Cluster dynamic simulations of materials under irradiation
Alloys used in nuclear applications are subjected to neutron irradiation, which introduces large amounts of vacancy and interstitial defects. Over time, these defects migrate, recombine and agglomerate with minor alloying elements to form small clusters. This affects the mechanical properties of ferritic steels and weakens them. In this context, the microstuctural evolution is to be simulated using the rate equation cluster dynamic method. However, this approach becomes ineffecient when several minor alloying elements need being taken into account. The difficulty comes from the huge number of cluster variables to describe. The project aims at optimizing the code efficiency on a distributed parallel architecture by implementing parallelized vector and matrix functions from SUNDIALS library. This library is used to integrate the ordinary differential equation describing the reactions between clusters. Another aspect of the work is more theoretical and involves reformulating the non-linear root-finding problem by taking advantage of the reversibility of most chemical reactions. This property should facilitates the implementation of direct and gradients iterative sparse solvers for symmetric definite positive matrices, such as the multi-frontal Cholesky factorization and the conjugate gradient methods, respectively. One avenue of research will consists of combining direct and iterative solvers, using the former as a preconditioner of the latter.
Simulation of PEMFC flooding phenomena
The proton exchange membrane fuel cell (PEMFC) is now considered as a relevant solution for carbon-free electrical energy production, for both transport and stationary applications. The management of the fluids inside these cells has a significant impact on their performance and their durability. Flooding phenomena due to the accumulation of liquid water are known to impact the operation of the cells, causing performance drops and also damages that can be irreversible. With the use of thinner channels in ever more compact stacks, these phenomena are becoming more and more frequent. The objective of this post-doc is to progress in the understanding of flooding in PEMFCs. The work will consist in analyzing the link between the operating conditions, the design of the channels and the materials used in the cell. It will be based on a two-phase flow modeling approach at different scales, from the local scale at the channel-rib level, up to, via an upscaling approach, the level of the complete cell. The study will also be based on numerous experimental results obtained at the CEA or in the literature.
High entropy alloys determination (predictive thermodynamics and Machine learning) and their fast elaboration by Spark Plasma Sintering
The proposed work aims to create an integrated system combining a computational thermodynamic algorithm (CALPHAD-type (calculation of phase diagrams)) with a multi-objective algorithm (genetic, Gaussian or other) together with data mining techniques in order to select and optimize compositions of High entropy alloys in a 6-element system: Fe-Ni-Co-Cr-Al-Mo.
Associated with computational methods, fast fabrication and characterization methods of samples (hardness, density, grain size) will support the selection process. Optimization and validation of the alloy’s composition will be oriented towards two industrial use cases: structural alloys (replacement of Ni-based alloys) and corrosion protection against melted salts (nuclear application)
Design of new extractant molecules for uranium and plutonium separation
The subject of this postoctoral position is related to the optimization of the process used to separate uranium and plutonium from spent nuclear fuels. In the so called PUREX process currently in operation at La Hague reprocessing plant in France, the TBP (tri-n-butylphosphate) is used as extractant in the solvent extraction system. This molecule shows high affinity for uranium and plutonium at oxidation states VI and IV and allows to reach high decontamination factors versus fission products. Nevertheless, the separation of U from Pu requires the use of reducing and anti-nitrous reagents to allow the back-extraction of Pu(III). In order to improve the process, researches are under way to design new extractant molecules which would allow to separate U and Pu without redox chemistry and with high selectivity versus fission products (Ru, Tc, Cs, lanthanides, etc) and other actinides (especially Np). The objective of the postdoctoral associate will be to select the molecules, to determine synthesis routes and to perform their synthesis using equipment available in the LCPE laboratory (micro-wave, flash chromatography, NMR, HPLC-MS, GC-HRMS) at the CEA Marcoule.
Continuum models calibration strategy based on a 3D discrete approach
In order to develop an identification strategy for continuum constitutive models devoted to quasi-brittle materials, suited for structural analysis, often realized arbitrarily, a model based on the discrete element method has been formulated. The discrete model is used to compensate the lack of experimental data required to calibrate the continuum model. Thanks to intrinsic predispositions with respect to fracture mechanisms, the discrete model can be used easily, and its efficiency has been proved. However, only 2D simulations have been undertaken so far, mostly due to computational costs limitations.
A 2D framework reduces extensively analysis possibilites with such model, in particular for reinforced structures where 3D effects are predominant. The purpose of the present post-doctoral work is to extend to 3D the discrete approach already developped in 2D. The developments will be integrated in the FEA code CAST3M-CEA developped by DEN/DANS/DM2S/SEMT. In the mean time, the discrete model will be optimized using available tools, such as solvers, available in the CAST3M-CEA environment. Depending on the computational costs improvements, even complete structures simulations might be considered.
At the end of this work, the developed numerical tool will allow to extend the identification stragegy to constitutive models including 3D effects, such as steel/concrete interface models (confinement) and concrete model (dilatancy).
Improvement of microfluidic tools for kinetic data measurement
The development and modeling chemical processes require the acquisition of many thermodynamic and kinetic data . Conventional methods for measuring these data generally involve significant amounts of reagents. In particular for the reactive crystallisation, where the stochastic nature of nucleation requires the realization of a large number of experiments . The subject is to continue the work already done on the development of a dedicated chip to measure rapid nucleation kinetics . Firstly , the validity of kinetic measurements obtained by microfluidics technique will be evaluated and optimized based on well known and non- radioactive chemical systems . The microfluidic tool will then be used to study the sensitivity of these reactions to various operating parameters ( supersaturation , impurities , additives, etc. . ), before considering its transposition to nuclear processes such as decontamination of radioactive effluents. Finally, a new chip design could be proposed for the measurement of kinetics of liquid-liquid extraction , in connection with the development of new hydrometallurgical processes.
Development of a compact XRF for online analysis dedicated to process monitoring.
X-ray fluorescence (XRF) spectrometry is a well-known analytical technique for elemental analysis in an industrial context. In a simplified way, this technique is based on the measurement of X-radiation characteristics that are emitted by the atoms rearranging their electron cloud following an external stimulus. This is a non-destructive measurement relevant for the determination of chemical elements within liquid and solid mixture. In the 90s, the work conducted by the CEA has shown the relevance of XRF for the measurement of heavy elements using L-edge, (U, Pu, Am, Np, Cm, Pb) as well as lighter ones (Zr, Mo, Sr) using K-edge. Low detection limits (few mg/l) have been reached and the method has been implemented industrially for monitoring several processes (for instance at La Hague plant). However, operating a XRF requires heavy and cumbersome equipment, especially a nitrogen-cooled detector and a large X-Ray generator.
Recently the technology has been significantly improved on two key issues:
• The X-rays sources, which were miniaturized,
• The detectors thanks to new type of semiconductor of small volumes, operating at room temperature with a convenient spectral resolution (CdZnTe crystals for instance).
In this framework, the proposed subject concerns new R&D studies on potentialities offered by these improvements, regarding two application fields:
• On-line monitoring in reprocessing process.
• Screening of the contaminant in the polluted soils before remediation in a decommissioning context
Multiscale Modelling of Radiation Induced Segregation
Irradiation produces in materials excess vacancies and self-interstials that eliminate by mutual recombination or by annihilation at sinks (surfaces, grain boudaries, dislocations).
It sustains permanent fluxes of point defects towards those sinks. In case of preferential transport of one componant of an alloy, the chemical composition is modified in the vicinity of the sinks: a Radiation Induced Segregation (RIS). Its modelling requires a good description of the alloy properties: its driving forces (derived from the thermodynamics) and its kinetic coefficients (the Onsager matrix). The objectif on this project is to combine (i) atomic models (Kinetic Monte Carlo simulations and Self-Consistent Mean Field), fitted on ab initio calculations, that provide the Onsager coeffcients and the driving forces and (ii) a Phase-Field modelling that will give a description of the evolution of the alloy under irradiation at much larger time- and space-scales. The approach will be applied to Fe-Cr and Fe-Cu alloys, already modelled at the atomic scale. RIS will be first modelled near grain boundaries, then near dislocation loops. Special attention will be paid to the effect of elastic stresses on the RIS.
Optimal management of a tertiary energy system
In the solution concerning residential or tertiary sites that consume and produce electrical energy , the objective is to optimize the use of energy based on economic criteria or constraints networks (adaptation of the consumption) without introducing perturbations of user comfort. The purpose of this position is to develop a solution for "optimal management of the use of solar energy in a tertiary building integrating EV charging stations and storage." according to three objectives:
- Minimize the cost of consumption based on a dynamic tarif - Maximize the use of solar energy - Minimize the power demand of the network. Taking into account the LCOS (levelised Cost Of Storage) of battery . The Post- Doc will contribute and participate in: - Specification of tertiary system - Development of algorithms for managing a tertiary system - Deploy and test the proposed solution.
Development of methods for U quantification in cells after exposure to uranium
This project fits into the transverse Toxicology Program, led by CEA, whose purpose is to address by multidisciplinary approaches, the potential effects on living organisms of elements of strategic interest to the CEA. The objective is to provide some understanding on the mechanisms of uranium toxicity and behavior, in connection with its speciation in cells. Indeed, the radionuclides speciation governs their bioavailability, accumulation, biodistribution, toxicity, detoxification mechanisms and their interaction at the molecular level.
The post-doctoral project (12 months) consists in:
- Developing methods to quantify U accumulated in the cells as well as endogenous content of trace elements after exposure of cells to uranium.
- Developing methods to determine the precise isotopic composition of U in the cells after their exposure.
The candidate will be in charge of developing chemical purification and measurement methods for precise elemental and isotopic analyses. The analyses will be performed using inductively coupled plasma quadrupole mass spectrometer (ICP- MS Q) or inductively coupled plasma multi- collection mass spectrometer of the latest generation (ICP- MS MC), to achieve the lowest level of uncertainties.