Auto-adaptive neural decoder for clinical brain-spine interfacing
CEA/LETI/CLINATEC invite applications for postdoctoral position to work on the HORIZON-EIC project. The project goal is to explore novel solutions for functional rehabilitation and/or compensation for people with sever motor disabilities using auto-adaptive Brain-Machine Interface (BMI) / neuroprosthetics. Neuroprosthetics record, and decode brain neuronal signal for activating effectors (exoskeleton, implantable spinal cord stimulator etc.) directly without physiological neural control command pass way interrupted by spinal cord injury. A set of algorithms to decode neuronal activity recorded at the level of the cerebral cortex (Electrocorticogram) using chronic WIMAGINE implants were developed at CLINATEC and tested in the frame of 2 clinical research protocols in tetraplegics in Grenoble and in paraplegics in Lausanne. The postdoctoral fellow will contribute to the next highly ambitious scientific breakthroughs addressing the medical needs of patients. The crucial improvement of usability may be achieved by alleviating the need of constant BMI decoder recalibration introducing an auto-adaptive framework to train the decoder in an adaptive manner during the neuroprosthetics self-directed use. Auto-adaptive BMI (A-BMI) adds a supplementary loop evaluating from neuronal data the level of coherence between user’s intended motions and effector actions. It may provide BMI task information (labels) to the data registered during the neuroprosthetics self-directed use to be employed for BMI decoder real-time update. Innovative A-BMI neural decoder will be explored and tested offline and in real-time in ongoing clinical trials.
Simulation of a porous medium subjected to high speed impacts
The control of the dynamic response of complex materials (foam, ceramic, metal, composite) subjected to intense solicitations (energy deposition, hypervelocity impact) is a major issue for many applications developed and carried out French Atomic Energy Commission (CEA). In this context, CEA CESTA is developing mathematical models to depict the behavior of materials subjected to hypervelocity impacts. Thus, in the context of the ANR ASTRID SNIP (Numerical Simulation of Impacts in Porous Media) in collaboration with the IUSTI (Aix-Marseille Université), studies on the theme of modeling porous materials are conducted. They aim to develop innovative models that are more robust and overcome the theoretical deficits of existing methods (thermodynamic consistency, preservation of the entropy principle). In the context of this post-doc, the candidate will first do a literature review to understand the methods and models developed within IUSTI and CEA CESTA to understand their differences. Secondly, he will study the compatibility between the model developed at IUSTI and the numerical resolution methods used in the hydrodynamics computing code of the CEA CESTA. He will propose adaptations and improvements of this model to take into account all the physical phenomena that we want to capture (plasticity, shear stresses, presence of fluid inclusions, damage) and make its integration into the computation code possible. After a development phase, the validation of all this work will be carried out via comparisons with other existing models, as well as the confrontation with experimental results of impacts from the literature and from CEA database.
Modeling of the Fission gas behaviour in a 4th generation nuclear fuel at low power level
French alternative energies and atomic energy commission (CEA) is still studying a sodium fast reactor (SFR) core with intrinsic safety [1]. In this reactor core, low linear heat rate induce a significant fission gas retention in the fuel. It is mandatory to describe accurately the thermomechanics of this concept in order to confirm its safety.
Current model used in the CEA as fuel performance code for SFR, GERMINAL, is based on an empirical approach which the calibration database is centered on fuel pins irradiated at a high linear heat rate, and also a low gas retention. This fellow aims to extend to SFR fuels an existing gas model, MARGARET, which has been developed for the pressurized water reactor (PWR) fuels. On issue will be the restructuring phenomenon, which is far more relevant in SFR than in PWR, this topic is raised in [4].
First step of the work will consist in the integration of the MARGARET gas model in the GERMINAL code throughout the PLEIADES platform. This task will need to couple variables associated to the resolution of equilibriums in various physics (thermal, mechanical, and gas swelling) in order to build the coupling scheme.
Second step of the work will be focused on the analysis of the mechanisms contributing to the gas swelling, using the post-irradiation experiments realized in the CEA Cadarache facility (LECA - Laboratoire d’Examens des Combustibles Actifs). Image analysis tools would be used in order to characterize the porosity distribution in the fuel. Based on these observations, it will be necessary to make the calibration of the MARGARET model in order to give a good assessment of the gas swelling and of the porosity distribution. Depending on the results, a second year dedicated to the extension of this gas model for the power transients would be possible.
Computational statistics for post-flight analysis in atmospheric reentry
The post-doctorate corresponds to the context of flight tests of an instrumented vehicle (space shuttle, capsule or probe) which enters into the atmosphere. The aim is to reconstruct, from measurements (inertial unit, radar, meteorological balloon, etc.), the trajectory and various quantities of interest, in order to better understand the physical phenomena and to validate the predictive models. We focus on Bayesian statistics, associated with Markov chain Monte Carlo (MCMC) methods. The post-doctoral fellow will develop and extend the proposed approach and will benefit from a scientific collaboration with Audrey Giremus, professor at the University of Bordeaux and specialist in the field. We will in particular try to increase the performance of high dimensional sampling. Special attention will be paid to the machine learning issue of the exploitation of an aerological database. The final objective will consist in developping an evolving software prototype dedicated to the post-flight analysis of flight tests, that exploits the various sources of information. The evaluations will be based on simulated and real data, with comparison to existing tools. The collaboration work will lead to scientific communications and publications.
Development and application of Inverse Uncertainty Quantification methods in thermal-hydraulics within the new OECD/NEA activity ATRIUM
Within the Best Estimate Plus Uncertainty methodologies (BEPU) for the safety analysis of the Nuclear Power Plants (NPPs), one of the crucial issue is to quantify the input uncertainties associated to the physical models in the code. Such a quantification consists of assessing the probability distribution of the input parameters needed for the uncertainty propagation through a comparison between simulations and experimental data. It is usually referred to as Inverse Uncertainty Quantification (IUQ).
In this framework, the Service of Thermal-hydraulics and Fluid dynamics (STMF) at CEA-Saclay has proposed a new international project within the OECD/NEA WGAMA working group. It is called ATRIUM (Application Tests for Realization of Inverse Uncertainty quantification and validation Methodologies in thermal-hydraulics). Its main objectives are to perform a benchmark on relevant Inverse Uncertainty Quantification (IUQ) exercises, to prove the applicability of the SAPIUM guideline and to promote best practices for IUQ in thermal-hydraulics. It is proposed to quantify the uncertainties associated to some physical phenomena relevant during a Loss Of Coolant Accident (LOCA) in a nuclear reactor. Two main IUQ exercises with increasing complexity are planned. The first one is about the critical flow at the break and the second one is related to the post-CHF heat transfer phenomena. A particular attention will be dedicated to the evaluation of the adequacy of the experimental databases for extrapolation to the study of a LOCA in a full-scale reactor. Finally, the obtained input model uncertainties will be propagated on a suitable Integral Effect Test (IET) to validate their application in experiments at a larger scale and possibly justify the extrapolation to reactor scale.
Thermo-aeraulic numerical simulation of an incineration reactor
An incineration and vitrification process devoted to the treatment of apha contaminated organic/metallic wastes originating from MOX production facilities is currently under development at the LPTI laboratory (Laboratoire des Procédés Thermiques Innovants) from the CEA of Marcoule. The development program relies on full scale mock-up investigation tests as well as 3D numerical simulation studies.
The thermo-aeraulic model of the incinerator reactor, developed with the Ansys-Fluent commercial software, is composed of several elementary bricks (plasma, pyrolysis, combustion, particle transportation).
The proposed work consists in improving the model, in particular as regards the pyrolysis and combustion components : chemical reactions, unsteady process… The degree of representativeness of the model will be assessed on the basis of a comparative study using experimental data coming from experiments carried out on the prototype reactor. Besides this development work, various parametric studies will be performed in order to evaluate the impact of various reactor design modifications.
So as to investigate the radiologic behaviour of the reactor during incineration of alpha contaminated wastes, a particle transport model (DPM) associated to a parietal interaction model will be implemented. The simulation results will be compared to experimental data obtained from the analysis of deposits collected on reactor walls (experimental tests are performed with actinides inactive surrogates).
Development and optimization of adaptive mesh refinement methods for fluid/structure interaction problems in a context of high performance computing
A new simulation code for structural and compressible fluid mechanics, named Manta, is currently under development at the french CEA. This code aims at both unifying the features of CEA’s legacy implicit and explicit codes and being natively HPC-oriented. With its many numerical methods (Finite Elements, Finite Volumes, hybrid methods, phase field, implicit or explicit solvers …), Manta enables the simulation of various static or dynamic kinds mechanical problems including fluids, structures, or fluid-structure interactions.
When looking for optimizing computation time, Adaptive Mesh Refinement (AMR) is a typical method for increasing numerical accuracy while managing computational load.
This postdoctoral position aims at defining and implementing parallel AMR algorithms in a high performance computing context, for fluid/structure interaction problems.
In a preliminary step, the functionalities for hierarchical AMR, such as cell refinement and coarsening, field transfers from parents to children cells, refinement criteria or hanging nodes management, will be integrated in Manta. This first work will probably rely on external libraries that should be identified.
In a second step, the distributed-memory parallel performances will be optimized. Especially, strategies for load balancing between the MPI processes should be studied, especially for fluid/structure interaction problems.
Finally, especially for explicit in time computations, one will have to define and implement spatially adapted time stepping to cope with the several levels of refinement and the different wave propagation velocities.
These last 2 points will give rise to some publications in specialized scientific journals.
Micro-scale modelling of the mass transfer induced by a vaporization-condensation process in a ceramic material under thermal gradient
The post-doctoral work concerns the mechanism of mass transfer induced by evaporation-condensation under a thermal gradient. In nuclear fuels, the presence of porosities, the very high temperatures combined with the strong thermal gradient activate this evaporation-condensation phenomenon. This results in a displacement of porosities towards the central hot part and a transfer of material in the opposite direction towards the external cold part. This phenomenon is currently modeled by a 2D homogenized approach at the fuel pellet scale in which the material transfer is computed by solving the advection equation coupled to the heat equation by the finite element method.
The post-doctoral fellow will have to set up a microscopic modeling of the vapor phase transfer phenomenon. This work will allow to improve the simulation of free volumes associated to cracks and thus, to justify the assumptions of the velocity law of porosities migration used in the 2D homogenized model.
The work to be carried out is decomposed in two main steps which are on the one hand, the formulation and the numerical implementation of the constitutive equations of the microscopic model, and on the other hand, the justification of the homogenized model. The post-doctoral fellow will work at the CEA Cadarache site in the framework of a collaboration between the research teams of the Department of Fuel Studies and the IUSTI of Aix-Marseille University on the simulation of material transfer in the vapor phase under a thermal gradient. A major advance expected from this work is to take into account the evolution of the geometry of porosities, induced by the material transfer, with techniques for tracking the movement of solid-gas interfaces. The results will be valorized by publications in scientific journals and participation in conferences.
Use and extension of the Alien solver library with the proto-application Helix
First, the post-doc candidate will have to integrate the solver Library Alien into Helix to carry out performance and usability assessments in iterative or direct solver configuration. These assessments will be done on different computer architecture from desktop computer to national supercomputer with thousands of cores.
In a second time, the candidate will deal with the possibility to add new functionalities in the Alien library to solve non-linear systems composed with equations and inequations to be able to solve, in an HPC context, mechanical problems like phase field problem or contact problems, problems often still opened in the community. The results will be compared to the classical test cases and benchmarks of the state of the art in the domain.
The candidate will join the Helix development team, formed by 3/4 developers for the moment in the laboratory LM2S (15 persons). A transversal program between CEA directions finances the post-doc and the candidate will collaborate with the Alien library developers at the DAM of CEA.
Detection of cyber-attacks in a smart multi-sensor embedded system for soil monitoring
The post-doc is concerned with the application of machine learning methods to detect potential cyber-security attacks on a connected multi-sensor system. The application domain is the agriculture, where CEA Leti has several projects, among which the H2020 project SARMENTI (Smart multi-sensor embedded and secure system for soil nutrient and gaseous emission monitoring). The objective of SARMENTI is to develop and validate a secure, low power multisensor systems connected to the cloud to make in situ soil nutrients analysis and to provide decision support to the farmers by monitoring soil fertility in real-time. Within this topic, the postdoc is concerned with the cyber-security analysis to determine main risks in our multi-sensor case and with the investigation of a attack detection module. The underlying detection algorithm will be based on anomaly detection, e.g., one-class classifier. The work has tree parts, implement the probes that monitor selected events, the communication infrastructure that connects the probes with the detector, and the detector itself.