Scaling Up Dislocation Dynamics Simulations for the Study of Nuclear Material Aging
Materials used in nuclear energy production systems are subjected to mechanical, thermal, and irradiation condition, leading to a progressive evolution of their mechanical properties. Understanding and modeling the underlying physical mechanisms involved is a significant challenge.
Dislocation Dynamics simulation aims to understand the behavior of the material at the crystal scale by explicitly simulating the interactions between dislocations, microstructure, and crystal defects induced by irradiation. The CEA, CNRS, and INRIA have been developing the NUMODIS calculation code for this purpose since 2007 (Etcheverry 2015, Blanchard 2017, Durocher 2018).
More specific work on zirconium alloys (Drouet 2014, Gaumé 2017, Noirot 2025) has allowed the validation and enhancement of NUMODIS's ability to handle these individual physical mechanisms by directly comparing them with experiments, through in situ tensile tests under a transmission electron microscope. However, these studies are limited by the current inability of the NUMODIS code to handle a sufficiently high and representative number of defects, and thus to obtain the mechanical behavior of the grain (~10 microns).
The objective of the proposed work is to implement new algorithms to extend the functionalities of the code, propose and test new numerical algorithms, parallelize certain parts still processed sequentially, and ultimately demonstrate the code's ability to simulate the deformation channeling mechanism in an irradiated zirconium grain.
The work will focus primarily on algorithms for calculating velocities, junction formation, and time integration, requiring both mastery of dislocation physics and the corresponding numerical methods. Algorithms for integration recently proposed by Stanford University and LLNL will be implemented and tested for this purpose.
Significant work will also be devoted to adapting the current code (hybrid MPI-OpenMP parallelism) to new computing machines that favor GPU processors, through the adoption of the Kokkos programming model.
Building on both previous experimental and numerical work, this study will conclude with the demonstration of NUMODIS's ability to simulate the channeling mechanism in an irradiated zirconium grain and to identify or even model the main physical and mechanical parameters involved.
At the interface between several fields, the candidate must have a good foundation in physics and/or mechanics, while being comfortable with programming and numerical analysis.
References:
1. Etcheverry Arnaud, Simulation de la dynamique des dislocations à très grande échelle, Université Bordeaux I (2015).
2. Blanchard, Pierre, Algorithmes hiérarchiques rapides pour l’approximation de rang faible des matrices, applications à la physique des matériaux, la géostatistique et l’analyse de données, Université Bordeaux I (2017).
3. Durocher, Arnaud, Simulations massives de dynamique des dislocations : fiabilité et performances sur architectures parallèles et distribuées (2018).
4. Drouet, Julie, Étude expérimentale et modélisation numérique du comportement plastique
des alliages de zirconium sous et après irradiation (2014).
5. Gaumé, Marine, Étude des mécanismes de déformation des alliages de zirconium
après et sous irradiation (2017).
6. Noirot, Pascal, Etude expérimentale et simulation numérique, à l'échelle nanométrique et en temps réel, des mécanismes de déformation des alliages de zirconium après irradiation (2025).
Rheology and Conduction of Functional Polymers for Embedded Electronics in 3D/4D Additive Manufacturing
This PhD project, conducted on the MAPP platform (CEA-Metz), focuses on the development of additive manufacturing (3D/4D) processes for the integration of smart materials. The aim is to overcome the limitations of traditional planar electronic architectures (PCBs, wafers) integration by enabling the direct-to-shape printing of electronic functions within 3D parts performed by Fused Deposition Modeling and Paste Extrusion Modeling. The research will address functional conductive polymers, composed of an organic matrix and metallic particles, whose conduction mechanisms (direct contact, tunneling effect, ionic conduction) are governed by the percolation threshold. The study will investigate the processing of these materials, their rheological and electrical behavior, and the exploitation of their resistive, piezoresistive and piezoelectric properties to design novel sensor (3D) and actuator (4D) functions. The doctoral candidate will benefit from advanced characterization facilities and the guidance of a multidisciplinary team with expertise in additive manufacturing, materials science, and microelectronics.
Synthesis of organic aerogels from polydicyclopentadiene derivatives
The study of inertial confinement fusion of the deuterium + tritium (DT) mixture has long been a research focus at the CEA. Experiments related to this topic, carried out within the Laser Mégajoule (LMJ) facility, require the use of materials with specific properties. This includes, among others, polymer foams (organic aerogels) used as pre-ignition targets. Such materials must combine very low density with sufficient mechanical strength to be compatible with the preparation process employed.
In this context, the objective is to develop CHx polymeric aerogels based on polydicyclopentadiene (pDCPD) and other polymers derived from ring-opening metathesis polymerization (ROMP), in order to produce materials that are (i) of low apparent density (target value in the project: below 50 mg/cc), (ii) homogeneous, (iii) exhibiting fine (open) nano-porosity, and (iv) machinable.
The proposed PhD work would focus on three main areas:
1. The synthesis of new (co-)monomers
2. The preparation of organic aerogels
3. The exploitation of data using AI (opportunity)
Effect of water radiolysis on the hydrogen absorption flux by austenitic stainless steels in the core of a nuclear pressurized water reactor
In pressurized water nuclear reactors, the core components are exposed to both corrosion in the primary medium, pressurized water at around 150 bar and 300°C, and to neutron flux. The stainless steels in the core are damaged by a combination of neutron bombardment and corrosion. In addition, radiolysis of the water can have an impact on the mechanisms and kinetics of corrosion, the reactivity of the medium and, a priori, the mechanisms and kinetics of hydrogen absorption by these materials. This last point, which remains unexplored, may prove problematic, as hydrogen in solid solution in steel can lead to changes in (and degradation of) the mechanical properties of the steel or induce premature cracking of the part. The pioneering work developed in this highly experimental thesis will focus on the impact of radiolysis phenomena on the mechanisms and kinetics of corrosion and, above all, hydrogen pick-up in 316L stainless steel exposed to the primary environment under irradiation. Hydrogen will be traced by deuterium, and neutron irradiation simulated by electron irradiation on particle accelerators. An existing permeation cell will be modified into a unique setup to allow in operando measurement by mass spectrometry of the deuterium permeation flux through a sample exposed to the simulated primary water under radiolysis conditions. The distribution of hydrogen in the material, as well as the nature of the oxide layers formed, will be analysed in detail using state-of-the-art techniques available at the CEA and in partner laboratories. The doctoral student will ultimately be required to (i) identify the mechanisms involved (corrosion and hydrogen entry), (ii) estimate their kinetics and (iii) model the evolution of hydrogen flux in the steel in connection with radiolysis activity.
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.
Understanding the mechanisms of oxidative dissolution of (U,Pu)O2 in the presence of Ag(II) generated by ozonation
The recycling of plutonium contained in MOx fuels, composed of mixed uranium and plutonium oxides (U,Pu)O2, relies on a key step: the complete dissolution of plutonium dioxide (PuO2). However, PuO2 is known to dissolve only with great difficulty in the concentrated nitric acid used in industrial processes. The addition of a strongly oxidizing species such as silver(II) significantly enhances this dissolution step—this is the principle of oxidative dissolution. Ozone (O3) is used to continuously regenerate the Ag(II) oxidant in solution.
Although this process has demonstrated its efficiency, the mechanisms involved remain poorly understood and scarcely documented. A deeper understanding of these mechanisms is essential for any potential industrial implementation.
The aim of this PhD work is to gain insight into the interaction mechanisms within the HNO3/Ag/O3/(U,Pu)O2 system. The research will be based on a parametric experimental study of increasing complexity. First, the mechanisms of generation and consumption of Ag(II) will be investigated in the simpler HNO3/Ag/O3 system. In a second phase, the influence of various parameters on the oxidative dissolution of (U,Pu)O2 will be examined. The results will lead to the development of a kinetic model describing the dissolution process as a function of the studied parameters.
At the end of this PhD, the candidate—preferably with a background in physical chemistry—will have acquired advanced expertise in experimental techniques and kinetic modeling, providing a strong foundation for a career in academic research or industrial R&D, both within and beyond the nuclear sector.
From angstroms to microns: a nuclear fuel microstructure evolution model whose parameters are calculated at the atomic scale
Controlling the behavior of fission gases in nuclear fuel (uranium oxide) is an important industrial issue, as fission gas release or precipitation limit the use of fuels at extended burn-ups. The gas behavior is strongly influenced by the material’s microstructure evolution due to the aggregation of irradiation-induced defects (gas bubbles, dislocation loops and lines). Cluster dynamics (CD) (a kind of rate theory model) is relevant for modelling the nucleation/growth of the defect clusters, there gas content and the gas release. The current model has been parameterized following a multiscale approach, based on atomistic calculations (ab initio or empirical potentials). This model has been successfully applied to annealing experiments of UO2 samples implanted with rare gas atoms and has emphasized the impact of the irradiation damage on gas release. The aim of this PhD thesis is now to improve the model, particularly the damage parameterization, and to extend its validation domain through in depth comparison of simulation with a large set of recently obtained experimental results, such as gas release measurement by annealing of sample implanted in ion beam accelerator, bubble and loop observation by transmission electrons microscopy of implanted or in-pile irradiated samples. This global analysis will finally yield an improved parameterization of the CD model.
The research subject combines a “theoretical” dimension (improving the model) with an “experimental” one (interpreting existing experiments or designing some new ones). The variety of techniques will introduce you into the experimental world and thus broaden your scientific skills. You will be welcomed at the Fuel Behavior Modeling Laboratory (part of the Institute for Research on Nuclear Systems for Low-Carbon Energy Production, IRESNE, CEA Cadarache), where you will benefit from an open environment rich in academic collaborations. You also have to manage collaborations for the experiments analysis, for the model development and for the specification of additional atomistic calculations. You will be at the interface of atomistic techniques, large-scale simulation and various experimental techniques. Therefore, You will develop a broad view of irradiation effects in materials and of multi-scale modelling in solids in general.
This project is an opportunity to contribute to the overall development of numerical physics applied to multi-scale modeling of materials, occupying a pivotal position and adopting a global viewpoint. This will allow experiencing yourself the way computed fundamental microscopic data finally helps solving complex practical issues.
Further readings:
Skorek et al. (2012). Modelling Fission Gas Bubble Distribution in UO2. Defect and Diffusion Forum, 323–325, 209.
Bertolus et al. (2015). Linking atomic and mesoscopic scales for the modelling of the transport properties of uranium dioxide under irradiation. Journal of Nuclear Materials, 462, 475–495.
Theoretical studies of orbital current and their conversion mechnism for leveraging spin-orbit torques based devices performances
The proposed PhD thesis aims at understanding and identifying the key parameters governing the conversion of orbital moments into spin currents, with the goal of enhancing the write efficiency of spin-orbit torque magnetic random-access memory (SOT-MRAM) devices. The work will employ a multiscale modeling approach comprising ab initio, tight-binding and atomistic calculations of the Orbital Hall Effect (OHE) and Orbital Rashba-Edelstein Effect (OREE). These phenomena exhibit larger magnitudes and diffusion lengths compared to their spin counterparts, Spin Hall Effect (SHE) and Rashba-Edelstein Effect (REE). Furthermore, they are present in a broader range of materials, including low-resistivity light metals. This opens very interesting opportunities for more efficient and conductive materials, potentially lifting the barriers limiting the technological deployment of SOT-MRAM.
This thesis will play a key role in a close collaboration between SPINTEC and LETI laboratories at CEA. The PhD student will conduct ab initio calculations at SPINTEC to unveil fundamental material characteristics to exploit the described orbitronic phenomena, and will construct multi-orbital Hamiltonians at LETI to calculate orbital and spin transport, in strong interaction/synergy with experimentalists working on SOT-MRAM development. The PhD will be co-supervised by M. Chshiev, K. Garello at Spintec and J. Li at LETI. This PhD project will be at the heart of collaborations with leading theoretical and experimental groups at national and international level.
Highly motivated candidates with a strong background in solid-state physics, condensed matter theory, and numerical simulations are encouraged to apply. The selected candidate will perform calculations using Spintec’s computational cluster, leveraging first-principles DFT-based packages and other simulation tools. Results will be rigorously analyzed, with opportunities for publication in international peer-reviewed journals.
Thermodynamic and experimental approach of the reactivity in multi-constituted Silicon-Metal-Carbon systems for ceramic brazing
The development of ceramic-based material assemblies plays a fundamental role in technological innovation in many engineering fields. The choice of materials and joining process must ensure a functional, reliable and durable assembly, whose properties comply with the specifications of the application.
The PhD thesis is part of the development of brazing alloys optimized for the joining of ceramics (primarily silicon carbide) considered for various applications in harsh environments, particularly in the field of energy. Indeed, the design of these materials requires a good knowledge of the reactivity at the liquid alloy / ceramic interface. In this context, the thesis will contribute to the development of a thermodynamic and experimental approach to predict and understand the reactivity in multi-constituted Si-Metal-Carbon systems. This work includes a study of the wetting and interfacial reactivity of selected alloys (wetting and brazing experiments, fine characterization of the interfaces by different techniques such as FEG-SEM, X-ray diffraction, TEM, XPS) with the support of thermodynamic modelling using the CALPHAD method. This highly experimental work will be carried out in a dynamic and collaborative environment.
Ductile fracture of irradiated materials under cyclic loadings : Experimental characterization, modelling and numerical simulation
Metal alloys used in industrial applications most often have a ductile fracture mode involving nucleation, growth, and coalescence of internal cavities. The cavities appear as a result of the rupture of inclusions and grow under mechanical loading until they join together, leading to the failure of the structure. Resistance to crack initiation and propagation results from this mechanism. The prediction of toughness therefore requires the modeling of the plasticity of porous materials. The behavior of porous materials has been extensively studied from an experimental, theoretical, and numerical point of view in the case of monotonic mechanical loading under large deformations, leading to constitutive equations that can be used to simulate ductile fracture of structures. The case of cyclic mechanical loading and / or involving low levels of deformation / low number of cycles has been comparatively little studied, even though this type of loading is of interest in industrial applications, for example in the case of earthquakes. In this thesis, the effect of oligocyclic loading on ductile fracture properties will be investigated systematically from an experimental, theoretical, and
numerical point of view. Test campaigns will be carried out on various materials used in nuclear applications and under different mechanical stress conditions in order to quantify the effect of oligocyclic loading on fracture deformation and toughness. At the same time, numerical simulations will be performed to obtain an extensive database on the plastic behavior of porous materials under cyclic loading, with a particular focus on the effects of elasticity, porosity, mechanical loading, and spatial distribution of cavities. These numerical simulations will be used to validate analytical models developed during the thesis to predict the evolution of porosity and yield stress. Finally, the models will be implemented in the form of constitutive equations and used to simulate experimental tests.