Behaviour of elastomeric seals in transport packaging for radioactive material

The mechanical and sealing behavior of elastomeric seals is a crucial issue for the safety of transport packaging for radioactive materials [1], [2]. The seals must ensure the containment enclosure's tightness over a wide temperature range to guarantee the confinement of gases and radioactive materials, even under accidental conditions.
At -40°C, the compression rate of the O-ring seal ensuring the tightness of the cover must remain sufficient, implying that its diameter should be large compared to the groove height [3]–[6]. Conversely, at high temperatures, efforts are made to prevent the seal's volume from exceeding that of the groove to avoid potential extrusion. Additionally, the residual deformation after compression (RDC), or the inability of the seal to return to its initial position after compression, must also be considered [7], [8].
These two criteria are challenging to reconcile, and the sizing of the groove/seal assembly can only result from a compromise since these requirements conflict. It is sometimes impossible to prevent the seal's volume from exceeding that of the groove, especially if the seal is subjected to high temperatures (e.g., 250°C). In such cases, as elastomers are considered incompressible materials, extrusion will occur if clearance is present, providing the seal with a volume to expand [9]–[12]. This phenomenon generally leads to the loss of the seal's physical integrity.
However, in transport packaging, the assembly between the flange and the containment cover generally does not allow clearance for the seal to expand, preventing extrusion. Yet, the high-temperature behavior of a seal in a constrained volume is poorly documented in the scientific literature [7]. Therefore, it is unknown whether the elastomer can become compressible or if it is, on the contrary, capable of lifting or deforming the groove and/or the metallic cover of the assembly, thereby compromising the packaging's integrity.
In this context, this thesis aims to advance our understanding of elastomeric seal properties with a thermomechanical approach (high and low temperatures), with a particular focus on two aspects: (a) better understanding the extrusion phenomenon when clearance is present and (b) better grasping the maintenance or loss of the concept of incompressibility of the elastomer material in a constrained volume.

The planned work is divided into several parts:
In collaboration with DTEL/SGPE, the study program will be defined, representative of the issues encountered in CEA's transport packaging: different seal and steel grades, dimensions, groove shapes, clamping forces, temperature range, etc.
An experimental protocol will be developed to characterize the behavior of seals in situ in representative screwed assemblies, providing either extrusion clearance or, conversely, a constrained volume. These experimental tests will be conducted at CETIM, benefiting from temperature control facilities, the design and creation of study-appropriate models with instrumentation, metrological control means, and sealing aspects linked to mechanics. CETIM also has extensive knowledge in the nuclear field, particularly with significant experience in studying issues related to the transport of radioactive material [2], [13].

After tests, the structure of the seals will be characterized at the atomic, microscopic, and mechanical scales using various techniques (SEM, microhardness, reaction force...).
These tests will be designed and interpreted using numerical simulations based on functional and material databases to model the extrusion phenomenon. With this method, and knowing the geometry and properties of the materials, it is possible to predict the extrusion pressure limit at different temperatures.
The experimental results will be compared to the calculations to optimize the design and parameters of the test devices.

The results obtained in this thesis will ultimately advance our understanding of elastomeric seal behavior and their ability to maintain tightness under extreme conditions. They will contribute to adopting a more innovative approach in designing transport packaging for radioactive materials, making them safer. Finally, the coupling between experiments and simulations will advance the numerical codes used to model the extrusion phenomenon.

This entire thesis work will be carried out through several collaborations:
• CETIM in Nantes,
• DES/DDSD/DTEL/SGPE at CEA Cadarache, and
• Gabriel Lamé Mechanics Laboratory (LaMé - EA 7494) - University of Tours
The doctoral student will be primarily based at CETIM in Nantes but will regularly visit CEA Cadarache and LaMé in Tours depending on the progress of each part of the work.

Battery-on-chip characterization and modeling by machine learning-assisted techniques

The physical mechanisms involved in the operation of a microbattery are still poorly understood and modelled. To study them, the CEA has a manufacturing and characterisation platform dedicated to lithium components.
The aim of this thesis is to develop a physical model to describe the performance of batteries (voltage, power output) depending on the conditions of use. The proposed methodology consists of :
1. Using 15,000 batteries per wafer as test vehicles. Measurements taken during battery cycling are compiled in a database.
2. Participating in the development of data processing programmes based on machine learning and Bayesian inference methods to highlight optimal cycling protocol parameters. the results are feed back into the physical and electrochemical model (validation/understanding/exploration).
3. Iteration with the manufacture and electrical testing of new battery architectures/designs.

Seeking the maximal active dopant concentration in Si using nanosecond laser annealing

In conventional CMOS technology, source & drain regions of transistors are formed by ion implantation of selected impurities (B, P) in silicon or SiGe alloy, and a subsequent thermal treatment to cure the crystal and electrically activate the dopants. In the case of 3D-sequential integration, an architecture in which at least tow levels of transistors are superimposed, the thermal budget for the fabrication of the upper level transistors is limited, to avoid any degradation of the bottom level. Classical annealings during a few seconds/ minutes at 600-1050°C are not anymore possible. One can choose to switch to Nanosecond Laser Annealing (NLA), enabling very short anneals with heat confined in the first tens of nanometers thanks to its UV laser and very short pulse duration. Depending on the amount of heat provided to the Si or SiGe layer by NLA, various phenomena can be encountered. When heat amount is sufficient, the layer can melt and solidify. On the other side, when heat amount does not exceed the melt threshold, solid phase epitaxial regrowth (SPER) can take place. In both cases, the extreme cooling rate gives access to high active dopant concentration, eventually beyond the solubility limit. However, maximal achievable active dose (phosphorus and boron in silicon, boron in SiGe) are not known, for both solid and liquid regimes.

Study of the influence of the ferrite additive manufacturing process on mechanical and magnetic properties

Traditional methods of manufacturing ceramic parts include costly processes such as slip casting, pressing or injection molding; they require specific equipment and expertise. When small quantities of ceramic parts, or prototypes, with specific properties are required, manufacturers are still forced to make costly investments. In addition, traditional manufacturing processes restrict design freedom and make it difficult to create internal channels, overhangs or lattice structures, for example. 3D printing opens up new prospects for innovation in the field of technical ceramics, by offering low-cost machines and opening up the field of possibilities for the design of complex parts impossible to obtain with molding methods.
This is the background to the subject of this thesis, on a ceramic material of interest to the CEA: (Ni-ZnFe2O4) ferrite. The CEA masters the manufacture of this material by traditional methods of powder pressing followed by sintering, but would like to extend its skills by producing parts with more complex geometries, with a reduced time between the design stage and the manufacture of a first prototype.
The work will involve optimizing the microstructure of ferrite implemented using Fused Deposition Modeling (FDM) 3D printing technology, then measuring mechanical and magnetic properties, as well as magneto-elastic effects. An analysis will be carried out to correlate the relationship between microstructure and material properties. The results will be compared with the conventionally developed material. This will highlight the influence of the manufacturing process on properties. Finally, a part with a complex geometry will be developed with the aim of understanding the difficulties associated with the change of scale. This stage will be accompanied by an initial assessment of the robustness of the process.

Discovery of new chromogenic probes for toxic using Chemistry-Trained Machine Learning

Today national and international situation justify new researches on the colorimetric detection of toxic and polluting gases (referred to as analytes in the following). For the already known and studied compounds, improvement of the detection capabilities involves increasing contrast and selectivity. For potential new analytes, it is also important to prepare for rapid identification of specific chromogenic probes. The objectives of the thesis will be to discover new chromogenic probes by using computational chemistry.
First stage of the thesis: Training of the model (ML/AI) on available database. This part of the thesis will focus on establishing a precise and robust model to classify the large experimental database available from our laboratory's previous work. This involves correlating the colorimetric results with the structures and chemical properties of the molecules described by state-of-the-art methods (e.g., https://pubs.acs.org/doi/10.1021/acs.chemrev.1c00107). At the end of this learning process, we will have a predictor (SVM, LCA, PCA…) validated on our data.
Second stage: Use of the predictor model to screen in silico several hundred thousand candidate probe molecules from commercial chemical libraries (and others), correlated with their chemical structure and property descriptions as in the first stage. After this initial screening, DFT prediction of the chromogenic response will be used to refine the selection of the best candidate molecules.
Third stage: Definition and implementation of an experimental chemical testing campaign. A fast organic synthesis platform HTE (high throughput experimentation) based on the miniaturization and parallelization of chemical reactions to optimize the implementation of synthesis reactions and tests, will save considerable time, while significantly increasing the number of possible combinations. HTE also enables the synthesis of libraries of analogous compounds. Following these massive tests, a second version of the predictor will be trained and will lead to the discovery of a new generation of chromogenic molecules.

Develoment of lithium mediated ammonia electrolyzer

Recent developments in electrochemical ammonia (NH3) synthesis using lithium (Li) metal deposition in THF-based electrolytes in the presence of protic species, reinvigorated the research interest in direct NH3 electrolyzes technology thanks to its surprisingly high performance in terms of synthesis rate and faradaic efficiency. However, the main drawback is poor energy efficiency due to minimum voltage requirements associated to Li metal deposition and H2 oxidation reactions on the opposite electrodes. In this project, we propose to study the nitridation reaction of Li-alloy forming metals that can enable the decrease in electolyzer voltage. This study will be performed using a 3-electrode electrochemical pressure cell and differential scanning calorimetry – thermogravimetric analysis under N2, H2 pressures. The goal here is to couple existing knowledge in chemical looping synthesis of ammonia with electrochemical synthesis. Porous (carbon or steel tissue) electrodes will be developed with nanoparticles of Li-alloy forming metals and their performance will be studied in an electrolyzer. The assumed 3-step reaction mechanism to form NH3 is as follows: Li deposition > nitridation > protonation. This mechanism is already a subject of discussion for pure Li metal which will be further complicated with the use of alloy forming metals. Therefore, we propose an in-depth study using x-ray photoemission spectroscopy. The ultimate objective of the project is to accelerate the direct NH3 electrolysis technology and address the Power-to-X needs of renewable electricity sources.

Development of a new generation of recyclable encapsulation films for photovoltaic modules

In the context of the energy transition, photovoltaic (PV) solar energy represents a growing share of the world's electricity production, and PV itself represents a growing share of the world's energy production. The massive production and deployment of PV modules is putting increasing pressure on the environment. In particular, because of the extraction of the raw materials required for their production and their disposal at the end-of-life. Recycling tackles both of these issues.
PV modules are made of layers of different natures laminated together. In the module central layers, PV cells are embedded in an elastomer, the encapsulant. This material plays several roles: barrier properties, mechanical protection, etc. Currently, the encapsulants used are generally cross-linked EVA copolymers, which makes recycling particularly difficult.
The aim of this thesis is to develop a vitrimer encapsulant for PV applications. Such an encapsulant, with exchangeable bonds, could drastically simplify recycling without compromising the integrity of the module in its lifetime. This work will start with the formulation of the encapsulant. It will go on with the characterization of its properties (thermo-reversibility, rheology and barrier properties), its extrusion into a film and its lamination in a PV module.
This development will be iterative, thus leading to tests under application representative conditions at various stages of development. It will rely on the resources and expertise of three laboratories LCMCP (Sorbonne Université), PIMM (ENSAM) et LITEN (CEA).

Development of a predictive power model for a photovoltaic device under spatial constraints

CEA is developing new cell and module architectures and simulation tools to assess the electrical performance of photovoltaic (PV) systems in their operating environment. One of these models, called CTMod (Cell To Module), takes into account not only the different materials making up the module, but also the different cell architectures. For space applications, the community wants to use terrestrial silicon-based technologies that can be integrated on flexible PVAs (Photovoltaic Assembly). The space environment imposes severe constraints. A relevant evaluation of performance at the start and end of a mission is therefore essential for their dimensioning.
The aim of this thesis is to correlate physical models of radiation-matter degradation in space with electrical models of photovoltaic cells. Performance degradations linked to the various electron, proton and ultraviolet (UV) irradiations of the space environment will be evaluated and validated experimentally. Linked to the CTMod Model, this new approach, never seen in the literature, will able to get a more accurate understanding of interactions between radiations and PVAs. These degradations result from non-ionizing energy deposition phenomena, quantified by the defect dose per displacement, and ionizing ones quantified by the total ionizing dose for protons and electrons. In the case of UV, the excitation of electrons in matter generates chain breaks in organic materials and colored centers in inorganic materials. Initially, the solar cell used in the model will be a Silicon cell, but the model can be extended to include other types of solar cell under development, such as perovskite-based cells.

Role of cement paste and fine aggregates in the triaxial behavior of concrete

Investigate electron and hole transport layers for high temperature stability III-V quantum dot photodiode devices

Colloidal Quantum Dots (QD) are novel building blocks for the fabrication of image sensors with high performance tunable light detection in the SWIR wavelength range but currently exhibit undesired degradation under high thermal stress. Thermal degradation can be significantly improved by optimizing the device materials (contacts, hole transport layer (HTL), electron transport layer (ETL) and encapsulation), film thicknesses, and deposition processes used to make quantum film (QF) photodiode devices. As such, a detailed investigation into many different HTL, ETL and top electrode materials will be pursued to find the best candidates to overcome the current limitations. Materials selection and deposition processes for these layers will be chosen and studied among a variety of existing materials developed at LETI. QD films with tunable absorption from 1-2.5 µm will be prepared by STMicroelectronics and CEA-IRIG in collaboration with other partners. The QD patterning step for the fabrication of the devices and the electro-optical testing will be performed internally at LETI with support from STMicroelectronics.

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