). In service, these components are subjected to high mechanical stress, neutron irradiation and exposure to the primary environment. Usually, this alloy shows very good resistance to intergranular cracking. However, there are microstructural and/or irradiation conditions which, by modifying the mechanical properties and plasticity mechanisms, make the material susceptible to intergranular cracking in the primary PWR environment.
In this context, the aim of this thesis will be to study the influence of microstructure (via different heat treatments) and irradiation on deformation localization and susceptibility to intergranular cracking in primary PWR media.
To this end, two grades will be tested, one deemed sensitive and the other not. In-situ SEM tensile tests on samples whose microstructure has been previously characterized by EBSD will be carried out to identify the types of intra- and intergranular deformation localization and their evolution. The non-irradiated state will be characterized as the reference state. In addition, exposure and intergranular cracking tests in the primary medium (coupons, slow tension, etc.) will be carried out on both grades and at different irradiation levels. The microstructure as well as surface and intergranular oxidation of the specimens will be characterized by various microscopy techniques (SEM, EBSD, FIB and transmission electron microscopy).
This thesis constitutes for the candidate the opportunity to address a problem of durability of metallic materials in their environment following a multidisciplinary scientific approach combining metallurgy, mechanics and physico-chemistry and based on the use of various cutting-edge techniques available at the CEA. The skills that he will thus acquire can therefore be valued during the rest of his career in the industry (including non-nuclear) or in academic institutions.
Development of a transport chemistry model for spent fuel in deep geological disposal under radiolysis of water
The direct storage of spent fuel (SF) represents a potential alternative to reprocessing as a means of managing nuclear waste. The direct storage of spent fuel in a deep geological environment presents a number of scientific challenges, primarily related to the necessity of developing a comprehensive understanding of the processes involved in the dissolution and release of radionuclides. The objective of this thesis is to develop a comprehensive scientific model that can accurately describe the intricate physico-chemical processes involved, such as the radiolysis of water and the interaction between irradiated fuel and its surrounding environment. The objective is to propose an accurate reactive transport model to enhance long-term predictions of storage performance. This thesis employs a back-and-forth process between modeling and experimentation, with the goal of refining the understanding of alteration mechanisms and validating hypotheses with experimental data. Based on existing models, such as the operational radiolytic model, the work will propose improvements to reduce the current simplifying assumptions. The candidate will contribute to major industrial and societal issues related to nuclear waste management and will help to provide solutions to the associated safety issues.
Study of wire additive manufacturing of a nuclear component with complex geometry
The general aim of the thesis is to study the feasibility of a component for the DEMO fusion reactor using Wire Additive Manufacturing (WAM). To achieve this, the PhD student will first design and manufacture demonstration parts representative of different sub-parts of the component in the laboratory's additive manufacturing cells. He or she will use CAD/CAM software to manufacture parts of increasing size and complexity, while ensuring repeatability.
These parts will be subjected to characterization work, firstly dimensional, to check their geometric conformity with the project specifications; but also microstructural and metallurgical, to guarantee manufacturing quality, in particular the absence of defects within the material (porosity, inclusions...) or metallurgical phases detrimental to its mechanical strength.
Finally, the PhD student will also be required to simulate the manufacture of certain parts using the finite element method, in order to analyze the evolution of parameters of interest, such as temperature, during manufacture, and to estimate the state of deformation and stress after manufacture. These simulations can be used to correct certain discrepancies between expected and actual results, within the framework of a calculation-test dialogue that will see the implementation of instrumentation also serving to validate the models. These simulations will be carried out using the Cast3M finite element code developed at CEA.
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Effect of plastic strain on brittle fracture: Decoupling of deformation induced dislocation structures and deformation induced microtexture evolution
In the nuclear field, the integrity of components must be ensured throughout their operating life, even in the event of an accident. The demand for justification of component resistance to the risk of sudden rupture is growing, and is being applied to a wide range of piping lines and equipment. The demonstration principle consists in showing that, even in the presence of a defect, the equipment is capable of withstanding the loads it is likely to be subjected to.
Particular attention is paid to brittle fracture by cleavage, because of its unstable and catastrophic nature, which immediately leads to the ruin of the component. Brittle fracture is sensitive to the level of plasticity and triaxiality at the crack tip, which explains the beneficial structural effect often observed on real components compared to laboratory specimens. The industrial challenge is to better understand the role of plasticity in relation to microtexture on brittle fracture, in order to improve current prediction criteria.
In the course of this thesis, the brittle fracture toughness of a ferritic steel will be evaluated after various types of mechanical pre-strain. By the end of the thesis, the candidate will have acquired solid skills in mechanical testing, microscopic analysis and numerical simulation. The work will be carried out between the LISN laboratory of the CEA and the materials center of the Ecole des Mines de Paris.
Study of the dynamics of molten salt fast reactors under natural convection conditions
Molten Salt Reactors (MSRs) are presented as inherently stable systems with respect to reactivity perturbations, due to the strong coupling between salt temperature and nuclear power, leading to a homeostatic behavior of the reactor. However, although MSRs offer interesting safety characteristics, the limited operational experience available restricts our knowledge of their dynamic behavior.
This research work aims to contribute to the development of a methodology for analyzing the dynamics of MSRs, with the goal of characterizing complex neutron-thermohydraulic coupling phenomena in an MSR operating in natural convection, identifying potentially unstable transient sequences, prioritizing the physical phenomena that cause these instabilities, and proposing simple physical models of these phenomena.
This work will contribute to the development of a safety-oriented methodology that will help MSR designers better understand and model the reactor dynamic behavior during transients, through dimensional analysis and the study of the flow stability. This methodology aims to define simple and robust criteria to ensure the intrinsic safety of a fast-spectrum MSR, depending on its design and operational parameters allowing compliance with the operating domain limits.
This PhD lies at the crossroad of theoretical analysis of the physical phenomena governing the MSR’s behavior, particularly the study of unstable regimes (oscillatory or divergent in nature) due to neutron-thermohydraulic coupling under natural convection conditions, and the development of analytical and numerical tools for conducting calculations to characterize these phenomena.
The PhD student will be based within a research unit dedicated to innovative nuclear systems. He/she will develop skills in MSR modelling and safety analysis, and will have the opportunity to present his/her work to the international MSR research community.