Accelerated development of materials resistant to molten chloride salts
The accelerated development of materials is a major challenge for all industries, and corrosion resistance is all the more important for resource conservation issues. This project therefore aims to estimate the corrosion resistance of FeNiMnCr alloys in chloride salt for use in molten salt nuclear reactors, in collaboration with the University of Wisconsin, which has demonstrated extensive expertise in the accelerated development of materials for molten fluoride and chloride salt reactors. As part of this post-doc, dozens of samples of quaternary FeNiMnCr model alloys will be synthesised by additive manufacturing at the University of Wisconsin, varying the composition in order to map the entire composition tetrahedron as accurately as possible. These samples, with a NiCr model alloy corroded in a wide range of molten chlorides salt chemistries, will then be corroded at the CEA. The aim of these experiments is, on the one hand, to obtain a large database on the corrosion of FeNiMnCr alloys in a very short time (1.5 years) and, on the other hand, to screen the effect of a wide range of salt compositions on a model NiCr alloy. Finally, these experiments will make it possible to target the best materials for studying their corrosion mechanisms.
Development of a capillary electrophoresis microsystem hyphenated to ICP-MS for isotopic and elemental analysis
The precise and accurate determination of isotopic and elemental compositions of samples by mass spectrometry is paramount in several research fields such as geoscience, environmental science, biology or the nuclear field. In order to avoid spectral or non-spectral interferences, it is necessary to perform chemical separations steps prior to analysis by mass spectrometry. In most cases, those separations are performed by liquid chromatography. Electrokinetic separation methods are particularly suitable to perform those separations due to the small volume of sample required and the small volume of waste produced, in the nL and µL range, respectively.
The main objective of this post-doctoral work is to integrate electrokinetic separations, presently performed in a glass capillary, in an analytical microsystem. This analytical microsystem, based on the microfluidic technology, will be used for the separation of elements and has to be hyphenated to an MC-ICP-MS without degrading the separation. The output of this work is an automated device capable of increasing throughput while keeping the same analytical performance as the original method. For nuclear samples, the device will reduce the dose received and the production of waste associated with the analytical protocol.
Understanding and modeling the thermodynamic and kinetic properties of MOX fuel in future reactors
This study is part of the Sodium-Cooled Fast Reactor projects. Uranium and plutonium dioxide (U,Pu)O2, known as MOX, is the reference fuel. During operation, fuel pellets are subjected to a high thermal gradient that induces mass transport, thermodiffusion, and vaporization phenomena, coupled with irradiation effects. Fuel performance codes are developed to simulate the behavior of fuel rods under nominal and incidental conditions, up to and including meltdown.
The objective of this study is to improve the thermokinetic model of MOX used in these codes. This model is based on the description of the U-Pu-O system using the CALPHAD method, coupled with a database of element mobilities developed using DICTRA software. The description of defects will be extended with the introduction of metal vacancies and oxygen clusters. The description of thermodynamic data (oxygen potential and heat capacity) and the phase diagram will also be improved by taking into account the most recent data. Finally, the mobility database, coupled with the Calphad model, will be improved to better describe diffusion in MOX. New experimental data as well as data calculated using atomic-scale calculation methods (molecular dynamics, ab initio) will be used.
Optimisation of the durability of metal alloys at high temperatures: exploration of new oxidation conditions
The aim of the OPTIMIST exploratory project is to increase the service life of metal alloys (alumina and chromia forming alloys) by forming a protective oxide layer, as is almost always the case to protect alloys from corrosion. The great originality of OPTIMIST will consist in forming an oxide layer with a minimum of 0D (point defects) and 2D (grain boundaries) structural defects. This objective will be based on two distinct strategies: the first will consist of forming a so-called endogenous oxide layer, i.e. by pre-oxidising the substrate by carefully choosing the pre-oxidation conditions (temperature, oxidising medium, oxygen partial pressure) in two types of Rhines Pack specifically developed at CEA/DES and IJL; the second will consist of forming a so-called exogenous oxide layer, i.e. created by a deposition technique: the HiPIMS recently commissioned at the CEA/INSTN. Different pre-oxidation conditions (for the endogenous layer) and process conditions (for the exogenous layer) will be investigated, then their 0D and 2D defects will be characterised at SIMaP using a novel combination of cutting-edge structural (TEM-ASTAR), chemical (atom probe, SIMS, nano-SIMS) and electronic (PEC PhotoelEctroChemistry) techniques. Finally, these characterised samples will be corroded in two environments (in air and in molten salts) at high temperatures to assess the effectiveness of the protection compared with conventional pre-oxidation. The stages of oxide growth, its stoichiometry and its microstructure (grain size and shape, nature of the grain boundaries) will thus be identified as a function of the endo and exogenous growth conditions so as to control them in order to achieve an oxide layer containing as few defects as possible.
Synthesis, Characterization, and Molecular Modeling of M-(A)-S-H
The main hydration product of Mg/silicate cements is magnesium silicate hydrate (M-S-H), whose composition evolves with time and environmental interactions [refs 1,2], with Mg/Si ratios ranging from 0.67 to 1.5, variable water content, and potential Al incorporation. Atomistic models of M-(A)-S-H remain largely unexplored [ref 4], and most of their properties are still unknown, making it difficult to establish composition–property relationships.
This project aims to elucidate the atomic-scale structure of (alumino)silicate magnesium hydrates (M-(A)-S-H) by combining experimental techniques and atomistic simulations, and to estimate their mechanical properties. The study will focus on M-(A)-S-H compositions relevant to nuclear applications or new low carbon cement matrices.
Impact of Microstructure in Uranium Dioxide on Ballistic and Electronic Damage
During reactor irradiation, nuclear fuel pellets undergo microstructural changes. Beyond 40 GWd/tU, a High Burnup Structure (HBS) appears at the pellet periphery, where initial grains (~10 µm) fragment into sub-grains (~0.2 µm). In the pellet center, under high temperatures, weakly misoriented sub-grains also form. These changes result from energy loss by fission products, leading to defects such as dislocations and cavities. To study grain size effects on irradiation damage, nanostructured UO2 samples will be synthesized at JRC-K, using flash sintering for high-density pellets. Ion irradiation experiments will follow at JANNuS-Saclay and GSI, with structural characterizations via Raman spectroscopy, TEM, SEM-EBSD, and XRD. The postdoc project will take place at JRC-K, CEA Saclay, and CEA Cadarache under expert supervision.
Thermochemical and thermodynamic study of chloride molten salts
In today’s climate emergency, access to clean and cheap energy is more important than ever. Several ways have been envisaged for several years now, but a number of technological issues still need to be overcome before they can be put into practice, as they represent breakthroughts. Whether for energy storage than for fourth generation nuclear reactors, molten salt environment used as coolant and/or as fuel is highly corrosive requiring a complexe choice of structural materials.
The aim of this subject proposed in the Corrosion and Materials Behavior Section is to study in depth the chemical properties of different chloride molten salts : the basic ternary salt (NaCl-MgCl2-CeCl3) but also the corrosion/fission/activation products that can be produced (MxCly with M=Cr, Fe, Te, Nd, Ni, Mo,…). The activity coefficients and solubility limits of these metallic elements will be determined using various techniques such as electrochemistry and Knudsen cell mass spectrometry. If required, this study can be completed by the phase transition temperature and heat capacity measurements using differential scanning calorimetry.
Adapting the Delayed Hydride Cracking (DHC) experience to irradiated materials
The objective of this study is to nuclearize the Delayed Hydride Cracking (DHC) experiment developed as part of Pierrick FRANCOIS PhD research (2020-2023). This experiment enables the reproduction of the DHC phenomenon in Zircaloy cladding under laboratory conditions to determine the material's fracture toughness in case of DHC: KI_DHC.
The term "nuclearize" refers to the adaptation of the experiment to test irradiated materials within dedicated shielded enclosures (called hot cells), where materials are handled using remote manipulators. The experimental protocols described in Pierrick FRANCOIS' thesis must therefore be modified, and ideally simplified, to allow for their implementation in hot cells. This will require close collaboration with the personnel responsible for the tests and the use of numerical simulation tools developed during the same PhD research.
The development of this hot cell procedure will be used by the postdoctoral researcher to assess the risk of HC during dry storage of spent fuel assemblies by quantifying the fracture toughness of irradiated claddings.
Optimizing phytotechnologies for the remediation of contaminated nuclear sites
CEA is recruiting a postdoctoral researcher for a research project aimed at optimizing phytotechnologies for the remediation of contaminated nuclear sites. This research is part of the risk management and remediation of contaminated soils, in particular those resulting from the decommissioning of nuclear facilities. The aim of the project is to develop an advanced mechanistic model of soil-plant transfers, in order to gain a better understanding of contaminant mobility in lightly contaminated soils, and to optimize the use of suitable plants to stabilize these contaminants.
Modeling the corrosion behavior of stainless steels in a nitric acid media with temperature
Controlling the aging of equipment materials (mainly stainless steel) of the spent nuclear fuel reprocessing plant is the subject of constant attention. This control requires a better understanding of the corrosion phenomena of steels by nitric acid (oxidizing agent used during the recycling stages), and ultimately through their modeling.
The materials of interest are Cr-Ni austenitic stainless steels, with very low carbon content. A recent study on Si-rich stainless steel, which was developed with the aim of improving the corrosion resistance of these steels with respect to highly oxidizing environments [1 , 2 ]; showed that the corrosion of this steel was thermally activated between 40 °C and 142 °C with different behavior below and above the boiling temperature (107 °C) of the solution [3]. Indeed, between 40°C and 107°C, the activation energy is 77 kJ/mol and above boiling point, it is much lower and is worth 20 kJ/mol. This difference may be due to a lower energy barrier or a different kinetically limited step.
The challenge of this post-doctoral subject is to have a predictive corrosion model depending on the temperature (below and beyond boiling). With this objective, it will be important to analyze and identify the species involved in the corrosion process (liquid and gas phase) as a function of temperature but also to characterize the boiling regimes. This model will be able to explain the difference in activation energies of this Si-rich steel below and above the boiling temperature of a concentrated nitric acid solution but will also make it possible to optimize the processes of the factory where temperature and/or heat transfer play an important role.