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.
Theoretical study of the physical and optical properties of some titanium oxide surfaces for greenhouse gas sensing applications
The international community is engaged in developing the policy to reduce greenhouse gases (GHGs) emission, in particular carbon dioxide (CO2), in order to reduce the risks associated to the global warming. Consequently, it is very important to find low-cost processes to dissociate and then capture carbon dioxide (CO2), as well as to develop low power, high performance sensors suitable to monitor GHGs reductions.A common and existing method for sensing the concentration of gases is achieved by using semiconducting metal oxides surfaces (MOS) like SnO2, ZnO, and TiO2. Moreover, one route to achieve CO2 dissociation is plasma assisted catalytic decomposition. However, surface defects, and in particular oxygen vacancies and charged trapped therein, play an important role in the (photo)reactivity of MOS. The way optical properties of surfaces are modified by such defects is not completely understood, nor is the additional effect of the presence of the gas. In some models, the importance of charge transfer is also emphasized.
In this Ph.D. work, theoretical methods will be used to model the surface with defects and predict the optical properties. The objective is threefold: To apply the theoretical frameworks developed at LSI for the study of defects to predict the defect charge states in bulk; To study the effect of the surface on the defect stability; to study bulk and surface optical properties, and find out spectroscopic fingerprints of the molecular absorption and dissociation near to the surface. Materials/gas under considerations are oxides like titanium oxide, eventually deposited on a layer on gold, and carbon dioxide. The theoretical method will be the time dependent density functional perturbation theory method (TDDFPT) developed at LSI in collaboration with SISSA, Trieste (Italy).
Ref.: I. Timrov, N. Vast, R. Gebauer, S. Baroni, Computer Physics Communications 196, 460 (2015).
Multiphysics modeling of fission gas behavior and microstructure evolution of nuclear fuels
The climate crisis demands urgent action and a rapid shift towards carbon-free technologies. This requires the development of advanced materials for more efficient electricity production and storage, including innovation in nuclear reactor fuels. To enhance the safety and efficiency of both current and future nuclear power plants, it is crucial to understand and predict fuel behavior under operating and accidental conditions.
A critical issue is related to fission gases produced upon nuclear fissions. These gases have low solubility and form small bubbles that grow from nanoscale to microscale during fuel operation, significantly impacting the fuel's overall properties. While experimental characterization is essential, numerical simulations complement this work by modeling bubble formation and growth, as well as the consequences in terms of changes in fuel properties. This approach is key to the design of next-generation, high-performance nuclear fuels.
This PhD project aims to advance simulation models for fission gas behavior within the polycrystalline structure of nuclear fuels, with a particular focus on uranium oxides. The PhD student will develop a physical model using the phase-field method, compute necessary input parameters, and conduct numerical simulations that replicate irradiation experiments performed in our department. Direct comparison between simulation results and experimental data will enable deeper insights into the underlying physics of gas behavior, including bubble formation, gas release, and fuel swelling. Additionally, this project will serve as validation for the INFERNO scientific code that will be used for these simulations on national supercomputers.
The research will be conducted at the Nuclear Fuel Department (DEC) of the IRESNE Institute(CEA-Cadarache), in collaboration with CEA fuel modeling and experimental characterization experts. The PhD student will have opportunities to share their findings through scientific publications and presentations at international conferences. Throughout the project, they will develop expertise in multiphysics modeling, numerical simulations, and scientific computing. These highly transferable skills will prepare them for a successful career in academic research, industrial R&D, or materials engineering.
References :
https://doi.org/10.1063/5.0105072
https://doi.org/10.1016/j.commatsci.2019.01.019