Modeling and dynamic studies of a space Nuclear Electric Propulsion system
Nuclear technology is key to enabling the establishment of scientific bases on the Moon or Mars, or for exploring deep space. Its use can take several forms (RTG, NTP among others), but this thesis focuses on Nuclear Electric Propulsion (NEP): heat produced by a nuclear reactor is converted into electricity to power an ionic propulsion engine. Various concepts have been studied in the past (PROMETHEUS, MEGAHIT and DEMOCRITOS, typically for Jupiter satellite exploration missions), while currently design studies are underway at CEA for a 100 kWe nuclear-electric NEP system.
The system of interest combines several specific design choices: uranium nitride fuel, direct gas cooling (helium-xenon mixture) and energy conversion system based on a Brayton cycle, as well as waste heat evacuation through thermal radiation. These choices address requirements to minimize mass and volume, and to ensure performance and reliability for the duration of the scientific mission. Analysis of the dynamic behavior of the nuclear-electric system is therefore crucial for project success. However, the issue of transient modeling of a complete spatial nuclear-electric system is very poorly addressed in the state of the art, especially for NEP.
The thesis objectives are therefore to research and develop physical models adapted to a NEP system, to propose an approach for their validation, and finally to implement them to analyze the dynamic behavior of the reactor and contribute to improving its design. Several mission phases will be studied: reactor startup in space, power variation transients for the ionic propulsion engine, reactor response in case of failure, and its potential shutdown with the problem of safe residual power evacuation.
The thesis will be conducted at IRESNE Institute (CEA Cadarache), in a stimulating scientific environment, and integrated into a team designing innovative nuclear reactors. CNES will also be involved in monitoring the work, particularly to define the ionic propulsion engine characteristics and exploration missions of interest for the nuclear-electric system. The thesis topic, combining modeling, fluid mechanics, thermodynamics, neutronics, and space mechanics, will lend itself to scientific communication and allow the development of key skills for an academic or industrial career.
Multi-physics modelling of a light water nuclear reactor operating under natural convection: study of innovative solutions for startup and power control
Among the most recent designs of water-moderated Small Modular Reactors (SMR), several concepts are characterized by natural convection in the primary circuit during normal and abnormal operation, with the aim of increasing the inherent safety of the design. The absence of primary pumps in this type of SMRs significantly complicates the start-up and power increase ramps. This requires the development of specific start-up procedures to heat up the primary water circuit and enable the reactor to reach its nominal conditions, in accordance with safety requirements. These kinds of procedures rely on simulations using validated models to understand the reactor behavior during these phases and define the accessible parameters domain.
The goal of this PhD project is to develop a numerical model capable of simulating the startup of an SMR operating in natural convection, and to contribute to the validation of this model. The PhD study also aims at developing a methodology for reactor control systems optimization, to attain a fast startup while remaining within the prescribed safety criteria.
The analysis of the reactor startup procedure entails two disciplines: thermal-hydraulics and neutronics, which requires the development of multi-physics coupled simulation tools. Three scientific calculation tools in particular will be coupled in the framework of the PhD study: CATHARE3 (reactor system thermal-hydraulics), FLICA5 (core thermal-hydraulics) and APOLLO3 (neutronics).
The PhD student will work in a team of neutron physicists and thermohydraulic engineers at the IRESNE Institute (CEA Cadarache). He/she will develop skills in nuclear reactor physics and modeling.
Diphasic thermoregulation system for ultra wide bandgap diamond semiconductors
The objective of this thesis is to study a diphasic thermoregulation system for ultra wide bandgap diamond semiconductors. One of the specific behavior of diamond semiconductors is the negative temperature coefficient of is on-state resistance. The thermoregulation proposed in this thesis aims to optimize the global losses of the system and to insure both temperature and electrical constraints between several diamond semiconductors in parallel.
Based on specifications that will be defined at the beginning of this theses (calories to dissipate, temperature range to control), the PhD candidate will have to:
- Define a temperature control strategy
- Define most appropriate materials and fluid of this system
- Design the thermoregulation system
- Realize and validate experimentally the proposed system
This thesis will tackle numerical simulation (component and thermoregulation system modelling) and experimental tests through the realization of a TRL 3-4 prototype of power converter system integrating diamond Schottky diodes.
The global objective to achieve is to put forward an innovative system modeled and experimentally demonstrated, where control strategy, dimensional and operative elements will be investigated and optimized.
Characterisation of reaction pathways leading to thermal runaway for new battery technologies
The development of all-solid-state cells is no longer a mere hypothesis today. As part of the Safelimove project, we assessed the safety of hybrid polymer cells of 1 Ah and 3 Ah, which led to a publication. Additionally, within the Sublime project, we evaluated the safety of 1 Ah sulfide-based cells (argyrodite), and a publication is currently being submitted.
With the arrival of these new cells, it becomes even more crucial to support their development with a detailed safety assessment and the identification of the complex mechanisms involved. Large-scale instruments such as synchrotrons and neutron reactors offer a powerful opportunity to achieve this goal, as they provide the best spatial and temporal resolutions. For example, thanks to fast X-ray radiography at ESRF, it is possible to visualize the inside of a cell during thermal runaway, thereby identifying the local impact of (electro)chemical reactions on the microstructure of components and validating our thermal runaway models. Moreover, with wide-angle X-ray scattering (WAXS), it is possible to monitor in situ the evolution of the crystalline structure of active materials during a very rapid thermal runaway reaction. Indeed, synchrotron radiation allows the acquisition of one diffractogram every 3 milliseconds. The neutron beam at ILL also enables us to track the evolution of lithium metal structure before, during, and after runaway. It is important to emphasize that these three techniques are currently mastered by the LAPS teams and have already led, or will lead, to publications.
Furthermore, new complementary techniques may be explored, such as studying the impact of thermal/mechanical stress on active materials using the BM32 beamline, or evaluating the oxidation states of metals via X-ray absorption spectroscopy (XAS) on ID26.
More conventional laboratory characterizations will also be carried out, such as DSC, TGA-MS, and XRD.
As part of our various collaborations, for the all-solid-state system, the active material of the positive electrode will most likely be NMC, or even LMFP in the event of supply difficulties. The electrolyte used will be sulphide-based, or even halide-based, while the anode will be composed of lithium metal or even a lithium alloy. If time permits, a post-Na-ion system will be considered from the second year onwards. Among other things, the thesis will aim to identify, based on the materials used, whether there are reactions prior to cathode destabilisation, whether the solid electrolyte reacts with the oxygen in the cathode or with the anode material, and whether these parallel reactions contribute to better or worse cell safety.
The three years of the PhD will be structured as follows: the first year will be dedicated to a literature review and the characterization of sulfide technology. Following the first milestones (1st CSI) and the evaluation of ongoing work on sulfides, the second year will focus either on sodium-ion technology or on further development of sulfide technology. Finally, the third year, in addition to the thesis writing, will concentrate more specifically on the impact of the identified materials on safety.
Mass transfers and hydrodynamic coupling: experimental investigation and models validation and calibration
With the energy transition and the paramount importance of the nuclear energy in this context, it is pivotal to understand the consequences of potential accident with core meltdown, as well as thinking about mitigation strategy.
During a nuclear severe accident with core meltdown a magma called corium can form a pool in the reactor lower head. The pool is not homogeneous and can stratify into multiple immiscible layers. The composition of the pool may evolve in time, due to progressive material assimilation. With the evolution of the global composition of the corium, the properties of the layers evolve. The vertical position of these layer may then change. This change comes with the creation of droplets from a layer which then cross the other one. The vertical order of the different layers as well as their movements have a significant impact on the heat fluxes imposed on the reactor vessel. A better understanding of these phenomena improves safety of both nowadays and future nuclear reactors.
Modelling work has been done, but it lacks validation and need calibration. Prototypical experiments (with actual materials present inside a reactor) are difficult to carry and are not foreseen in the near future. This PhD aims at experimentally studying the mass transfer between a droplet and its surrounding as well as the droplet creation. The planned experimental setup will use a water-based system which allow for local measurement. The goal is to validate, calibrate the existing model, and potentially create new ones. The final goal being to capitalize the work into PROCOR software platform. The experimental setup will be constructed and operated in LEMTA laboratory in University of Lorraine, where the student will work.
The PhD work will be mainly experimental but will also require software use for calibration, validation and for the design of the experimental setup. This work will be conducted in close collaboration between the laboratories LMAG in CEA/IRESNE (Cadarache) and LEMTA in University of Lorraine (Nancy). The student will work in LEMTA, where the experiments will be conducted, while being part of the CEA. The student will benefit from LEMTA’s expertise in building of experimental setup, transport phenomena in fluids and metrology, and from LMAG’s expertise in mass transfer, physical modeling and simulation in the scope of nuclear severe accidents. The student will regularly interact with CEA team which will follow the work closely. The student will therefore have to regularly go to CEA Cadarache.
The PhD student will be integrated to a dynamic environment comprised of researchers and other PhD students. The PhD candidate needs to be knowledgeable in transport phenomena, and needs to have a taste for experimental sciences.
Understanding the signals emitted by moving liquids
Elasticity is one of the oldest physical properties of condensed matter. It is expressed by a constant of proportionality G between the applied stress (s) and the deformation (?): s = G.? (Hooke's law). The absence of resistance to shear deformation (G' = 0) indicates liquid-like behavior (Maxwell model). Long considered specific to solids, shear elasticity has recently been identified in liquids at the submillimeter scale [1].
The identification of liquid shear elasticity (non-zero G') is a promise of discoveries of new liquid properties. For example, do we know that a confined liquid changes temperature under flow? Yet no classical model (Poiseuille, Navier-Stokes, Maxwell) predicts the effect because without long-range correlation between molecules (i.e. without elasticity), the flow is dissipative, therefore athermal. For a change in temperature to be flow induced (without a heat source), the liquid must have elasticity and this elasticity must be stressed [1,2].
The PhD thesis will explore how the mechanical energy of the flow is converted in a thermal response [2]. We will exploit the capacity of conversion to develop a new generation of microfluidic devices (patent FR2206312).
We will also explore the impact of the wetting on the liquid flow, and reciprocally, we will examine how the liquid flow modifies the solid dynamics (THz) of the substrate [3]. Powerful methods only available in Very Large Research Facilities such as the ILL will be used to probe the non-equilibrium state of solid phonons. Finally, we will strengthen our existing collaborations with theoreticians.
The PhD topic is related to wetting, macroscopic thermal effects, phonon dynamics and liquid transport.
1. A. Zaccone, K. Trachenko, “Explaining the low-frequency shear elasticity of confined liquids" PNAS, 117 (2020) 19653–19655. Doi:10.1073/pnas.2010787117
2. E. Kume, P. Baroni, L. Noirez, “Strain-induced violation of temperature uniformity in mesoscale liquids” Sci. Rep. 10 13340 (2020). Doi : 10.1038/s41598-020-69404-1.
3. M. Warburton, J. Ablett, P. Baroni, JP Rueff, L. Paolasini, L. Noirez, “Identification by Inelastic X-Ray scattering of bulk alteration of solid dynamics due to Liquid Wetting”, J. of Molecular Liquids 391 (2023) 123342202
Multi-modal in situ nuclear magnetic resonance analysis of electrochemical phenomena in commercial battery prototypes
Advancing electrochemical energy storage technologies is impossible without a molecular-level understand-ing of processes as they occur in practical, commercial-type devices. Aspects of the battery design, such as the chemistry and thickness of electrodes, as well as configurations of current collectors and tabs, influence the electronic and ionic current density distributions and determine kinetic limitations of solid-state ion transport. These effects, in turn, modulate the overall battery performance and longevity. For these reasons, optimistic outcomes of conventional ‘coin’ cell tests often do not converge into high-performance commercial cells. Safety concerns associated with high energy density and flammable components of batteries are another subject paramount for conversion from fossil to green energy sources.
Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) are exceptionally sensitive to the structural environment and dynamics of most elements in active battery materials.
Recently, plug-and-play NMR and surface-scan MRI methods have been introduced. In the context of fun-damental electrochemical research, merging two innovative complementary concepts within one multi-modal (NMR-MRI) device would enable diverse analytical solutions and reliable battery performance metrics for academia and the energy sector.
In this project, an advanced analytical framework for in situ analysis of fundamental phenomena such as sol-id-state ion transport, intercalation and associated phase transitions, metal plating dynamics, electrolyte deg-radation and mechanical defects in commercial Li- and Na-ion batteries under various operational conditions will be developed. A range of multi-modal (NMR-MRI) sensors will be developed and employed for deep analysis of fundamental electrochemical processes in commercial battery cells and small battery packs.