Geophysical site characterizations of European seismic monitoring stations and contributions to the development of best-practices for noninvasive site characterization methods
In-situ geophysical characterizations of earthquake recording stations are essential toward the effective use of ground motion records by the earthquake engineering community to mitigate seismic hazards. The current methods used for seismic site characterization rely on array-based recordings of surface-waves. These methods determine the site dispersion curves from surface-wave phase velocity versus frequency (or wavelength) relations before inverting the curves to model the site shear wave velocity (VS) versus depth profiles. VS profiles are then used to calculate the site’s time-averaged VS of the upper 30 m (VS30) for use by engineers in developing ergodic GM regression models, and/or in combination with the site GM recordings, to directly estimate the site-specific seismic response. In recent years, an important effort was initiated by several countries to improve the state-of-knowledge about geophysical site characterization methods with the aim toward robust and consistent characterizations (VS, VS30, etc.). Much work still remains to be done with respect to the characterization of site conditions at network stations and the improvement of the methods selected and their implementation. The proposed post-doc position will consist of participation in the current effort of seismic station characterization, as well as the optimization of their implementation: 1/ The post-doctoral candidate is expected to participate in geophysical surveys of seismic stations located in European, 2/ The candidate is expected to improve acquisition parameters in order to optimized subsequent surveys 3/ Finally, the candidate is expected to be involved within the COSMOS guidelines project and assist organizers within the project advancement and guidelines writing coordination.
Study of a transient regime of helium dispersion to simulate an accidental release of hydrogen from a fuel cell.
CEA and industrial partners want to improve their knowledge, models and risk mitigation means for the conséquences of an accidental release of hydrogen from a H2 Fuel Cell. The dispersion of helium as a replacement for hydrogen takes place in a private garage and the transient state will be studied. Different scenarios of release are considered: from a cubic idealized fuel cell, then with different aspect ratios and finally with varying main dimension. The goal is to study some scaling effects. For the first case, we will measure helium concentration with katarometres and possibly velocity fields with PIV methods. Then mitigation processes will be tested. At last comparisons with models and numerical simulations will be performed.
Structural characterization, reactivity and physico-chemical properties of Pu(IV) colloidal suspensions
Pu(IV) is known to be highly prone to hydrolysis leading to the formation of extremely stable Pu(IV) colloidal suspensions (known as intrinsic colloids). The lack of knowledge concerning the speciation and reactivity of these Pu colloids hinders the development of reliable models allowing to predict their behavior in industrial and environmental systems. The behavior of these colloidal species towards dissolution, complexation, or aggregation has been very poorly described in the literature. It thus appears essential to study and characterize Pu(IV) colloids and in particular their surface charge properties which ensure their stability and their interactions with their environment. This pioneering project in the nuclear field aims to study and characterize colloidal Pu(IV) suspensions whose size, concentration and dispersion medium can be controlled by our approaches. It comprises: (i) the preparation of intrinsic Pu(IV) colloidal suspensions and the study of their chemical and sonochemical reactivity; (ii) the electrophoretic characterization of various colloidal suspensions and the study of their behavior under the influence of an electric field; (iii) the characterization of their multi-scale structural properties by small and large angle scattering (SAXS / WAXS) coupled with EXAFS / XANES measurements (MARS line, SOLEIL synchrotron).
Analysis of low abundance 144Ce and 106Ru isotopes by mass spectrometry
The aim of this project is to develop the high precision analysis of 144Ce and 106Ru by mass spectrometry in irradiated samples for the qualification of neutronic calculation codes. These two isotopes are present at low abundances in the samples of interest and display significant isobaric interferences with 144Nd and 106Pd respectively. To complete this project, the candidate will carry out the appropriate analytical developments in conventional laboratory on inactive samples. Then the procedure will be transposed in the active laboratory for validation with the analysis of real samples. In the case of 144Ce, the implementation of a coupling between high performance liquid chromatography (HPLC) and ICPMS-MC, combined with the isotope dilution technique for the precise determination of atomic contents is envisaged. For 106Ru, the 101Ru concentration will first be determined by ICPMS-Q and the 101Ru/106Ru ratio will be determined by HPLC/ICPMS-Q or HPLC/ICPMS-MC coupling to remove the 106Pd/106Ru interference.
Simulation of supercritical helium flows in the cooling circuits of tokamaks
Future fusion reactors such as tokamaks (ITER, DEMO) will have to demonstrate the safety of their systems, validated by thermal hydraulic codes. To meet this requirement, the CATHARE code has been chosen as scientific computing tool. The work will consist in adapting the CATHARE code to helium at low temperatures and then to benchmark it with other thermal hydraulic codes used by the DRF (Direction de la Recherche Fondamentale), as well as with experimental data available at CEA Grenoble.
The study will be threefold. The first phase will be dedicated to a literature survey on the thermal hydraulics of helium, featured by closing equations for monophasic helium (friction and heat transfer coefficients). In a second step, the engineer will implement these laws in the code and perform some validations tests. The last part will focus a benchmark based on three applications: the study of a cryo-pump, the study of a supercritical helium discharge and the study of a superconducting cable.
Development of flexible solar panel for space application
Traditional solar panels used to power satellites can be bulky with heavy panels folded together using mechanical hinges. Smaller and lighter than traditional solar panels, flexible solar array consists of a flexible material containing photovoltaic cells to convert light into electricity. Being flexible, the solar array could roll or snap using carbon fiber composite booms to deploy solar panels without the aid of motors, making it lighter and less expensive than current solar array designs.
On the other hand, satellite trends are shifting away from one-time stints and moving towards more regular use in a constellation setting. In the last years, the desire increased to mass-produce low-weight satellites. Photovoltaic arrays companies are challenged on their capacity to face these new needs in terms of production capacity and versatility. And this is exactly where space photovoltaics can learn from terrestrial photovoltaics where this mass production and low-cost shift occurred years ago.
To tackle these new challenges, the Liten institute started to work on these topics two years ago. In the frame of this post-doc, we propose the candidate to work on the development of an innovative flexible solar panel architecture, using high throughput assembly processes. We are looking for a candidate with a strong experience in polymers and polymers processing, along with an experience in mechanics. A previous experience in photovoltaic will be greatly appreciated.
Numerical quality analysis of simulation codes with CADNA, Verificarlo and Verrou
Numerical codes rely on floating-point arithmetic to represent real numbers and the operations applied to them. However, in general, real numbers cannot be exactly represented by floating-point numbers. The finite precision of the floating-point arithmetic may lead to round-off errors that may accumulate. With the increasing computational power, the algorithm complexification and the coupling of numerical codes, it is crucial to quantify the numerical robustness of an application or an algorithm.
CADNA [1], Verificarlo [2] and Verrou [3] are dedicated tools that allow estimating the round-off errors propagation and measuring the numerical accuracy of the obtained results. The objective of this work is to use these three tools on GYSELA [4, 5], a simulation code used to characterize the plasma dynamics in Tokamak, and PATMOS [6], a mini-app representative of a Monte Carlo neutron transport code. This analysis will be aimed at assessing the numerical robustness of these two applications or some of their algorithms. In addition to the analysis of the numerical quality, these tools will also be used to see whether it is possible to lower the precision (simple or even half precision instead of double) of some algorithms, thus improving the memory footprint and/or performances (vectorization, communications). Beyond the lessons learnt on the two analyzed codes, a second objective will be the elaboration of a methodology that could be more generic and be applied more broadly to other codes.
Development of multiphysics tools dedicated to the modeling of FSR and associated studies.
The sodium group of DM2S (department of CEA Saclay) develops numerical coupling tools in order to realize accidental case studies (fast transient). The physical domains concerned are neutronics, thermo-hydraulics and mechanics. The subject of this post-doc deals within this framework.
The aim is to carry out several studies: the integration of a coupling within the CORPUS platform, to carry out studies in order to test (and introduce) in the coupling the impact of the deformation of the assemblies by the Temperature on the flow of liquid sodium, the use of the neutronic cross sections generated by the code APOLLO3, the study of other accidental cases, and extend the modeling to the subchannel and pin scales.
Development of a computational framework dedicated to model order reduction by certified reduced basis method.
Many engineering fields require to solve numerically partial differential equations (PDE) modeling physical phenomenon.
When we focus on a mathematical model that describes the physical behavior of a system based on one or more parametrized PDEs (geometrical or physical parameters), it may be desirable to rapidly and reliably evaluate the output of the model (quantity of interest)
for different parameter values.
The real-time context, needed to perform command-control, and contexts asking many evaluations of model outputs (typically for optimization methods or uncertainty and sensitivity analysis) lend themselves perfectly.
The certified reduced basis method is an intrusive reduction method beacause, unkike non-intrusive methods, the reduction is based on the projection of operators associated to physical model PDEs.
This method allow to obtain rapidly, for a given set of parameter values, an approximation of the evaluation of the model output.
One of the strengths of the method is the "certified" aspect to estimate the approximation error of the model output evaluation.
The goal of the post-doctorate is to develop a computational framework for the certified reduced basis method. This framework should be based on the TRUST platform (https://sourceforge.net/projects/trust-platform/) developed at CEA and will be generic enough to be used to deal with different types of problems (linear or not, stationary or not, coercive or not...)
The framework will be used in the case of a two-fluid mixing model.