Influence of ionization density in water on fluorescent solutes. Application to the detection of alpha radiation
The location and rapid identification, at a distance, of sources of alpha and beta particle emissions on surfaces or in wet cavities or solutions, in nuclear facilities undergoing decommissioning or to be cleaned up, is a real challenge.
The aim of the proposed PhD project is to develop a concept for the remote detection of fluorescence light from water radiolysis processes on molecules or nano-agents. Temporal characterization using fluorescence lifetime measurements will enable detection to be attributed to a type of radiation, depending on its linear energy transfer (LET). In the Bragg peak of alpha radiation, where the TEL is maximal, the ionization density due to this TEL influences the fluorescence lifetime. However, dose rate effects also need to be considered.
Molecules and nanoparticles that are candidates for forming fluorescent products and are sensitive to the ionization density and radicals produced in traces at very short times will be identified by guided bibliography work, then tested and compared by measurements. Spectral measurements (absorption and fluorescence) and fluorescence lifetimes of the corresponding fluorescent species will be carried out using the multi-channel (16-channel) TCSPC (Time Corelated Single Photon Counting) method. Ion beams or alpha particles from sealed sources will be used for proof-of-concept. Ion beams or alpha particles from sealed sources will be used for proof-of-concept in the CEA clean-up/dismantling program.
Thermohydraulic modelisation of a steam generator and chemical species propagation
Steam generators are essential components of nuclear reactors whose main function is heat exchange. The chemical species present in steam generators are the cause of many parasitic phenomena (clogging, fouling, sludge deposition, etc.). Numerical simulation of species transport, taking into account the migration of chemical species and exchanges between species, both intra- and inter-phase, will allow a better understanding and better management of these problems. Numerical resolution of species transport systems presents real difficulties, in particular the management of the appearance and total disappearance of certain species, high void rates, as well as rapidly excessive calculation times.
While relying on the new code for nuclear components developed at STMF, the thesis will address the following three main scientific issues:
• Upstream, the analysis of numerical methods allowing in particular the management of evanescence, as mentioned above, and thermo-hydraulic modeling at high void rates. For this, we will rely on the PolyMAC and PolyVEF numerical schemes, already implemented in the component code.
• The physical modeling of a steam generator in the new component code, via the addition (in C++) of correlations specific to steam generators, the completion of the state laws already available, etc..
• The determination of the major chemical species to be transported, in order to be able to take into account both thermo-hydraulics and chemistry. The algorithmic coupling between thermo-hydraulics and chemistry, taking into account feedback, being the long-term objective.
While benefiting from the existing parallelization of the component code, the thermo-hydraulic and chemical modeling will be done taking into account the constraints on computation times.
Chemo-mechanical modeling of the coupling between carbonation, rebar corrosion and cracking in cementitious materials
Rebar corrosion is one of the main causes of premature degradation of concrete infrastructures, including in the nuclear sector, where concrete is extensively used in containment structures and waste storage facilities. Carbonation, caused by the penetration of CO2 into the concrete, lowers the pH of the pore solution, promoting rebar corrosion. This corrosion leads to the formation of expansive products that can cause cracking in the material. The proposed thesis work, developed as part of a European collaborative project between CEA Saclay, École des Mines de Paris - PSL, and IRSN, aims to develop a numerical model to simulate these phenomena. The model combines a reactive transport code (Hytec) and a finite element code (Cast3M) to study the local effects of carbonation-induced corrosion on concrete cracking. This project will benefit from parallel experimental work to gather data for parameter identification and model validation. The first part of the research will focus on modeling the carbonation of cementitious materials under unsaturated conditions, while the second part will address the corrosion of rebar caused by the pH drop induced by carbonation. The model will describe the growth of corrosion products and their expansion, inducing stress within the concrete and potential microcracking.
This research project is aimed at a PhD student wishing to develop their skills in materials science, with a strong focus on multi-physical and multi-scale modeling and numerical simulations. The thesis will be carried out principally at CEA Saclay and at École des Mines de Paris – PSL (Fontainebleau).
Improving the predictivity of large eddy simulations using machine learning guided by high-fidelity simulations
This thesis aims to explore the application of machine learning techniques to improve turbulence modeling and numerical simulations in fluid mechanics. More specifically, we are interested in the application of artificial neural networks (ANNs) for large eddy simulation. The latter is a modeling approach that focuses on the direct resolution of large turbulent structures, while modeling small scales by a subgrid-scale model. It requires a certain ratio of total kinetic energy to be resolved. However, this ratio may be difficult to achieve for industrial simulations due to the high computational cost, leading to under-resolved simulations. We aim to improve the latter by focusing work along two main axes: 1) Using ANNs to build generic sub-mesh models that outperform analytical models and compensate for coarse spatial discretization; 2) Training ANNs to learn wall models. One of the main challenges is the ability of the new models to generalize correctly in configurations different from those used during training. Thus, taking into account the different sources and quantification of uncertainties plays a vital role in improving the reliability and robustness of machine-learned models.
HW/SW Contracts for Security Analysis Against Fault Injection Attacks on Open-source Processors
This thesis focuses on the cybersecurity of embedded systems, particularly the vulnerability of processors and programs to fault injection attacks. These attacks disrupt the normal functioning of systems, allowing attackers to exploit weaknesses to access sensitive information. Although formal methods have been developed to analyze the robustness of systems, they often limit their analyses to hardware or software separately, overlooking the interaction between the two.
The proposed work aims to formalize hardware/software (HW/SW) contracts specifically for security analysis against fault injection. Building on a hardware partitioning approach, this research seeks to mitigate scalability issues related to the complexity of microarchitecture models. Expected outcomes include the development of techniques and tools for effective security verification of embedded systems, as well as the creation of contracts that facilitate the assessment of compliance for both hardware and software implementations. This approach could also reduce the time-to-market for secure systems.
Cryptographic security of RISC-V processor enclaves with CHERI
CHERI (Capability Hardware Enhanced RISC Instructions) is a solution for securing the processor against spatial and temporal memory leaks by transforming any pointer into a capability that clearly defines the access limits to the data or instructions addressed.
In this thesis, we propose to enrich CHERI and its control-flow integrity capabilities on a RISC-V application processor, by protecting instructions right up to their execution against any type of modification. Secondly, based on authenticated memory encryption, we will study the possibility of using CHERI to define secure enclaves enabling cryptographic isolation between processes. The processor will be modified so that each process is encrypted with its own key and can have a secure life cycle. All keys must be efficiently protected in hardware.
Contact : olivier.savry@cea.fr
Topologic optimization of µLED's optical performance
The performance of micro-LEDs (µLEDs) is crucial for micro-displays, a field of expertise at the LITE laboratory within CEA-LETI. However, simulating these components is complex and computationally expensive due to the incoherent nature of light sources and the involved geometries. This limits the ability to effectively explore multi-parameter design spaces.
This thesis proposes to develop an innovative finite element method to accelerate simulations and enable the use of topological optimization. The goal is to produce non-intuitive designs that maximize performance while respecting industrial constraints.
The work is divided into two phases:
Develop a fast and reliable simulation method by incorporating appropriate physical approximations for incoherent sources and significantly reducing computation times.
Design a robust topological optimization framework that includes fabrication constraints to generate immediately realizable designs.
The expected results include optimized designs for micro-displays with enhanced performance and a methodology that can be applied to other photonic devices.
Combining over and underapproximation of memory abstractions for low-level code analysis
Rice's theorem stating that no method can automatically tell whether a property of a program is true or not has led to the separation of verification tools into two groups: sound tools operating by over-approximation, such as abstract interpretation, are able to automatically prove that certain properties are true, but are sometimes unable to conclude and produce alarms; conversely, complete tools operating by under-approximation, such as symbolic execution, are able to produce counter-examples, but are unable to demonstrate whether a property is true.
*The general aim of the thesis is to study the combination of sound and complete methods of programanalysis, and in particular static analysis by abstract interpretation and the generation of underapproximated formulae by symbolic execution*.
We are particularly interested in the combination of over- and sub-approximating abstractions, especially for memory. The priority applications envisaged concern the analysis of code at the binary level, as achieved by the combination of the BINSEC and CODEX analysis platforms, so as to automatically discover new security vulnerabilities, or prove their absence.
Study and evaluation of silicon technology capacities for applications in infrared bolometry
Microbolometers currently represent the dominant technology for the realization of uncooled infrared thermal detectors. These detectors are commonly used in the fields of thermography and surveillance. However, the microbolometer market is expected to grow explosively in the coming years, particularly with their integration into automobiles and the proliferation of connected devices. The CEA Leti LI2T, a recognized player in the field of infrared thermal detectors, has been transferring successive microbolometer technologies to the industrial partner Lynred for over 20 years. To remain competitive in this growing market for microbolometers, the laboratory is working on breakthrough microbolometers incorporating CMOS components as the sensitive element. In this context, the laboratory has initiated studies focusing on temperature-dependent silicon technology capabilities, with promising initial results not reported in the literature. The thesis topic fits into this context and aims to demonstrate the interest of these components for microbolometric applications. It will therefore cover the analytical modeling of these components and their associated physical effects, as well as the reading of such a component in a microbolometer imager approach. A reflection on technological integration will also be conducted. The student will benefit from several already realized technological lots to experimentally characterize the physical effects and familiarize themselves with the subject. To understand the encountered phenomena, the student will have access to the laboratory's entire test set-ups (semiconductor parameter tester, noise analyzer, optical bench, etc.) as well as the numerical analysis Tools (Matlab/Python, TCAD simulations, SPICE simulations, Comsol, etc.). By the end of the thesis, the student will be able to address the question of the interest of these components for microbolometric applications.