Turbulence synthetization methods in porous media from detailed simulations for multi-scale simulations of nuclear cores
The production of electricity through nuclear energy plays a crucial role in the energy transition due to its low carbon impact. To continuously improve safety and performance, it is essential to develop new knowledge and tools.
The core of a nuclear reactor consists of thousands of fuel rods traversed by a turbulent flow. This flow can cause vibrations, leading to wear. Two flow scales are identified: a local scale, where the fluid interacts with the rods, and a global scale, representing the flow distribution within the core. The local scale requires CFD simulations and fluid-structure coupling, while the global scale can be modeled using averaged approaches, such as porous media simulations.
Coupled fluid-structure interaction (FSI) simulations at the CFD scale are limited to small domains. To overcome this limitation, multi-scale approaches are required, combining large-scale porous media simulations and detailed small-scale CFD simulations. The goal of the thesis is to develop methods for synthesizing turbulence from the results of porous media simulations to improve boundary conditions for CFD simulations. The candidate will first study how existing turbulence models can provide details on turbulent flow at the component scale, and then how to synthesize turbulence for local CFD simulations.
This PhD project is the subject of a collaboration between the IRESNE Institute (CEA) and the ASNR (main execution site of the thesis) in Cadarache. Funding is provided by a MSCA Doctoral Network. The PhD student will be integrated into a network of 17 PhD students. To be eligible, the candidate must have resided no more than 12 months in the last 36 months in France.
Mass transfers and hydrodynamic coupling: experimental investigation and models validation and calibration
In the context of the energy transition and the crucial role of nuclear power in a low-carbon energy mix, understanding and then mitigating the consequences of any accident leading to a reactor core meltdown, even a partial meltdown, is an imperative research direction.
During a core meltdown accident, a pool of molten material, known as corium, can form at the bottom of the reactor vessel. The composition of the pool can change over time. The corium bath is not homogeneous and can stratify into several immiscible phases. As the overall composition of the corium changes, so do the properties of the different phases. The vertical stratification order of the phases may change, leading to a vertical rearrangement of the phases. During this rearrangement, one phase passes through the other in the form of drops. The order of the phases and their movements are of prime importance, as they have a major influence on the heat flows transmitted to the tank. A better understanding of these phenomena will enable us to improve the safety and design of both current and future reactors.
Initial models have already been produced, but they lack validation and calibration. Prototype experiments are difficult to set up and none are planned in the short term. This thesis proposes to fill this gap by carrying out an experimental study of the phenomenon using a water-based simulating system that allows local instrumentation and large-scale test campaigns. The aim is to validate and calibrate the existing models, and even develop new ones, with a view to capitalising on these results in the PROCOR software platform, which is used to estimate the probability of a reactor vessel breach. The experimental set-up would be built and operated at the LEMTA laboratory at the University of Lorraine, where the PhD student would be seconded. In terms of experiments, two cases will be studied, the single drop case, and the stratified case with drop formation via Rayleigh-Taylor instabilities.
The work will be mainly experimental, with a component involving the use of code for calibration and validation, and may include a modelling component. It will be carried out entirely at the LEMTA laboratory in Nancy. The PhD student will benefit from LEMTA's expertise in the development of simulating experimental devices, fluid transfers and metrology. They will be part of a dynamic environment made up of researchers and other PhD students. The candidate should have knowledge of transfer phenomena (mass transfer in particular), as well as a definite interest in experimental science.
Multiphysic modeling of sintering of nuclear fuel pellet: effect of atmosphere on shrinkage kinetics
Uranium dioxide (UO2) fuels used in nuclear power plants are ceramics, for which solid-phase sintering is a key manufacturing step. The sintering stage involves heat treatment under controlled partial O2 pressure that induces coarsening of UO2 grain and then consolidation and densification of the material. Grain growth induce material densification and macroscopic shrinkage of the pellet. If the green pellet (powder obtained by pressing, manufacturing step before sintering) admit a highly heterogeneous density, this gradient leading to differential shrinkage and the appearance of defects. Furthermore, the sintering atmosphere, i.e., the gas composition in the furnace, impacts grain growth kinetics and thus the shrinkage of the pellet. Advanced simulation is the key to improving understanding of the mechanisms observed as well as optimizing manufacturing cycles.
The PhD thesis aims at developing a Thermo-chemo-mechanical modeling of sintering to simulate the impact of the gas composition and properties on the pellet densification. This scale will enable us to take into account not only the density gradients resulting from pressing, but also the oxygen diffusion kinetics that have a local impact on the densification rate, which in turn impacts the transport process. Therefore, a multiphysics coupling phenomenon has to be modelled and simulated.
This thesis will be conducted within the MISTRAL joint laboratory (Aix-Marseille Université/CNRS/Centrale Marseille CEA-Cadarache IRESNE institute). The PhD student will leverage his results through publications and participation in conferences and will have gained strong skills and expertise in a wide range of academic and industrial sectors.