Crystalline materials for the selective extraction of monovalent metal cations: understanding the link between the crystalline structure and the selectivity

The selective extraction of monovalent metal cations from aqueous solutions have complex compositions is a key step in many energy-related fields. In this work, specific adsorbents for Cs, to decontaminate effluents produced by the nuclear industry, and for Li, to extract this strategic metal for the development of batteries, will be studied. Due to their modularity in terms of porosity and structure, crystalline oxides (as zeolites) are promising for the selective extraction of such cations. With a view to understand the role of their microstructure on their sorption/desorption performances and mechanisms, identify the selective sorption sites within these crystal structures is crucial.
For that purpose, the objective of this research work is, on the one hand, to synthesize crystal structures allowing the selective sorption of Cs or Li. Then, by using fine characterization techniques at the atomic scale as well as structures reconstruction effort, we will identify the location of selective sorption sites within these materials and, in this way, better understand their sorption mechanisms and properties.
For this post-doctoral position, we are looking for a PhD in material science with strong skills in synthesis and characterization of crystalline materials by X-ray diffraction. Experience in the study of crystalline oxides, such as zeolites, would be an advantage.

Conditioning of waste from a NaCl-MgCl2 molten salt reactor

In recent years, interest in molten salt nuclear reactors (MSR) has been revived in France and abroad, and the use of chlorinated salts is now being considered. Irrespective of the technological issues to their development, the credibility of this approach depends on the controlled management of the final waste produced during the operation of such reactors. This involves a conditioning stage, which needs to be developed in line with the nature of the waste in question.
The conditioning of two types of waste, resulting from the treatment of spent fuel from a NaCl-MgCl2 molten salt reactor according to different scenarios, is the subject of this post-doctorate, which will be structured in two distinct directions.
The first part of the post-doctorate is dedicated to the vitrification of solutions with complex compositions and enriched in magnesium compared to the usual vitrified fluxes. For this purpose, an aluminoborosilicate matrix is being considered. In order to validate the feasibility of such vitrification, it is essential to assess the microstructure, structure and chemical durability of the resulting glasses, with regard to the expected magnesium contents. Thus, a series of aluminoborosilicate glasses with variable magnesium content will be developed and characterized. The study of these glasses alteration in aqueous solution will be coupled with their structural characterization (Raman, RMN).
The second line of the post-doctorate is dedicated to the conditioning of chlorinated waste, in particular alkaline and alkaline-earth chlorides. In this case, the conditioning method currently favored is ceramization, and will be the subject of a bibliographic study. The chosen route(s) will be tested and characterized, and its containment performance determined.
Skills required: materials science, glass, ceramics, taste for experimentation. Knowledge of Raman will be appreciated.

Study of the decontaminating melting of end-of-life nuclear fuel claddings

Currently, in France, UOx fuel cladding is made of zirconium and tin or niobium based alloys. At the end of the use of nuclear fuels in the reactor, the fuel claddings contaminated with actinides (An), fission products (FP) and activation products (AP) are stored awaiting their storage in a geological site.

It is planned to decontaminate these claddings, once the spent fuel has been removed, to make them less radioactive, with the aim of decategorizing them.
Studies on radioactive sheaths have shown that the use of a fluoride-based slag allows decontamination of An and FP towards the slag. The scientific obstacle lies in the decontamination in AP, and more precisely in the preliminary stage of Zr/Nb separation.
In addition, the management of fluoride slags should be avoided due to the strong tendency of fluorine to corrosion. The use of oxide slags would be an alternative way to be privileged, constituting the second challenge of this research project.

The objectives of the post-doctorate will initially be to formulate slags. Then, Zr alloy decontamination, Zr/Nb separation and separation of activation products will be studied.

The experiments developed, in a non-radioactive laboratory, will be based on a literature review carried out by the post-doc and on the results of thermodynamic calculations performed at the LM2T of CEA Saclay.
The missions of the post-doc will be to develop slags according to the operating conditions that he/she will have defined (composition, temperature), to characterize them using physico-chemical analyzes (SEM/EDS, XRD, TDA/TGA) and to test them, in the presence of duct sections, for decontaminating melting. The post-mortem analysis will allow to assess their decontamination capacity and to optimize, if necessary, the interactions at the interfaces and/or their chemical composition.

Mitigation of Alkali Silica Reaction in concrete used for radwaste stabilization and solidification

Electricity production from nuclear power plants generates radioactive wastes, the management of which represents a major industrial and environmental concern. Thus, low- or intermediate - level radioactive aqueous waste streams may be concentrated by evaporation, and immobilized with a Portland cement, before being sent to disposal. Nevertheless, interactions may occur between some components of the waste and the cement phases or aggregates, and decrease the stability of the final waste forms. Thereby, the formation of a gel-like product has been recently observed on the surface of some cemented drums of evaporator concentrates which were produced in the 80’s in Belgium. This product results from a reaction between silica from the aggregates and the very alkaline pore solution of the concrete. However, its composition and rheological properties differ from those reported for alkali-silica gels in civil engineering. Extensive work has been performed to better understand the processes involved in the gel formation within the cement-waste forms and characterize its properties. Based on these results, the post-doctoral project will be focussed on the mitigation of alkali silica reaction in cement-waste forms. Two approaches will be more particularly investigated by decreasing the water saturation ratio of concrete and/or the pH of its pore solution using supercritical carbonation.
This project is intended for a post-doctoral fellow wishing to develop skills in materials science, with an interest in advancing the field of cement chemistry and improving the conditioning of radioactive waste. It will be performed in collaboration with ONDRAF-NIRAS, the Agency in charge of radioactive waste management in Belgium, and will build upon the expertise of two laboratories at CEA Marcoule: the Cements and Bitumen for Waste Conditioning Laboratory for materials elaboration and characterization, and the Supercritical and Decontamination Laboratory.

Rhelogical properties of molten crystallized glass

Formulation of nuclear waste conditioning glass results from a compromise between waste loading, glass technological feasibility and its long-term behavior. Up to now borosilicate glasses formulated at CEA and elaborated at La Hague plant by Orano to condition nuclear waste are homogeneous when molten. That means that today glass formulation is determined such as solubility limits of each constituting elements of waste aren’t exceeded in order to avoid phase separation (implying typically Mo, S, P) and/or crystallization (implying typically Fe, Ni, Cr, Zn, Al, Ce, Cs, Ti…) leading to a two-phase molten glass (liquid-liquid or liquid-solid).Today CEA would like to explore the impact of solid particles in suspension in the molten glass and in the final glass canister on respectively the glass technological feasibility and its long-term behaviour.
The proposed study focuses on the molten glass technological feasibility. The presence of solid heterogeneities in the melt is known to lead to a modification of some of its physical properties – notably its rheology, as well as thermal and electrical conductivities, and can generate settling phenomena. Yet these properties are in the heart of vitrification process control and modelling. This study will then investigate the impact of crystals in the molten glass on vitrification process control and modelling.

Modeling of the spent fuel alteration mechanisms in a water-saturated environment with temperature effect

Modeling the alteration of spent nuclear fuel in the eventuality of an underwater interim storage in pools or a deep geologic disposal is essential for long-term prediction. In the event of a failed spent fuel assembly, corrosion processes can lead to a deterioration of the failed rod and to a radionuclide release into water. A geochemical model coupling chemistry to transport (reactive transport) was the subject of first developments in connection with deep geological disposal conditions using the CHESS-HYTEC code developed by the Ecole des Mines de Paris. This model makes it possible to take into account the main alteration mechanisms and associated kinetics while relying on robust thermodynamic data. It remains important to pursue these developments by studying the effect of temperature between 20 and 70 °C. Adapting this model to other alteration conditions like an underwater of spent fuel in dedicated pools for several decades is also a short-term objective.

Advanced tandem time of flight mass spectrometry for microelectronic applications

The CEA LETI seeks to recruit a post-doctoral researcher to work on the development of advanced time of flight secondary ion mass spectrometry applications (TOF-SIMS). The candidate will work on a new TOF-SIMS instrument equipped with tandem MS spectrometry, in-situ FIB and Argon cluster sputtering. The research project will be focused around the following topics

• Developing methods to correlate TOF-SIMS with AFM, XPS and Auger
• Improving the sensitivity and efficiency of fragmention of the tandem MS spectrometer
• Developing 3D FIB-TOF-SIMS applications and improving the spatial resolution.
The candidate will also have access to the wide range of state of the art instruments present on the nanocharacterisation platform as well as bespoke samples coming from the advanced technology branches developed at the LETI. The candidate will also benefit from a collaboration with the instrument supplier.

Strudy and processing of C/SiC composites

For different applications, we are looking for materials having superior mechanical properties at high temperature (1000 ° C or higher) and that are resistant to oxidation. The family of ceramic matrix composite materials (CMC), especially C / SiC, seems the most relevant to our needs. However, it is necessary to conduct studies to determine the most efficient solutions among the wide variety of fibrous architectures and possible matrix microstructures, while taking into account the constraints related to available processes and targeted geometries. This work will be conducted in collaboration with other CEA laboratories.

Nano-silicon/graphene composites for high energy density lithium-ion batteries

This postdoctoral fellowship is part of the Graphene Flagship Core 2 H2020 european project (2018-2020) on the energy storage applications of graphene. In lithium-ion batteries, graphene associated to nanostructured silicon in a proper composite helps increase the energy capacity. Indeed graphene wraps silicon, reducing its reactivity with electrolyte and the formation of the SEI passivation layer. It also maintains a high electrical conductivity within the electrode.
The study will compare two technologies: graphene-silicon nanoparticles and graphene-silicon nanowires. The former composite, already explored in the above mentioned project, will be optimized in the present study. The latter is a new kind of composite, using a large scale silicon nanowire synthesis process recently patented in the lab. The postdoc will work within two laboratories: a technological research lab (LITEN) with expertise in batteries for transportation, and a fundamental research lab (INAC) with expertise in nanomaterial synthesis.
The postdoc will synthesize silicon nanowires for his/her composites at INAC. Following LITEN know-how, she/he will be in charge of composite formulation, battery fabrication and electrochemical cycling. He/she will systematically compare the electrochemical behavior of the nanoparticle and nanowire based silicon-graphene composites. Comparison will extend to the mechanism of capacity fading and SEI formation, thanks to the characterization means available at CEA Grenoble and in the European consortium: X-ray diffraction, electronic microscopy, XPS, FTIR, NMR spectroscopies. She/he will report her/his work within the international consortium (Cambride UK, Genova Italy, Graz Austria) meetings.
A 2-year post-doctoral position is open.
PhD in materials science is requested. Experience in nanocharacterization, nanochemistry and/or electrochemistry is welcome.
Applications are expected before May 31st, 2018.

Physisorption of chemical species on sensitive surfaces during transfer in controlled mini-environment in microelectronics industry

A characterization platform based on the connection concept between process and characterization tools through the use of a transfer box under vacuum was implemented allowing a quasi in-situ characterization of substrates (wafers) of the microelectronics. Currently, this transfer concept based simply on static vacuum inside a carrier box is satisfactory regarding the residual O or C on the surface of especially sensitive materials (Ge, Ta, Sb, Ti…) and the MOCVD layers growth on GST or III/V surfaces. Its optimization for more stringent applications (molecular bonding, epitaxy…) in terms of contamination surface prevention requires studies the understanding of the physico-chemical evolution of the surfaces.
The proposed work will be focused on physico chemical studies of the evolution and molecular contamination of surfaces during transfers and will take place in clean room. XPS, TD-GCMS and MS coupled to the carrier itself (to be implemented) will be used to address the sources (wall, seals, gaseous environment…) of the adsorbed chemical species implied and to determine the physisorption mechanisms on the substrates. The studied surfaces will be sensitive to the contaminants in such a way than the box environment impact will be extracted and studied parameters will be the nature of polymer seal used, the carrier box thermal conditioning, the vacuum level, the use of low pressure gaseous environment in the carrier (gas nature, pressure level…).