Generation of Cesium silicate micro-particles from Fukushima
Microscopic in size, but large in environmental impact, cesium microparticles hold one of the keys to understanding the Fukushima nuclear accident. Following the Fukushima Daiichi accident, these cesium-rich silicate glass microparticles (CSMP) were discovered in the environment, carrying a significant portion of the radioactivity. Very poorly soluble in water, they differ from those observed at Chernobyl. A previous thesis demonstrated that these CSMPs could be the result of the interaction between corium and concrete during a severe accident, via small-scale experiments. The study made it possible to reproduce similar particles, made of amorphous silica with crystalline nano-inclusions. However, the results need to be refined, particularly with regard to the presence of zinc and calcium. The proposed thesis aims to explore the physicochemical mechanisms leading to the synthesis of these CSMPs. Laboratory experiments will recreate the corium-concrete interaction conditions, representative of Fukushima, in order to optimize the compositions and improve the modeling of the releases of these particles in current severe accident assessment tools.
Development of multiscale and multiview correlation techniques for monitoring large-scale dynamic tests
Experimental data obtained on large-scale specimens plays an important role in the study of structural integrity. Detailed interpretations of these tests require extensive instrumentation of the models. In addition to conventional data acquisition systems, digital image correlation (DIC) techniques can be used to measure displacement fields and extract quantities of interest (e.g. damage field). The aim of this thesis is to develop a multi-view, multi-scale digital image correlation (DI2M) technique for monitoring large-scale dynamic tests. We will focus on the behavior of reinforced concrete structures subjected to dynamic loading. The finite element model updating (FEMU) technique will be used to identify non-linear phenomena in the process zone around cracks. FEMU will be coupled with DI2M analyses, which can also be used to measure boundary conditions. The use of DI techniques to calculate acceleration fields will also be studied. A numerical framework will be proposed for performing modal analysis based on calculated fields. Ultimately, these tools could be integrated into a test/calculation dialogue procedure, providing precise information on the mechanical properties of structural elements and their evolution (e.g. damage) induced by seismic loading.
Experimental and theoretical studies of the fission fragment excitation energy and angular momentum generation
The discovery of nuclear fission in 1939 profoundly changed our understanding of nuclear physics. The fission reaction is the splitting of heavy nuclei, such as uranium 235, into two lighter nuclei, together with the release of a large amount of energy. Many years of research have led to the development of nuclear fission models, from which evaluated nuclear data files are derived. These files are essential inputs to reactor simulations; yet, their quality needs to be improved.
This PhD thesis aims to study the generation of angular momentum and the excitation energy of fission fragments from both experimental and theoretical standpoints. These studies will not only improve our understanding of the underlying process and our models, but also enhance the predictive power of simulation tools, particularly those used to predict gamma heating in reactors. Part of the work will involve finalizing the analysis of data acquired as part of a recent thesis. The student will take part in complementary experimental campaigns at the nuclear reactor of the Institut Laue-Langevin (ILL), using the LOHENGRIN spectrometer to measure isomeric ratios and the kinetic energy distributions of fission fragments.
The doctoral student will be based in a nuclear and reactor physics unit. He/she will develop skills in nuclear physics, data analysis, and computer programming. The programming languages used will be C++ and Python. Professional perspectives include academic research, R&D organisations, nuclear industry, and possibly also data scientist positions.
Analyzis and modelling of ions-catalyst-ionomer interactions in an AEM electrolyzer cell
CEA/Liten is a research organization on new energies. It offers a PhD on the production of green hydrogen by electrolysis of water using a new technology. The 3 types of water electrolysis to produce hydrogen from electricity are: high temperature electrolysis, low temperature alkaline electrolysis, low temperature PEM electrolysis (proton exchange membrane). All these types of electrolysis have their advantages and disadvantages. Very recently, a new type of electrolysis was born: low temperature electrolysis with AEM membrane (OH- anion exchange). It is a compromise between PEM and alkaline electrolysis to benefit from the advantages of these 2 technologies. First prototypes of such a device exist at the CEA and are studied at the cell or stack scale but the mechanisms involved in the electrochemical and chemical reactions at smaller scales within the electrodes are still poorly understood. In particular, the interactions (ion exchanges, ionic potentials) between the ionomer of the active layer, the membrane and the solution of water and diluted KOH are poorly understood. The objective of the thesis is 1/ to study these mechanisms and to quantify them by developing elementary experiments then, 2/ to model them and implement these models in an existing in-house electrolyzer code and finally 3/ to simulate polarization curves to validate all the models of the code including those developed by the doctoral student.
This thesis will span 2 laboratories: an experimental laboratory and a simulation laboratory in which the student will find all the skills necessary to achieve these objectives. This thesis is linked to several projects involving people from the CEA and other French university laboratories. The student will therefore be in a working environment where this theme is booming.
The candidate is required to have good knowledge of electrochemistry and polymer chemistry and to have notions of modeling and use of software such as Comsol.
Modeling of Critical Heat Flux Using Lattice Boltzmann Methods: Application to the Experimental Devices of the RJH
The Lattice Boltzmann Methods (LBM) are numerical techniques used to simulate transport phenomena in complex systems. They allow for the modeling of fluid behavior in terms of particles that move on a discrete grid (a "lattice"). Unlike classical methods, which directly solve the differential equations of fluids, LBM simulates the evolution of distribution functions of fluid particles in a discrete space, using propagation and collision rules. The choice of the lattice in LBM is a crucial step, as it directly affects the accuracy, efficiency, and stability of the simulations. The lattice determines how fluid particles interact and move within space, as well as how the discretization of space and time is performed.
LBM methods exhibit natural parallelism properties, as calculations at each grid point are relatively independent. Although classical CFD methods based on the solution of the Navier-Stokes equations can also be parallelized, the nonlinear terms can make parallelism more difficult to manage, especially for models involving turbulent flows or irregular meshes. Therefore, LBM methods allow, at a lower computational cost, to capture complex phenomena. Recent work has shown that it is possible, with LBM, to reproduce the Nukiyama cooling curve (boiling in a vessel) and thus accurately calculate the critical heat flux. This flux corresponds to a mass boiling, known as the boiling crisis, which results in a sudden degradation of heat transfer.
The critical heat flux is a crucial issue for the Jules Horowitz Reactor, as experimental devices (DEX) are cooled by water in either natural or forced convection. Therefore, to ensure proper cooling of the DEX and the safety of the reactor, it is essential to ensure that, within the studied parameter range, the critical heat flux is not reached. It must therefore be determined with precision.
In the first part of the study, the student will define a lattice to apply LBM methods on an RJH device in natural convection. The student will then consolidate the results by comparing them with available data. Finally, exploratory calculations in forced convection (from laminar to turbulent flow) will be conducted.
Portable GPU-based parallel algorithms for nuclear fuel simulation on exascale supercomputers
In a context where the standards of high performance computing (HPC) keep evolving, the design of supercomputers includes always more frequently a growing number of accelerators or graphics processing units (GPUs) that provide the bulk of the computing power in most supercomputers. Due to their architectural departures from CPUs and still-evolving software environments, GPUs pose profound programming challenges. GPUs use massive fine-grained parallelism, and thus programmers must rewrite their algorithms and code in order to effectively utilize the compute power.
CEA has developed PLEIADES, a computing platform devoted to simulating nuclear fuel behavior, from its manufacture all the way to its exploitation in reactors and its storage. PLEIADES can count on an MPI distributed memory parallelization allowing simulations to run on several hundred cores and it meets the needs of CEA's partners EDF and Framatome. Porting PLEIADES to use the most recent computing infrastructures is nevertheless essential. In particular providing a flexible, portable and high-performance solution for simulations on supercomputers equipped with GPUs is of major interest in order to capture ever more complex physics on simulations involving ever larger computational domains.
Within such a context the present thesis aims at developing and evaluating different strategies for porting computational kernels to GPUs and at using dynamic load balancing methods tailored to current and upcoming GPU-based supercomputers. The candidate will rely on the tools developed at CEA such as the thermo-mechanical solver MFEM-MGIS [1,2] or MANTA [3]. The software solutions and parallel algorithms proposed with this thesis will eventually enable large 3D multi-physics modeling calculations of the behavior of fuel rods on supercomputers comprising thousands of computing cores and GPUs.
The candidate will work at the PLEIADES Fuel Scientific Computing Tools Development Laboratory (LDOP) of the department for fuel studies (DEC - IRESNE, CEA Cadarache). They will be brought to evolve in a multidisciplinary team composed of mathematicians, physicists, mechanicians and computer scientists. Ultimately, the contributions of the thesis aim at enriching the computing platform for nuclear fuel simulations PLEIADES.
References :[1] MFEM-MGIS - https://thelfer.github.io/mfem-mgis/[2]; Th. Helfer, G. Latu. « MFEM-MGIS-MFRONT, a HPC mini-application targeting nonlinear thermo-mechanical simulations of nuclear fuels at mesoscale ». IAEA Technical Meeting on the Development and Application of Open-Source Modelling and Simulation Tools for Nuclear Reactors, June 2022, https://conferences.iaea.org/event/247/contributions/20551/attachments/10969/16119/Abstract_Latu.docx, https://conferences.iaea.org/event/247/contributions/20551/attachments/10969/19938/Latu_G_ONCORE.pdf; [3] O. Jamond et al. «MANTA : un code HPC généraliste pour la simulation de problèmes complexes en mécanique », https://hal.science/hal-03688160
Study of the amorphous intermediate states during the precipitation of actinides oxalate
Growing energy needs and the climate emergency require a rapid transition to completely carbon-free energy, by mixing renewable energies and sustainable nuclear power. In this context, the precipitation of plutonium and uranium in the form of oxalate constitutes a key step in the industrial process of recycling spent fuel. A detailed understanding of the crystallization mechanisms of these oxalates thus constitutes a major challenge for better management of these operations.
However, it is now widely accepted that ions in solution assemble into crystals via a series of non-crystalline transient states, which fundamentally contradicts all classical nucleation theories used in precipitation models. In particular, we have demonstrated in recent years that rare earth oxalate crystals (Eu, Nd, Ce, Tb), some used to experimentally simulate the recycling of uranium and plutonium, form via liquid, reagent-rich nanodroplets which separate from the aqueous solvent. This behavior modifies the view hitherto retained for the precipitation of these oxalates and leads us to question the behavior of actinide oxalates.
The aim of this thesis is to confirm or refute that transient mineral droplets also form during the formation of uranium and plutonium oxalates, and to determine whether crystallization transients impact the precipitation models used to calibrate the recycling process of nuclear fuel. This study will not only impact precipitation processes used in recycling, but will also advance a fundamental question about long-debated “non-classical” crystallization.
Thermomechanical behaviour at high temperature of an irradiated nuclear ceramic
This thesis is part of the studies on pellet-cladding interactions in nuclear fuel rods used in NPP. The operator must ensure and demonstrate the integrity of rods in any situations. The mechanical stresses on the clad, the first safety barrier, are linked to the viscoplastic properties of the fuel. It is therefore necessary to know these behaviors and their evolution in operation.
The topic proposed will focuse on the characterization, in hot lab, of an irradiated fuel. One of the main difficulties is that the irradiated fuels in a reactor are multi-cracked, which makes their mechanical characterization particularly complex. However, an ongoing thesis (2022-25) has reached different steps: (i) the design of a specific thermomechanical testing machine, (ii) the partial qualification of this device, (iii) the implementation of tools and cracked sample extraction method, (iv) and a whole system model (digital twin).
The thesis will be the continuation of this work and will be built in four stages on three experimental platforms available at the CEA:
1. Getting the knowledge and improving existing digital and experimental tools,
2. Implementation of the device in hot-cell on an existing furnace,
3. Thermomechanical testing on irradiated fuel, a world first time in these conditions.
The tests will require dedicated post-processing based on simulation-experiments comparisons. Once the experimental base is sufficiently developed and interpreted, it will then be possible to confirm or revise the irradiated fuel behaviour laws. A link with the microstructure of materials could be addressed.
Throughout these stages, the PhD student will draw on skills and expertise of laboratories of the Fuel Research Department (IRESNE Institute, CEA Cadarache) and on a academic collaboration. This thesis also fits into the framework of the European project OPERA HPC and is a major issue.
The PhD student should have a strong taste for the experimental approach and some facilities for the use of digital tools. Knowledge of materials science is the minimum required. During the three years, the PhD student will improve his multiphysical skills in experimental device design and high-temperature material behavior, as well as in numerical simulation, which will facilitate his professional integration.
Bottom-up study of Ionic Transport in Unsaturated Hierarchical Nanoporous Materials : application to cement-based materials
Ion transport is critical in determining the durability of cement-based materials and, therefore, the extension of service life of concrete (infra)structures. Transport phenomena determine the containment capacity of concrete, which is crucial in the design and asset management of concrete infrastructures for energy production. Under most service conditions, concrete exists in unsaturated conditions. Anomalous transport has been associated with cement-based materials, and the reasons behind such deviations from the expected behavior of other porous materials may stem from nanoscale processes.
Research efforts have aimed to correlating material composition and microstructure to transport properties and durability. However, to date, the majority of predictive modeling of durability does not explicitly account for nanoscale processes, which are fundamental in determining transport properties. Recent advances have been made in quantifying the behavior of confined water in various phases present in cement systems. Calcium silicate hydrates (C-S-H) are the main hydrated phase in cement-based materials and present nanopores in the micro and mesopore range. The effects of desaturation remain however to be fully worked out. A fundamental understanding of transport processes requires a multiscale framework in which information from the molecular scale reverberates across other relevant scales (in particular, the mesoscale associated with C-S-H gel porosity (~nm), capillary porosity, and interfacial transition zone (~µm) up to the macroscopic scale of industrial application in cement-based materials).
The goal of this PhD work is to evaluate the ionic transport of chlorides, a critical species for the durability of concrete, under non-saturated conditions by combining small-scale simulations, multiscale modelling and experimentation in a bottom-up approach. The work will focus on the C-S-H. The project aims to characterize the effects of desaturation on the nanoscale processes driving transport of chlorides.
Modeling of complexation equilibria of actinides in nitric medium. Application to the PUREX process
The PAREX+ code is a major tool in the field of separation chemistry. It allows for the modelling and simulation of separation processes base on solvent extraction. In this code, the distribution of interest species between the aqueous and organic phases is calculated at every point in the process, both in steady and transitory states. The aim of this thesis is to improve this distribution model. To achieve this, a better understanding of the phenomena involved in the organic and aqueous phases is necessary, as well as a new approach to incorporate them into the model. This thesis thus combines experimental work and modeling. The student will join a supervisory team composed of experts in separation chemistry and modeling. His work will be valued through the publication of papers and participation in international conferences. At the end of this thesis, the student will have solid knowledge in the field of solvent extraction and its modeling, which he can leverage with industry or research organizations in the nuclear field or in other areas of separation chemistry (separation of rare earths or hydrometallurgy).