Validation of a Model-Free Data Driven Identification approach for ductile fracture behavior modeling
This research proposes a shift from traditional constitutive modeling to a Data-Driven Computational Mechanics (DDCM) framework which has been recently introduced [1]. Instead of relying on complex constitutive equations, this approach utilizes a database of strain-stress states to model material behavior. The algorithm minimizes the distance between calculated mechanical states and database entries, ensuring compliance with equilibrium and compatibility conditions. This new paradigm aims to overcome the uncertainties and empirical challenges associated with conventional methods.
As a corollary tool for simulations DDCM, Data-Driven Identification (DDI) has emerged as a powerful standalone method for identifying material stress responses [2, 3]. It operates with minimal assumptions about while being model-free, this making it particularly suitable for calibrating complex models commonly used in industry.
Key objectives of this research include adapting DDCM strategies for plasticity [4] and fracture [5], enhancing DDI for high-performance computing, and evaluating constitutive equations. The proposed methodology involves collecting full-field measurement maps from an heterogeneous test, utilizing High-Speed cameras and Digital Image Correlation. It will adapt DDCM for ductile fracture scenarios, implement a DDI solver in a high-performance computing framework, and conduct an assessment of a legacy constitutive model without uncertainties. The focus will be on 316L steel, a material widely used in nuclear engineering.
This thesis is the result of a collaboration between several labs at CEA ans Centrale Nantes which are prominent in computational and experimental mechanics, applied mathematics, software engineering and signal processing.
[1] Kirchdoerfer, Trenton, and Michael Ortiz. "Data-driven computational mechanics." Computer Methods in Applied Mechanics and Engineering 304 (2016): 81-101.
[2] Leygue, Adrien, et al. "Data-based derivation of material response." Computer Methods in Applied Mechanics and Engineering 331 (2018): 184-196.
[3] Dalémat, Marie, et al. "Measuring stress field without constitutive equation." Mechanics of Materials 136 (2019): 103087.
[4] Pham D. et al, Tangent space Data Driven framework for elasto-plastic material behaviors, Finite Elements in Analysis and Design, Volume 216, 2023, https://doi.org/10.1016/j.finel.2022.103895.
[5] P. Carrara, L. De Lorenzis, L. Stainier, M. Ortiz, Data-driven fracture mechanics, Computer Methods in Applied Mechanics and Engineering, Volume 372, 2020, https://doi.org/10.1016/j.cma.2020.113390.
Activated conductive materials for energy conversion and energy storage through capacitive effect
Energy production from renewable sources requires efficient storage systems to address imbalances between supply and demand. This project aims to develop cost-effective supercapacitors using composite electrodes derived from industrial by-products. Mineral binders, such as geopolymers or Alkali Activated Materials (AAM), made conductive by dispersing carbon black, are being studied for energy storage or heat generation applications. Based on a recently filed patent, we propose a detailed study of these conductive composites. Their performance will be evaluated depending on formulation and shaping parameters. Additionally, the porous network and the dispersion of conductive charges in the material will be thoroughly characterized. Finally, material shaping tests will be conducted, and supercapacitors will be assembled to study the impact of the process (3D printing) and geometries.
Study of the synthesis and thermodynamic properties of the (An,Zr)O2 and (Zr,An)SiO4 compounds
In the event of a serious nuclear accident, the fuel in the reactor core may melt, resulting in the formation of a compound known as corium. Cases of major accidents and prototypical corium formation experiments have identified the formation of key compounds such as mixed oxides (U,Zr)O2 formed by interaction of the fuel with the zircaloy cladding and silicates (Zr,U)SiO4 formed by interaction of the corium with structural materials. In the case of MOx, (U,Pu)O2 fuels, corium formation could lead to the formation of equivalent phases with significant plutonium contents. However, experimental thermodynamic data on such compounds, which would enable their behaviour to be assessed, are currently non-existent. In this context, determining the conditions for synthesising such compounds with a good degree of purity is essential for acquiring such data. The synthesis of (Zr,Pu)O2 and (Zr,Pu)SiO4 solid solutions is therefore an essential first step before studying (Zr,U,Pu)O2 and (Zr,U,Pu)SiO4 systems.
The aim of this PhD thesis will be to determine the conditions suitable for the synthesis of these compounds, to carry out a series of characterisations enabling their purity to be assessed and their thermodynamic properties to be established. To achieve this, experiments will be carried out on the ATALANTE facility and a multi-technique characterisation approach will be chosen (XRD, Raman and infrared spectroscopies, SEM, synchrotron characterisation techniques, etc.). Solubility tests in a controlled environment will then be set up and calorimetric measurements carried out as part of international collaborations.
Measurement and evaluation of the energy dependence of delayed neutron data from 239Pu
This PhD proposal aims to measure and characterize the delayed neutron emissions from the fission of 239Pu. This actinide is involved in various reactor concepts, and the nuclear data available remains insufficient, particularly with fast neutrons. The project has a strong experimental focus, with multiple measurement campaigns at the MONNET electrostatic accelerator from JRC Geel, in which the candidate will actively participate.
The first phase focuses on the intercomparison of the neutron flux measurement methods (dosimetry, fission chamber, long-counter detector and recoil proton scintillator) which will be confronted to Monte-Carlo simulations of neutron emission from charged particle interactions (D+T, D+D, p+T). This work will ensure proper neutron flux characterization, a crucial step for the project.
Next, the candidate will replicate the delayed neutron measurements for ²³8U using an existing target in order to verify the results from a 2023 experimental campaign.
Finally, the candidate will measure the delayed neutron yields and group abundances for ²³?Pu in a neutron energy range from 1 to 8 MeV. The objective is to produce an energy-dependent evaluation, integrated into an ENDF file, to be tested on reactor calculations (beta-eff, power transients, absorber efficiency calibration, etc.). These measurements will complement a thermal spectrum study conducted at ILL in 2022, forming a coherent model for ²³?Pu from 0 to 8 MeV.
This project will contribute to the OECD/NEA's JEFF-4 nuclear data file, addressing a strong demand from the nuclear industry (highlighted by the IAEA) to improve the precision of multiplicity measurements and delayed neutron kinetic parameters, thus enhancing reactor safety and reducing safety margins.
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.
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.
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).
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.