Development of a multiphysics stochastic modelling for liquid scintillation measurements
The Bureau international des poids et mesures (BIPM) is developing a new transfer instrument named the "Extension of the International Reference System" (ESIR), based on the Triple-to-Double Coincidence Ratio (TDCR) method of liquid scintillation counting with a specific instrumentation comprising three photomultipliers. The aim is to enable international comparisons of pure beta radionuclides, certain radionuclides that decay by electron capture, and to facilitate international comparisons of alpha emitting radionuclides.
The TDCR method is a primary activity measurement technique used in national laboratories. For the activity determination, its application relies on the construction of a model of light emission requiring knowledge of the energy deposited in the liquid scintillator. Depending on the decay scheme, the combination of different deposited energies can be complex, particularly when it results from electronic rearrangement following electron capture decay. The stochastic approach of the RCTD model is applied by randomly sampling the different ionizing radiation emissions following a radioactive decay. The recent addition of modules for automatically reading nuclear data (such as those available in the Table des Radionucléides) in radiation/matter simulation codes (PENELOPE, GEANT4), means that all possible combinations can be rigorously taken into account. The stochastic approach makes it possible to consider the actual energy deposited in the liquid scintillation vial, taking into account interactions in the instrumentation as a whole.
The aim of this thesis is to develop a multiphysics stochastic approach using the GEANT4 radiation/matter simulation code, to be applied in particular to the BIPM's ESIR system. The choice of the Geant4 code offers the possibility of integrating the transport of ionizing particles and scintillation photons. This development is of great interest for radioactivity metrology, with the aim of ensuring metrological traceability to a larger number of radionuclides with the BIPM's ESIR system. The thesis will be carried out in collaboration with the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), which already has experience in developing a stochastic model with the GEANT4 code for its instrumentation dedicated to the TDCR method at the Laboratoire National Henri Becquerel (LNE-LNHB).
HPC Parallel Integrodifferential Solver for Dislocation Dynamics
Context : Understanding the behavior of metals at high deformation rate [4] (between 104 and 108 s-1) is a huge scientific and technologic challenge. This irreversible (plastic) deformation is caused by linear defects in the crystal lattice : these are called dislocations, which interact via a long-range elastic field and contacts.
Nowadays, the behavior of metals at high deformation rate can only be studied experimentally by laser shocks. Thus, simulation is of paramount importance. Two approaches can be used : molecular dynamics and elastodynamics simulations. This thesis follows the second approache, based on our recent works [1, 2], thanks to which the first complete numerical simulations of the Peierls-Nabarro Equation (PND) [5] was performed. The latter equation describes phenomena at the scale of the dislocation.
PND is a nonlinear integrodifferential equation, with two main difficulties : the non-locality in time and space of the involved operators. We simulated it thanks to an efficient numerical strategy [1] based on [6]. Nevertheless, the current implementation is limited to one CPU –thus forbidding thorough investigations on large-scale systems and on long-term behaviors.
Thesis subject : There are two main objectives :
- Numerics. Based on the algorithmic method of [1], implement a HPC solver (High Performance Computing) for the PND equation, parallel in time and space, with distributed memory.
- Physics. Using the solver developped, investigate crucial points regarding the phenomenology of dislocations in dynamic regime. For exploiting the numerical results, advanced data-processing techniques will be employed, potentially enhanced by resorting to AI techniques.
Depending on the time remaining, the solver might be employed for investigating dynamic fractures [3].
Candidate profile : The proposed subject is multidisciplinary, between scientific computing, mechanics, and data-processing. The candidate shall have a solid background in scientific computing applied to Partial Differential Equations. Mastering C++ with OpenMP and MPI is recommended. Moreover, interest and knowledge in physics –especially continuum mechanics- will be a plus.
The PhD will take place at the CEA/DES/IRESNE/DEC in Cadarache (France), with regular journeys to Paris, for collaboration with CEA/DAM and CEA/DRF.
[1] Pellegrini, Josien, Shock-driven motion and self-organization of dislocations in the dynamical Peierls model, submitted.
[2] Josien, Etude mathématique et numérique de quelques modèles multi-échelles issus de la mécanique des matériaux. Thèse. (2018).
[3] Geubelle, Rice. J. of the Mech. and Phys. of Sol., 43(11), 1791-1824. (1995).
[4] Remington et coll., Metall. Mat. Trans. A 35, 2587 (2004).
[5] Pellegrini, Phys. Rev. B, 81, 2, 024101, (2010).
[6] Lubich & Schädle. SIAM J. on Sci. Comp. 24(1), 161-182. (2002).
Dynamic Assurance Cases for Autonomous Adaptive Systems
Providing assurances that autonomous systems will operate in a safe and secure manner is a prerequisite for their deployment in mission-critical and safety-critical application domains. Typically, assurances are provided in the form of assurance cases, which are auditable and reasoned arguments that a high-level claim (usually concerning safety or other critical properties) is satisfied given a set of evidence concerning the context, design, and implementation of a system. Assurance case development is traditionally an analytic activity, which is carried out off-line prior to system deployment and its validity relies on assumptions/predictions about system behavior (including its interactions with its environment). However, it has been argued that this is not a viable approach for autonomous systems that learn and adapt in operation. The proposed PhD will address the limitations of existing assurance approaches by proposing a new class of security-informed safety assurance techniques that are continually assessing and evolving the safety reasoning, concurrently with the system, to provide through-life safety assurance. That is, safety assurance will be provided not only during initial development and deployment, but also at runtime based on operational data.
Multiscale dynamics of a slender structure with frictional singularities: application to a fuel assembly
The dynamic modeling of complex structures may require to take into account phenomena occurring at very different scales. However, a full refined modeling of this type of structure generally leads to prohibitive calculation costs. Multiscale modeling then presents an alternative solution to this problem, taking into account each phenomenon at the most appropriate scale.
We are interested here in slender structures subjected to mechanical stresses with frictional contacts between the structure and the retaining elements. The behavior of slender structures is in general represented by beam models, but accurately taking into account all the local contact/friction requires massive 3D models.
The originality of the work proposed here is to build an efficient multiscale and multimodel approach between beam and massive models which makes it possible to locally take into account the friction contact of slender structures. We are therefore moving towards the use of local multigrid (or multilevel) methods which naturally allow a non-intrusive multiscale coupling. The accuracy of these methods depends on the choice of transfer operators between scales, which must be carefully defined. It will also be necessary to take into account the incompatibility of the meshes supporting the models on the various relevant scales. Hence, the final model will consist in an enriched beam model taking into account local contact phenomena.
The developed model will be compared with experimental results obtained during test campaigns already carried out, and with reference numerical solutions, of increasing complexity, intended to finely validate the relevance of the proposed multiscale approach.
The strong potential of the targeted multiscale approaches, applied in this subject to the nuclear field, could be exploited by the candidate for other industrial issues such as those of aeronautics or the automotive industry.
This thesis will take place within the framework of the joint MISTRAL laboratory between the CEA and the LMA (Laboratoire de Mécanique et d’Acoustique) in Marseille. The PhD student will carry out the major part of his thesis within the CEA (IRESNE institut, Cadarache) in teams specialized in numerical methods and dynamic modeling of complex structures. The doctoral student will travel regularly to Marseille to discuss with the university supervisors.
Development of a Multilayer Encapsulation System for the Production of Core-Shell Microcapsules Suitable for Organoid Growth
Every year, 20 million people worldwide are diagnosed with cancer, with 9.7 million succumbing to the disease (Kocarnik et al., 2021). Personalized treatment could significantly reduce the number of deaths. This thesis addresses this challenge by proposing the development of organoids derived from patient biopsies to optimize treatments.
The bioproduction of encapsulated cells in biopolymers is a rapidly growing field, with applications in personalized medicine, research, drug screening, cell therapies, and bioengineering. This thesis aims to contribute to these fields by focusing on the multilayer encapsulation of cells in biopolymers with a wide range of viscosities.
The inner layer (core) provides an optimal environment for the maturation and survival of cells or organoids, while the outer layer (shell) ensures mechanical protection and acts as a filtering barrier against pathogens.
This new thesis aims to design, develop, and study—both analytically and numerically—the architecture of a dual-compartment nozzle for the high-frequency production of monodisperse core-shell capsules. It builds upon a previous thesis completed in 2023, which focused on the detailed characterization and development of a predictive model for the generation of single-layer microcapsules using centrifugal force alone.
The formation and ejection mechanisms of multilayer capsules are complex, involving the rheological properties of biopolymers, centrifugal force, surface tension, and interfacial dynamics. The nozzle architecture must account for these properties.
The first part of this thesis will focus on understanding the multilayer formation and ejection mechanisms of microcapsules as a function of nozzle geometry. This will allow the prediction and control of capsule formation based on the rheological properties of the biopolymers. The second part will involve developing an automated system for the aseptic production of capsules. Finally, biological validation will assess the functionality and reliability of the developed technology.
To achieve the objectives of this study, the candidate will first conduct analytical and numerical studies, design the ejection nozzles, and leverage the laboratory's expertise for their fabrication. Fluidic tests on prototypes will help optimize the design, leading to the development and testing of a fully operational microcapsule production system.
The ideal candidate will have a background in physics, engineering, and fluid mechanics, with a strong inclination for experimental approaches. Prior experience in microfluidics or biology would be a valuable asset.
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, and positron annihilation spectroscopy. 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 the candidate into the experimental world and thus broaden his scientific skills. The candidate will also have to manage collaborations for the experiments analysis, for the model development and for the specification of additional atomistic calculations. He will be at the interface of atomistic techniques, large-scale simulation and various experimental techniques. Therefore, he 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 oneself 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.
Assisted generation of complex computational kernels in solid mechanics
The behavior laws used in numerical simulations describe the physical characteristics of simulated materials. As our understanding of these materials evolves, the complexity of these laws increases. Integrating these laws is a critical step for the performance and robustness of scientific computations. Therefore, this step can lead to intrusive and complex developments in the code.
Many digital platforms, such as FEniCS, FireDrake, FreeFEM, and Comsol, offer Just-In-Time (JIT) code generation techniques to handle various physics. This JIT approach significantly reduces the time required to implement new simulations, providing great versatility to the user. Additionally, it allows for optimization specific to the cases being treated and facilitates porting to various architectures (CPU or GPU). Finally, this approach hides implementation details; any changes in these details are invisible to the user and absorbed by the code generation layer.
However, these techniques are generally limited to the assembly steps of the linear systems to be solved and do not include the crucial step of integrating behavior laws.
Inspired by the successful experience of the open-source project mgis.fenics [1], this thesis aims to develop a Just-In-Time code generation solution dedicated to the next-generation structural mechanics code Manta [2], developed by CEA. The objective is to enable strong coupling with behavior laws generated by MFront [3], thereby improving the flexibility, performance, and robustness of numerical simulations.
The doctoral student will benefit from guidance from the developers of MFront and Manta (CEA), as well as the developers of the A-Set code (a collaboration between Mines-Paris Tech, Onera, and Safran). This collaboration within a multidisciplinary team will provide a stimulating and enriching environment for the candidate.
Furthermore, the thesis work will be enhanced by the opportunity to participate in conferences and publish articles in peer-reviewed scientific journals, offering national and international visibility to the thesis results.
The PhD will take place at CEA Cadarache, in south-eastern France, in the Nuclear Fuel Studies Department of the IRESNE Institute [4]. The host laboratory is the LMPC, whose role is to contribute to the development of the physical components of the PLEIADES digital platform [5], co-developed by CEA and EDF.
[1] https://thelfer.github.io/mgis/web/mgis_fenics.html
[2] MANTA : un code HPC généraliste pour la simulation de problèmes complexes en mécanique. https://hal.science/hal-03688160
[3] https://thelfer.github.io/tfel/web/index.html
[4] https://www.cea.fr/energies/iresne/Pages/Accueil.aspx
[5] PLEIADES: A numerical framework dedicated to the multiphysics and multiscale nuclear fuel behavior simulation https://www.sciencedirect.com/science/article/pii/S0306454924002408
Optimizing the estimation of the mass of the nuclear material by advanced statistical methods
In order to comply with safety and security standards for nuclear waste storage and non-proliferation treaties, producers of waste containing uranium or plutonium often need to measure the amount of nuclear materials in their radioactive waste. The radiological characterization of nuclear materials by passive and active neutron measurement is one of the historical research activities of the Nuclear Measurement Laboratory (LMN) of the CEA/IRESNE Institute.
Proportional counters filled with 3He or covered with boron are the reference detectors used for these techniques, which are reference tools for measuring plutonium or uranium. In passive measurement, neutron coincidence makes it possible to discriminate spontaneous fission events associated in particular with 240Pu from neutrons resulting from (a, n) reactions. In active measurement, the active neutron interrogation technique (DDT) provides information on the amount of fissile isotopes inside a waste package.
In order to reduce the sensitivity of neutron measurement techniques to matrix attenuation and contaminant localization effects, one of the objectives of the thesis is to study the coupling of different types of measurements, such as channel-by-channel measurement, emission tomography or high-energy X-ray radiography, within a framework of advanced statistical methods. The thesis also aims to evaluate the contribution of advanced statistical methods, such as regression algorithms, Bayesian approaches (among which the Gaussian process), and neural networks, to reduce the uncertainty associated with the plutonium mass.
Particular attention will be paid to the treatment of heterogeneities in the matrix and the distribution of the radioactive contaminant. The influence of these heterogeneities can be particularly difficult to quantify, requiring not only the use of advanced statistical methods, but also an in-depth experimental study using the SYMETRIC neutron measurement station of the CEA/IRESNE Institute.
The thesis work will be carried out at the CEA site of Cadarache Nuclear Measurement Laboratory, which is a professional laboratory, expert in non-destructive methods of radiological, elementary and physical characterization of objects whether radioactive or not. It is equipped with leading technological platforms, located in the TOTEM facility (neutron and gamma measurements) and the INB Chicade (SYMETRIC platforms for neutron measurement and CINPHONIE for high-energy RX imaging). Finally, the doctoral student will work in a collaborative environment where the different teams interact closely with each other.
Simplified modelling of calcination in a rotating tube
As part of the reprocessing of uranium oxide spent fuel, the final high-level liquid waste is packaged in glass using a two-stage process, calcination followed by vitrification. Calcination gradually transforms the liquid waste into a dry residue, which is mixed with preformed glass in a melting furnace. The calciner consists of a rotating tube heated by a resistance furnace. The calcined solutions consist of nitric acid and compounds in their nitrate form or insolubles in the form of metal alloys. In order to improve control of the calciner, it is proposed to model it.
The modelling will consist of creating and then coupling three models:
- A thermodynamic model to represent the transformations undergone by the material. This part will almost certainly involve ATD and ATG measurements, coupled with a design of experiments type approach (1st year).
- A material flow model. The literature already contains very simplified principles for representing the flow in a rotating tube calciner, but we will have to be innovative, in particular by defining tests to characterise the flow of the material during the calcination process (2nd year).
A thermal model that will take into account exchanges between the furnace and the calciner tube as well as exchanges between the material and the tube. The exchange coefficients will have to be characterised (1st year).
Combining these three models (3rd year) will give rise to an initial simplified calcination model. This model will be used to help control the calcination stage and also to train operators to control this apparatus.
You will be working in the LDPV, a multidisciplinary team (process, chemistry, fluid mechanics, modelling, mechanics, induction) comprising 16 engineers and technicians. A team with 30 years' experience in vitrification processes, recognised both nationally and internationally.
Development of thin film negative electrodes for Li-free all-solid-state batteries
The aim of this work is to develop 'Li-free' negative electrodes for new generations of high energy density all-solid-state lithium batteries. The function of this type of electrode is to provide a significant gain in energy density in the battery, to facilitate its manufacture by eliminating the need to handle lithium metal and, most importantly, to enable the formation of a homogeneous, dendrite-free lithium film when the battery is charged.
These electrodes will be based on the functionalisation of a metal collector with thin-film materials comprising at least one lithiophilic material (typically a compound that can be alloyed with lithium) and an inorganic ionic conductor. These electrodes are prepared by physical vacuum deposition processes such as sputtering or thermal evaporation. It will therefore be necessary to study the influence of the composition and structure of the lithiophilic layer on the nucleation and growth mechanism of the lithium film and on the evolution of the electrode during charge/discharge cycles. The role of chemical/mechanical interactions with the ionic conducting layer will also be investigated.
This work, which is part of a national CEA/CNRS joint project, will be carried out at the CEA Tech site in Pessac, which has a full range of vacuum deposition and thin film characterisation equipment, in close collaboration with ICMCB CNRS in Bordeaux. It will benefit from the many characterisation resources (confocal optical microscopy, SEM/cryo FIB, ToF-SIMS, SS-NMR, µ-XRD, AFM,...) available in the various partner laboratories involved in the project.