Hydrogen and ammonia combustion within porous media: experiments and modelling
- Context
Current energy prospects suggest the use of hydrogen (H2) and ammonia (NH3) as carbon-free energy carriers to achieve neutrality by 2050. NH3 offers advantages like high energy density and safe storage but faces combustion challenges such as narrow flammability and high NOx emissions. Interestingly, some H2 can be obtained by partial cracking of NH3 to create blends of more favourable combustion properties, with open questions regarding pollutant emissions and unburnt NH3 content.
- Challenges
Porous burners show promise for safe and low-pollutant combustion of NH3/H2 blends. However, material durability issues and the complexity of flame stabilization pose significant hurdles. Fortunately, recent advances in additive manufacturing enable the precise tailoring of porous matrices, but the experimental characterization remains difficult due to the opacity of the solid matrix.
- Research objectives
The PhD candidate will operate an experimental bench at CEA Saclay to conduct combustion experiments with NH3/H2/N2+air mixtures in various porous burners. Key tasks will include designing new burner geometries, comparing experimental results with numerical simulations, and advancing the modelling of porous burners using 1D Volume-Averaged Models and asymptotic theory. Experimental measurements will include hotwire anemometry, infrared thermometry, output gas composition analysis, chemiluminescence, and laser diagnostics. The porous burners will be manufactured using 3D printing techniques with materials such as stainless steel, inconel, alumina, zirconia, and silicon carbide.
The research aims to develop more robust and efficient porous burners for NH3/H2 combustion, enhancing their practical application in achieving carbon neutrality. The candidate will contribute to advancing the field through experimental data, innovative designs, and improved modelling techniques.
Modeling of the fall of a drop in a volume, in support of the system code CATHARE
This thesis focuses on the study of droplet fall in free volumes, as part of the continuous improvement of the physical models in the CATHARE code, used for safety studies of Pressurized Water Reactors. The current models are based on the work of Ishii and Zuber, who model the fall velocity of droplets in a two-phase fluid. The objective of the thesis is to refine the precision of this model by incorporating additional parameters and validating it through experiments such as those of Dampierre and CARAYDAS. The PhD candidate will be required to develop a more representative mechanistic model, based on experimental data or CFD simulations if necessary. The innovation lies in developing a more accurate model of droplet fall processes, paving the way for specific applications such as spray modeling, and thus contributing to the validation of the CATHARE code in additional fields.
Study of of the thermodynamic of K2CO3-CO2-H2O for the development of NET and SAF technologies
.Bioenergy with Carbon Capture and Storage (BECCS) uses biomass energy while capturing the carbon dioxide released by the process, resulting in negative emissions into the atmosphere. The reference process in Europe uses potassium carbonate but at atmospheric pressure [1], whereas its sequestration or hydrogenation into sustainable molecules requires high pressures.
The thesis consists in acquiring new thermodynamic and thermo-chemical data at high temperature/pressure [2] required for the energy optimization of such a process, and integrating them into a thermodynamic model.
The overall process will then be reassembled in order to quantify the expected energy gain.
The thesis will be carried out at the Thermodynamic Modeling and Thermochemistry Laboratory (LM2T), in collaboration with LC2R (DRMP/SPC) for the experimental part.
References :
[1]K. Gustafsson, R. Sadegh-Vaziri, S. Grönkvist, F. Levihn et C. Sundberg, «BECCS with combined heat and power: assessing the energy penalty,» Int. J. Greenhouse Gas Control, vol. 110, p. 103434, 2021.
[2] S. Zhang, X. Ye et Y. Lu, «Development of a Potassium Carbonate-based Absorption Process with Crystallization-enabled High-pressure Stripping for CO2 Capture: Vapor–liquid Equilibrium Behavior and CO2 Stripping Performance of Carbonate/Bicarbonate,» Energy Procedia, 2014
Effect of microstructure and irradiation on susceptibility to intergranular cracking of alloy 718 in PWR environment.
Alloy 718, a nickel-based alloy, is used in fuel assemblies for pressurized water reactors (PWRs). In service, these components are subjected to high mechanical stress, neutron irradiation and exposure to the primary environment. Usually, this alloy shows very good resistance to intergranular cracking. However, there are microstructural and/or irradiation conditions which, by modifying the mechanical properties and plasticity mechanisms, make the material susceptible to intergranular cracking in the primary PWR environment.
In this context, the aim of this thesis will be to study the influence of microstructure (via different heat treatments) and irradiation on deformation localization and susceptibility to intergranular cracking in primary PWR media.
To this end, two grades will be tested, one deemed sensitive and the other not. In-situ SEM tensile tests on samples whose microstructure has been previously characterized by EBSD will be carried out to identify the types of intra- and intergranular deformation localization and their evolution. The non-irradiated state will be characterized as the reference state. In addition, exposure and intergranular cracking tests in the primary medium (coupons, slow tension, etc.) will be carried out on both grades and at different irradiation levels. The microstructure as well as surface and intergranular oxidation of the specimens will be characterized by various microscopy techniques (SEM, EBSD, FIB and transmission electron microscopy).
This thesis constitutes for the candidate the opportunity to address a problem of durability of metallic materials in their environment following a multidisciplinary scientific approach combining metallurgy, mechanics and physico-chemistry and based on the use of various cutting-edge techniques available at the CEA. The skills that he will thus acquire can therefore be valued during the rest of his career in the industry (including non-nuclear) or in academic institutions.
Development of a transport chemistry model for spent fuel in deep geological disposal under radiolysis of water
The direct storage of spent fuel (SF) represents a potential alternative to reprocessing as a means of managing nuclear waste. The direct storage of spent fuel in a deep geological environment presents a number of scientific challenges, primarily related to the necessity of developing a comprehensive understanding of the processes involved in the dissolution and release of radionuclides. The objective of this thesis is to develop a comprehensive scientific model that can accurately describe the intricate physico-chemical processes involved, such as the radiolysis of water and the interaction between irradiated fuel and its surrounding environment. The objective is to propose an accurate reactive transport model to enhance long-term predictions of storage performance. This thesis employs a back-and-forth process between modeling and experimentation, with the goal of refining the understanding of alteration mechanisms and validating hypotheses with experimental data. Based on existing models, such as the operational radiolytic model, the work will propose improvements to reduce the current simplifying assumptions. The candidate will contribute to major industrial and societal issues related to nuclear waste management and will help to provide solutions to the associated safety issues.
Study of wire additive manufacturing of a nuclear component with complex geometry
The general aim of the thesis is to study the feasibility of a component for the DEMO fusion reactor using Wire Additive Manufacturing (WAM). To achieve this, the PhD student will first design and manufacture demonstration parts representative of different sub-parts of the component in the laboratory's additive manufacturing cells. He or she will use CAD/CAM software to manufacture parts of increasing size and complexity, while ensuring repeatability.
These parts will be subjected to characterization work, firstly dimensional, to check their geometric conformity with the project specifications; but also microstructural and metallurgical, to guarantee manufacturing quality, in particular the absence of defects within the material (porosity, inclusions...) or metallurgical phases detrimental to its mechanical strength.
Finally, the PhD student will also be required to simulate the manufacture of certain parts using the finite element method, in order to analyze the evolution of parameters of interest, such as temperature, during manufacture, and to estimate the state of deformation and stress after manufacture. These simulations can be used to correct certain discrepancies between expected and actual results, within the framework of a calculation-test dialogue that will see the implementation of instrumentation also serving to validate the models. These simulations will be carried out using the Cast3M finite element code developed at CEA.
Translated with DeepL.com (free version)
Effect of plastic strain on brittle fracture: Decoupling of deformation induced dislocation structures and deformation induced microtexture evolution
In the nuclear field, the integrity of components must be ensured throughout their operating life, even in the event of an accident. The demand for justification of component resistance to the risk of sudden rupture is growing, and is being applied to a wide range of piping lines and equipment. The demonstration principle consists in showing that, even in the presence of a defect, the equipment is capable of withstanding the loads it is likely to be subjected to.
Particular attention is paid to brittle fracture by cleavage, because of its unstable and catastrophic nature, which immediately leads to the ruin of the component. Brittle fracture is sensitive to the level of plasticity and triaxiality at the crack tip, which explains the beneficial structural effect often observed on real components compared to laboratory specimens. The industrial challenge is to better understand the role of plasticity in relation to microtexture on brittle fracture, in order to improve current prediction criteria.
In the course of this thesis, the brittle fracture toughness of a ferritic steel will be evaluated after various types of mechanical pre-strain. By the end of the thesis, the candidate will have acquired solid skills in mechanical testing, microscopic analysis and numerical simulation. The work will be carried out between the LISN laboratory of the CEA and the materials center of the Ecole des Mines de Paris.
Study of the dynamics of molten salt fast reactors under natural convection conditions
Molten Salt Reactors (MSRs) are presented as inherently stable systems with respect to reactivity perturbations, due to the strong coupling between salt temperature and nuclear power, leading to a homeostatic behavior of the reactor. However, although MSRs offer interesting safety characteristics, the limited operational experience available restricts our knowledge of their dynamic behavior.
This research work aims to contribute to the development of a methodology for analyzing the dynamics of MSRs, with the goal of characterizing complex neutron-thermohydraulic coupling phenomena in an MSR operating in natural convection, identifying potentially unstable transient sequences, prioritizing the physical phenomena that cause these instabilities, and proposing simple physical models of these phenomena.
This work will contribute to the development of a safety-oriented methodology that will help MSR designers better understand and model the reactor dynamic behavior during transients, through dimensional analysis and the study of the flow stability. This methodology aims to define simple and robust criteria to ensure the intrinsic safety of a fast-spectrum MSR, depending on its design and operational parameters allowing compliance with the operating domain limits.
This PhD lies at the crossroad of theoretical analysis of the physical phenomena governing the MSR’s behavior, particularly the study of unstable regimes (oscillatory or divergent in nature) due to neutron-thermohydraulic coupling under natural convection conditions, and the development of analytical and numerical tools for conducting calculations to characterize these phenomena.
The PhD student will be based within a research unit dedicated to innovative nuclear systems. He/she will develop skills in MSR modelling and safety analysis, and will have the opportunity to present his/her work to the international MSR research community.
Development of a multiscale / multimodel boundary condition
In the context of thermalhydraulics, Computational Fluid Dynamics (CFD) codes are widely used for design and safety analysis. CFD codes solve the Navier-Stokes equations in three dimensions. They mostly rely on the Reynolds-averaged formulation of the Navier-Stokes equations. This approach allows for a detailed representation of the flow while requiring a limited numbers of hypotheses (turbulence models, law of the wall). A fine spatial discretisation is needed in order to achieve good prediction capabilities. This implies a large number of control volumes. The computational resources necessary to carry out a calculation at the industrial scale, such as a two-phase flow transient on the entire primary side of a nuclear reactor, are often prohibitive by present-day standards.
In order to cut the computational cost, a coarser spatial discretisation can be retained. Depending on the case of interest, the best practise guidelines of the RANS approach might not all be respected. Further hypotheses need to be added in order to maintain the quality of the model’s predictions. Such models may include pressure drops, heat transfer correlations or mixing terms. This approach is often referred to as a porous media approach.
Regardless of the method, the system of interest is often restricted to an open-loop model, which requires boundary conditions for the equation system to be solved.
Multi-scale coupling methods aim at using each approach where it best suited. The rationale is to reduce the computational burden while capturing the relevant physical phenomena.
Multi-scale coupling can be either one-way or two-way. In a one-way coupling, boundary conditions obtained from a first calculation are used as boundary conditions for another calculation. There is no feedback from the second calculation on the first one. In a two-way coupling, the coupled codes exchange data in the form of boundary conditions, usually at each time step. There is feedback between the two codes. Two-way is the method that is selected in the following.
The boundary conditions used in the standard approach are developed for cases were only macroscopic data are available, flow rate and temperature at the inlet, pressure at the outlet. In the context of a multi-scale coupling, data that are more detailed can be available such as velocity and pressure fields. This thesis work aims at developing boundary conditions, which can take benefit of all the available data in order to make the coupling as seamless as possible.
As an example, in case of two code instances, each one solving a portion of a physical domain relying on the same discretisation and modelling options, the results obtained from these two instances should be identical to that of a single code instance relying on the same discretisation and modelling options solving the entire domain.
Mapping the tower of nuclear Effective Field Theory
The ability of nuclear models to accurately predict the rich phenomenology emerging in nuclei (whether for fundamental purposes or nuclear data applications) is conditioned by the possibility to construct a systematically improvable theoretical framework, i.e. with controlled approximations and estimation of associated uncertainties and biases. This is the goal of so called ab initio methods, which rely on two steps:
1 - The construction of an inter-nucleon interaction in adequation with the underlying theory (quantum chromodynamics) and adjusted in small systems, following effective field theory paradigm.
2 - The resolution of nuclear many-body problem to a given accuracy (for structure or reactions observables). This provides predictions in all nuclei of interest and includes the uncertainty propagation stemming from the interaction model up to nuclear data predictions.
This PhD thesis mostly deals with Step 1. The goal of the thesis is to construct a family of ab initio interactions by developing a new adjustment procedure of the low energy constants (including the evaluation of covariances for sensitivity analysis). The adjustment will include structure data but also reaction observables in light systems. This will open the door to a new evaluation of p+n->d+gamma cross sections (which have large uncertainties despite their importance for neutronics applications) in the context of state-of-the-art effective fields theories.
The thesis will be done in collaboration between CEA/IRESNE (Cadarache) and IJCLab (Orsay), the PhD student will spend 18 months in each laboratories. Professional perspectives are academic research and R&D labs in nuclear physics.