Manufacturing of analytical microsystems by thermoplastics micro-milling

Micro-fabrication techniques and especially micro-milling can be used to manufacture microsystem prototypes with a timescale of days, as close as possible to the application. Polymethylmethacrylate (PMMA - Commercial name Plexiglas) is a commonly used material for microsystems fabrication, limited however by its chemical incompatibility with acids and solvents.
The goal of this post-doctoral project is to study the feasibility of machining thermoplastic materials other than PMMA and to optimise the associated manufacturing parameters. The post-doctoral project will start with the selection of materials in line with the aimed applications (optical and physico-chemical properties). The materials will be selected among the thermoplastics family (PC, POM, PS, HDPE, PEEK, PVC, PP, PTFE, ULTEM, etc).
The optimisation of the micro-milling step will be realized by varying parameters such as the tool rotation speed, the feed rates, the depth of cut, etc. The surfaces and channels will be characterized by optical or mechanical profilometry, optical microscopy and/or scanning electron microscopy.

Thermodynamic study of the Nb-O-Zr system for the nuclear fuel elements recycling

The first step of nuclear material recycling consists in a section-cutting process of the fuel assemblies leading to shells.
Nuclear materials in the cut sections are dissolved in acid solutions whilst structural as well as cladding materials are rinsed and then compacted in CSD-C containers for a final storage in CIGEO.
The REGAIN project aims at studying the feasibility of an alternative solution: the objective is to investigate the possibility to optimize the nuclear and cladding materials management by reducing the radiological source term. The idea is to proceed to a sequence of decontamination steps in order to minimize the waste volume: The first step consists in removing minor actinides and fission products and the second one in the separation of zirconium from structural activation products.
In order to feed the industrial process study, a part of the REGAIN project aims at collecting raw data, which will be used by the other work packages of the project.
In this framework, CEA proposes a post-doctoral position with the purpose of developing a thermodynamic database for the Nb-O-Zr system starting from literature data as well as using experimental informations obtained within the first stages of the project. It will be also possible to include a selection of key fission products into the existing database. The candidate may also be asked to complete the existing data by an experimental campaign to obtain a complete set of data for the modelling. The scientific approach will be based on the CALPHAD method: this method allows developing a thermodynamic database by the definition of an analytical formulation of the thermodynamic potential, which will be used to calculate phase diagrams as well as thermodynamic properties of multi-components systems.

Development of a simulation tool for the pitting process of a stainless steel used for the storage of nuclear waste

Structural nuclear waste is compacted in patties, stacked in a stainless steel container. In these compacting boxes are placed various metal-type materials with the addition of organic matter, including chlorinated waste. By radiolytic degradation, these can lead to the formation of hydrogen chloride HCl. During the storage phase, relative humidity may be present within the container, which, added to the HCl, may lead to a phenomenon of condensation, resulting, on the surface of the materials, of acid and concentrated into chloride ions condensates. In contact with this acid and chloride electrolyte, a pitting phenomenon is likely to begin on the surface of a stainless steel. This is a local phenomenon that can lead to the piercing of the material in extreme cases. The initiation of this phenomenon depends on several factors: the morphology of the electrolyte, its composition and its evolution over time.
If nowadays this phenomenon is well known, modeling it remains a major challenge because it is a coupled multi-physics and multi-parameter problem. Many questions remain open, particularly at the level of the physical and chemical laws to be used or how to represent the corrosion process?
The objective of the post-doctorate is to develop a tool under COMSOL capable of simulating the initiation and the evolution over time of a pit on the surface of a stainless steel. The approach will be based on a mechanistic modeling of the processes (material transport process and all the chemical and electrochemical reactions).
The post-doctorate will take place in several actions:
1- make a state of the art of the bibliography in order to understand the pitting phenomenon and to identify the laws necessary for modeling.
2-simulate the spread of the pit in a chloride environment in order to establish a propagation criterion.
3-the pitting initiation will be implemented in order to obtain a complete tool capable of simulating the pitting process

Separation microsystem coupled to mass spectrometry for on-line purification and characterisation of nuclear samples

The miniaturisation of analytical steps commonly carried out in laboratories offers many advantages and particularly in the nuclear sector, where the reduction of material consumption and waste production is of major interest. In this context, one of our laboratory’s focus area is the miniaturisation of analytical tools, particularly chromatographic separation techniques. The aim of this project is to reduce the scale of the purification steps of nuclear samples by solid phase extraction chromatography, prior to the analytical processes. Obtaining these miniaturised extraction devices is based on the in situ synthesis and anchoring of monoliths, in the channels of cyclic olefin copolymer (COC) microsystems. Since this material is chemically inert, COC functionalisation strategies are currently under development to covalently graft reactive sites on its surface, before locally anchoring actinide-specific monoliths in the micro-channels. The aim is to design and fabricate chromatographic extraction microsystems in COC, and to implement them for chemical purification and mass spectrometry measurements, both off-line and on-line.

Assessment of bimetallic bonds for molten salt nuclear reactors: understanding the microstructure-mechanical behavior relationships

Nickel-based alloys are the natural candidates for corrosion-resistant metallic materials, but their mechanical behavior and resistance to irradiation may not be satisfactory. To overcome this difficulty, the CEA with its partners in the ISAC project is developing generic solutions for bimetallic components that allow the surface of a reference material known for its good behavior under irradiation to be functionalized by a thick deposit of a nickel-based grade.
Two reference materials were selected for this study, the austenitic steel 316L(N) and the martensitic steel with phase transformation Fe-9Cr T91. The behavior of these two grades under irradiation is well known and controlled. The process used for deposition is thick TIG welding which is close to additive manufacturing methods. It has the advantage of being generic and simple to implement on very different parts (ferrules, plates, inside tubes ...). The purpose of this post doc is to evaluate the relevance of this bimetallic concept for MSR.

Migration of radioactive aerosols through a cracked-concrete wall

In the framework of the extension of the operating life of the nuclear powerplants, EDF is regularly called to reassess the adequacy of the design provisions with regard to safety. Concerning the radiological consequences in case of a severe accident, the safety demonstration studies consider that in the presence of cracks in the containment, all the radionuclides are released through the containment. In order to better assess the margins provided by this assumption, EDF addresses this study to obtain a better knowledge about the pathway of radionuclides within the cracked concrete in the event of a severe accident and to estimate the retention rate of radionuclides present in the form of aerosols, according to their type and flow conditions.
The objective of the study is to design, prototype and validate a laboratory-scale test model dedicated to the migration of radioactive aerosols under high temperature (until ~ 150°C) and pressure (until 4 bars in relative) conditions through a cracked concrete specimen.

Automatic machine learning identification of nanoscale features in transmission electron microscopy images

Imaging nanoscale features using transmission electron microscopy (TEM) is key to predicting and assessing the mechanical behaviour of structural materials in nuclear reactors or in the fields of nanotechnology. These features, visible by phase contrast (nanobubbles) or diffraction contrast (dislocation loops or coherent precipitates), are prime candidates for automation. Analysing these micrographs manually is often tedious, time-consuming, non-universal and somehow subjective.

In this project, the objective is to develop a Python-based framework for data treatment of transmission electron microscopy (TEM) images.
Machine Learning approaches will be implemented in order to tackle the following tasks:
- Data collection: The success of any machine learning approach is linked to the database quality. In this project, a huge database is available. Four microscopists are involved in the project and will continuously enrich the database with images containing easily recognizable features.
- Denoising and finding the defect contour both through existing open-access software and in-house developed descriptors. Representative ROI (region-of-Interest) will be generated on images.
- Design of the Convolutional Neural Network (CNN) Architecture and model training: A collective feature map will be generated for the entire images in order to identify some representatives ROI. Each ROI is then overlaid to the original feature map and is passed to the CNN for individual region classifications. Secondly, recent advances in image segmentation will be placed in the core engine of the workflow.
- Model performance metrics: The aim is to reach a compromise between the training time and the detector performance.
The process will be applied to nanometer-sized features formed under irradiation in nuclear oriented materials (Co-free high entropy alloys (HEA), UO2) and precipitates in materials with a technological interest (coherent Cr precipitates in Cu).

Implementation of a sensor allowing the online monitoring of the corrosion of stainless steels in a hot and concentrated nitric acid medium

The control of materials (mainly stainless steel) aging of the spent nuclear fuel reprocessing plant is the subject of permanent attention. Some installations at La Hague plant will have to be replaced very soon. In this context, it is important for the industry to develop sensors that are resistant to concentrated nitric acid (˜ 2.5 mol / L) and temperature (from ambient to 130 °C), allowing the online monitoring of the corrosion.
The aim of this work is to manufacture one sensor for the detection of corrosion of the steel intended for handling by the operators of the plant. In case of a positive response, the second sensor is used.
The challenges of this work are essentially technological since it will develop or use materials adapted to concentrated and hot nitric acid media.
The laboratory is specialized in the corrosion study in extreme conditions. It is composed of a very dynamic and motivated scientific team.