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.
Interfacial friction modelling for rod bundle geometry in thermohydraulic system code CATHARE
The thermohydraulic system code CATHARE, developed in CEA with EDF, Framatome and ASNR, permits to simulate normal and accidental behaviours of the hydraulic circuit of a Pressured Water Reactor (PWR). This code is used as a reference in France for transient simulation in nuclear reactor, and is especially used as a support for licensing by EDF and Framatome.
Former studies show the need to improve the validity of the interfacial friction modelling for rod bundle geometry at low pressure or for high hydraulic diameter conditions. Moreover, the current interfacial friction modelling for rod bundle geometry is based on numerous simplifications and a calibration against some steam-water at high temperature experimental data. A new interfacial friction model for Cathare could nowadays be developed using more comprehensive models found in the literature and be calibrated against a larger experimental database
This doctorate aims to improve the interfacial friction modelling for rod bundle geometry by studying the physical phenomena associated with this problem. This work will help implement a more comprehensive model in the CATHARE code following the thesis, thus extending the code's validity conditions to new applications.
Modeling of Wall Condensation Phenomena and Liquid Film Interactions
In this thesis, we focus on modeling mass and energy transfer associated with wall condensation in a turbulent flow of a vapor–noncondensable gas mixture. The flow is two-phase and turbulent, where forced, mixed, and natural convection modes may occur. The framework of this work relies on the RANS approach applied to the compressible Navier–Stokes equations, in which wall condensation is described using semi-analytical wall functions developed in a previous doctoral study cite{iziquel2023}. These functions account for the different convection modes as well as suction and species interdiffusion effects, but neglect the presence of a liquid film.
In the literature, the influence of film formation and flow on mass and heat transfer is often neglected, since it is generally assumed that, in the presence of noncondensable gases, the resistance of the gaseous layer to vapor diffusion is much greater than the thermal resistance of the liquid film.
The objective of this thesis is to improve the prediction of heat and mass transfer by investigating, beyond the thermal resistance of the condensate, the dynamic effect of the liquid and its interaction with the gaseous diffusion layer during wall condensation. The study will first consider laminar film flow, and then attempt to extend the analysis to the turbulent regime.
In the gas phase, the wall-function model developed in cite{iziquel2023} for a binary mixture of vapor and a single noncondensable gas will be extended to mixtures of vapor and $n>1$ noncondensable gases (N2, H2, …), in order to address hydrogen risk issues.
The validation of the implemented models will be carried out using results from separate-effect (SET) and coupled-effect (CET) experiments available in the literature (Huhtiniemi cite{huhti89}, COPAIN, ISP47-MISTRA, ISP47-TOSQAN, RIVA). Comparisons at the CFD scale, using wall functions for condensation neglecting the film, will be performed on benchmark cases from the literature and condensation experiments (COPAIN) to assess the impact of this assumption as well as the improvement provided by the new model in terms of accuracy and computational cost.
Design of asynchronous algorithms for solving the neutron transport equation on massively parallel and heterogeneous architectures
This PhD thesis work aims at designing an efficient solver for the solution to the neutron transport equation in Cartesian and hexagonal geometries for heterogeneous and massively parallel architectures. This goal can be achieved with the design of optimal algorithms with parallel and asynchronous programming models.
The industrial framework for this work is in solving the Boltzmann equation associated to the transportof neutrons in a nuclear reactor core. At present, more and more modern simulation codes employ an upwind discontinuous Galerkin finite element scheme for Cartesian and hexagonal meshes of the required domain.This work extends previous research which have been carried out recently to explore the solving step ondistributed computing architectures which we have not yet tackled in our context. It will require the cou-pling of algorithmic and numerical strategies along with programming model which allows an asynchronousparallelism framework to solve the transport equation efficiently.
This research work will be part of the numerical simulation of nuclear reactors. These multiphysics computations are very expensive as they require time-dependent neutron transport calculations for the severe power excursions for instance. The strategy proposed in this research endeavour will decrease thecomputational burden and time for a given accuracy, and coupled to a massively parallel and asynchronousmodel, may define an efficient neutronic solver for multiphysics applications.
Through this PhD research work, the candidate will be able to apply for research vacancies in highperformance numerical simulation for complex physical problems.
One-sided communication mechanisms for data decomposition in Monte Carlo particle transport applications
In the context of a Monte Carlo calculation for the evolution of a PWR (pressurized water reactor) core, it is necessary to compute a very large number of neutron-nucleus reaction rates, involving a data volume that can exceed the memory capacity of a compute node on current supercomputers. Within the Tripoli-5 framework, distributed memory architectures have been identified as targets for high-performance computing deployment. To leverage such architectures, data decomposition approaches must be used, particularly for reaction rates. However, with a classical parallelization method, processes have no particular affinity for the rates they host locally; on the contrary, each rate receives contributions uniformly from all processes. Access to decomposed data can be costly when it requires intensive use of communications. Nevertheless, one-sided communication mechanisms, such as MPI RMA (Message Passing Interface, Remote Memory Access), make these accesses easier both in terms of expression and performance.
The objective of this thesis is to propose a method for partial data decomposition relying on one-sided communication mechanisms to access remotely stored data, such as reaction rates. Such an approach will significantly reduce the volume of data stored in memory on each compute node without causing a significant degradation in performance.
HPC two-phase simulations with lattice Boltzmann methods and adaptative mesh refinement
CEA/STMF develops computational fluid dynamics (CFD) codes in thermohydraulics that aim to quantify mass and energy transfers in nuclear cycle systems such as reactors and management devices of radioactive wastes. This thesis focuses on Lattice Boltzmann Methods (LBM) adapted to Adaptive Mesh Refinement (AMR) inside a generic computing environment based on Kokkos and executable on multi-GPU supercomputers. The proposed work consists in developing LB methods in the Kalypsso-lbm code to simulate coupled partial differential equations (PDEs) modelling incompressible two-phase and multi-component flows such as those encountered in downstream cycle devices. Once the developments have been completed, they will be validated with reference solutions. They will allow a comparison of various interpolation methods between blocks of different sizes in the AMR mesh. A discussion will be held on the refinement and de-refinement criteria that will be generalized for these new PDEs. Finally, benchamrks of performance will quantify the contribution of AMR for 3D simulations when the reference simulation is performed on a static and uniform mesh. This work will use supercomputers which are already operational (e.g., Topaze-A100 from CEA-CCRT), as well as the future exascale supercomputer Alice Recoque depending on the progress of its installation.
Novel architecture and signal processing for mobile optical telecommunications
Free-Space Optical Communications (FSO) rely on transmitting data via light between two distant points, eliminating the need for fibers or cables. This approach is particularly valuable when wired connections are impractical or prohibitively expensive.
However, these links are highly susceptible to atmospheric conditions—fog, rain, dust, and thermal turbulence—which attenuate or distort the light beam, significantly degrading communication quality. Current solutions remain costly and limited, both in terms of optical compensation hardware and signal processing algorithms.
Within this framework, the thesis aims to design high-performance, robust mobile optical links capable of adapting to dynamic and disturbed environments. The study will focus on leveraging Silicon-based Optical Phased Arrays (OPAs)—a technology derived from low-cost LiDAR systems—offering a promising path toward compact, integrated, and cost-effective architectures.
The primary focus of the research will be developing advanced algorithmic approaches for signal processing and compensation. The PhD candidate will be tasked with designing a dedicated simulation environment to evaluate and validate architectural choices and algorithmic strategies before practical experimentation.
The overarching goal is to propose an integrated, flexible, and reliable architecture that ensures uninterrupted optical communication in motion, with potential applications in aerospace, space, and terrestrial domains.
Modeling the impact of defects in Steel–Concrete Structures. Identification of critical defects through metamodeling and optimization algorithms
To meet growing constructability challenges, steel–concrete (SC) structures are emerging as a promising alternative to conventional reinforced concrete structures. These elements are composed of infill concrete, two external steel plates, and steel shear studs that ensure composite action. While such structures present a clear interest due to their overall mechanical behavior, the presence of the steel plates prevents visual inspection of the concrete casting quality. It is therefore essential to characterize the impact of possible defects. This is the context of the proposed PhD research. Building upon recent results obtained in the laboratory, the goal is to develop a numerical framework to account for defects in steel–concrete structures. The thesis will be structured in several stages: validation of a modeling strategy for the mechanical behavior of defect-free SC structures, introduction of defects in the simulations and assessment of the applicability of the numerical approach, development of a metamodel and sensitivity analysis, and identification of critical defect configurations through optimization algorithms. One of the operational objectives of this doctoral work is to provide a tool capable of identifying critical defect configurations (size, position, and number) with respect to a given target quantity of interest (such as loss of strength or reduction in average stiffness). The research will therefore rely on the use and further development of state-of-the-art numerical tools in the fields of finite element modeling, optimization techniques, sensitivity analysis, and metamodeling. The thesis will be carried out within a rich collaborative environment, notably in partnership with EDF.
Integrated optical functions on microbolometer focal planes for uncooled infrared imaging
Thermal infrared imaging (wavelengths 8-14 µm) is a growing field, particularly in industry, transportation, and environment. It relies on a detection technology, microbolometers, for which CEA-Leti is at the forefront of the global state of the art. Integrating advanced optical functions directly onto the detectors is a very promising approach for improving performance, compactness, and cost in future infrared cameras.
The optical functions under consideration include spectral filtering, polarimetry, wavefront correction, and more. Some aim to enrich the image with information essential for applications such as absolute thermography (temperature and emissivity measurement), identification for automated scene interpretation (machine vision), gas detection, and others.
The proposed work will include the design, fabrication, and electro-optical characterization of functionalized microbolometer arrays. Using 3D electromagnetic simulation tools, the design of these optical functions will take into account the compatibility with our microbolometer technologies and the capabilities of our microfabrication facilities. Fabrication will take place in the CEA-Leti cleanrooms by dedicated personnel, but the candidate will participate in defining and monitoring the work. Finally, optical and electro-optical characterizations will be performed in our laboratory, if necessary with the development of dedicated characterization benches.
Effects of structural heterogeneities on air flow through reinforced concrete walls
The containment building represents the third barrier to confinement in nuclear power plants. Its role is to protect the environment in the event of a hypothetical accident by limiting releases to the outside. Its function is therefore closely linked to its tightness. Traditionally, the estimation of the leakage rate is based on a sound knowledge of transfer properties (such as permeability), combined with a chained (thermo-)hydro-mechanical simulation approach. While the mechanical behavior of the structure is now broadly well understood, progress is still needed in the comprehension and quantification of fluid flow. This is particularly true in the presence of heterogeneities (cracks, honeycombs, construction joints, reinforcements, cables, etc.), which represent situations that can locally disturb permeability. This is the context of the present PhD topic.
The work will consist, through a methodology combining experimental testing and numerical simulation, in improving the representation of fluid flow by explicitly accounting for the impact of heterogeneities. An initial analysis will define an experimental plan, which will then be carried out. The results will be analyzed in order to empirically characterize the influence of each type of heterogeneity tested on transfer properties. A simulation approach, exploiting the experimental findings, will then be developed using finite element and discrete methods. Finally, the applicability of the methodology to a real-scale structure will be assessed, while explicitly accounting for uncertainties regarding the presence and impact of such heterogeneities (probabilistic approach).The PhD will therefore rely on state-of-the-art experimental and numerical tools and methods, and will be conducted in a rich collaborative context (CEA, ASNR, EDF).