Multiphysics modeling of an experimental sintering furnace
In the scope of the development and improvement of the performance of low-carbon energy sources, the CEA has a software platform for modeling the behavior of nuclear fuel from its manufacture to its use in the reactor. Sintering, a key step in fuel fabrication is the heat treatment process used to consolidate and densify nuclear fuel to form the solid solution U1_yPuyO2-x. The sintering cycle generally comprises a rise in temperature with a linear ramp, a constant temperature plateau and a controlled cooling, with possibly a continuous adaptation of the oxygen potential through the oxidation-reduction buffer imposed by the H2 over H2O ratio of the carrier gas to reach the target oxygen-metal ratio. A first modeling of an industrial sintering furnace was carried out using the OpenFOAM software suite and the C++ finite elements library DIFFPACK. A second step aims to validate the models used in the simulation of this industrial furnace based on a separate effects approach and the modeling of a laboratory sintering furnace. This post-doctorate will be carried out at CEA Cadarache within the multiscale modeling laboratory (LM2Cà of the fuel studies department. This work will be carried out in close collaboration with the teams of experimenters from the Solid Chemistry and Actinide Materials Development Laboratory (LSEM) of CEA Marcoule who are developing and operating the experimental furnace. The collaboration will focus on the modeling input data (furnace geometry, temperature and atmospheric conditions) and the measurements to be compared with the simulation data. The post-doctoral student will evolve in a stimulating environment, within a dynamic laboratory where about fifteen doctoral and post-doctoral students are already working, in contact with experts in fuel physics modeling and in collaboration with experimenters. The work can be enhanced by presentations at conferences and the writing of articles.
Numerical studies of laser plasma interaction in intermediate field on Laser Megajoule
In the Inertial Confinement Fusion experiments (ICF), intense laser beams cross a gas filled hohlraum. The gas is fully ionized and laser beams then propagate into a sub-critical plasma where laser plasma instabilites can develop. Optical smoothing techniques enable to break both spatial and temporal coherences so that both spatial and temporal scales of the beam become smaller than those required for the development of the instabilites. The breaking of spatial coherence is done thanks to the use of a phase plate which spreads the laser energy in a multitude of light grains called speckles. The breaking of temporal coherence is done by using a phase modulator which widens the spectrum and by dispersing each frequency with a grating. It is essential to know the statistical properties of speckles (width, lenght, contrast, coherence time, velocities ...) to be able to predict the instabilities levels which can depend on time and on the distance of propagation of the beam. .
For the sake of simplicity, the laser plasma instabilities are very often studied at the best focus of the beam. However, in the FCI experiments, laser beams are focused near the laser entrance hole of the hohlraum whose length is about 1 cm. The development of instabilities can then occur before the best focus (outside the hohlraum) and mainly beyond the best focus (far inside the hohlraum). The goal of this post-doctoral contract is to study the development of instabilities when it occurs in the intermediate field (far from the best focus of the beam) and to assess the efficiency of different smoothing options on Lase MagaJoule (LMJ) to limit these instabilities. We will especially study propagation instabilities (self-focusing, forward stimulated Brillouin scattering) and stimulated Brillouin backscattering. This work will be done thanks to numerous existing numerical codes and diagnostic tolls.
Natural convection at high Ra numbers for nuclear safety: 2nd year
Thermal exchanges at very high Rayleigh numbers (Ra) exist on geophysical scale, at civil engineering scale and increasingly in industrial applications and here particularly in the energy sector. At this point, we mention the cooling of solar panels or the heat removal from nuclear power plants under accidental conditions. In fact, the passive safety concept of Small Modular Reactors (SMR) is based on the transfer of residual heat from the reactor to a water pool in which the reactor is placed. Since the outer reactor vessel is very high, heat exchange occurs by natural convection at Rayleigh numbers (Ra) between 1010 and 1016. Reliable heat transfer correlations exist to date only up to about Ra < 1012 with very high uncertainties in the extrapolation to higher Ra. Understanding the heat transfer at very high Ra is thus of fundamental and practical interest. The associated challenges are twofold:
• Numerical challenges: CFD codes cannot model turbulent heat transfer at very high Ra with sufficient accuracy and appropriate calculation time. Improved physical and numerical models are required, which use high performance computing (HPC) capabilities.
• Experimental challenges: Detailed experiments are essential for code validation. Since experiments in water require impractical huge dimensions, cryogenic experiments with helium are planned at CEA, based on the interesting physical properties of this fluid in the range of 5 K (high thermal expansion associated to low viscosity and thermal conduction).
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.
Design of 2D Matrix For Silicum Quantum computing with Validation by Simulation
The objective is to design a 2D matrix structure for quantum computing on silicon in order to consider structures of several hundred physical Qubits.
In particular the subject will be focused on:
- The functionality of the structure (Coulomb interaction, RF and quantum)
- Manufacturing constraints (simulation and realistic process constraint)
- The variability of the components (Taking into account the variability parameter and realistic defectivity)
- The constraints induced on the algorithms (error correction code)
- Scalability of the structure to thousands of physical Qubits
The candidate will work within a project of more than fifty people with expertise covering the design, fabrication, characterization and modeling of spin qubits as well as related disciplines (cryoelectronics, quantum algorithms, quantum error correction, …)
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.