Systematic study of the neutron scattering reactions on structural materials of interest for nuclear reactor applications
Elastic and inelastic scattering reactions on structural materials have a significant impact on the simulation of neutron transport. The nuclear data of structural materials of interest for nuclear reactors and criticality studies must be known with good precision over a wide incident neutron energy range, from a few tens of meV to several MeV. The thesis proposal aims to carry out a systematic study of the scattering reactions above the resolved resonance range up to 5 MeV. In this energy range, neither the R-Matrix formalism nor the statistical Hauser-Feshbach model are valid for structural materials. A new formalism will be developed by using high-resolution measurements of the scattering angular distributions. This work will focus more precisely on measurements already done at the JRC-Geel facility (sodium [1], iron [2]) and will be extended to other elements studied within the framework of the IAEA/INDEN project, such as copper, chromium and nickel. As part of this thesis, the experimental database will be complemented by new measurements on the copper isotopes (Cu63 and Cu65). The measurements will be carried out at JRC Geel GELINA facility with the ELISA detector. Concerning the copper isotopes, integral benchmarks from the ICSBEP database revealed several issues in the nuclear data libraries, which provide contradictory integral feedbacks on the nuclear data of U235. For example, the ZEUS benchmarks, which is routinely used to study the capture cross section of U235 in the fast neutron energy range, are very sensitive to the nuclear data of copper. This type of benchmark will provide an ideal framework for quantifying the impact of any new formalism developed to evaluate the nuclear data of structural materials.
This study will allow the PhD student to develop skills in experimental and theoretical nuclear physics, as well as in neutron physics. The results will be communicated to the JEFF working group of the Nuclear Energy Agency (OCDE/AEN).
[1] P. Archier, Contribution à l’amélioration des données nucléaires neutroniques du sodium pour le calcul des réacteurs de génération IV, Thèse, Université de Grenoble, 2011.
[2] G. Gkatis, Study of neutron induced reaction cross sections on Fe isotopes at the GELINA facility relevant to reactor applications, Thèse, Université Aix-Marseille, 2024.
AI based prediction of solubilities for hydrometallurgy applications
Finding a selective and efficient extractant is one of the main challenges of hydrometallurgy. A comprehensive screening is impossible by the synthesis/test method due to the high number of possible molecules. Instead, more and more studies use quantum calculations to evaluate the complexes stabilities. Still, some important parameters such as solubility are lacking in this model.
This project thus aims to develop an AI based tool that provides solubility values from the molecular structure of any ligand. The study will first focus on 3 solvants: water, used as a reference as AI tools already exist, 3 M nitric acid to mimic nuclear industry applications and n-octanol, organic solvent used to measure the partition coefficient logP. The methodology follows 4 steps:
1) Bibliography on existing AI tools for solubility prediction yielding the choice of the most promising method(s)
2) Bibliography on existing databases to be complemented by the student's in-lab solubility experiments
3) Code generation and training of the neural network on the step 2 databases
4) Checking the accuracy of the predictions on molecules not included in the databases by comparing the calculated results with in-lab experiments
Behavior of nanocavities under mechanical loading: from understanding physical mechanisms to homogenizing nanoporous materials
Nanocavities - typically a few nm to a few tens of nm in size - are often observed in metals, for example in high-temperature applications due to the condensation of vacancies or in metal alloys used in nuclear reactors due to irradiation. The presence of these nanocavities degrades the mechanical behaviour of materials and contributes to fracture. It is therefore necessary to determine the physical mechanisms associated with the behaviour of these nanocavities under mechanical loading and to obtain homogenised models describing the macroscopic behaviour of these nanoporous materials. The results available in the literature remain limited to date, particularly with regard to the representativeness of the simulations carried out and the models proposed for the applications of interest. This includes for example considering crystal defects surrounding the cavities, the effect of cyclic loading and the localisation of nanocavities at grain boundaries. The objectives of this thesis are therefore to determine the behaviour of nanocavities under mechanical loading and the associated physical mechanisms by considering realistic situations with respect to applications, to develop physically-based analytical models to describe the behaviour of nanocavities under mechanical loading, and finally to propose homogenised models adapted to nanocavities that can be used to simulate the failure by growth and coalescence of cavities. The targeted applications are those related to metal alloys under irradiation, but the elements of understanding obtained and the models developed could be used in a broader context. In order to achieve these objectives, Molecular Dynamics (MD) simulations will be performed, analysed from the elastic theory of dislocations and used to propose relevant homogenised models for nanoporous materials.
Study of the transitions of flow regimes in post-burnout
Dispersed two-phase flows are part of many fluid systems such as the cooling of nuclear reactors. Depending on the heat flux in the reactor core, the flow rate, the subcooling or the pressure, different flows may occur: single phase, bubbly or annular flows (with a liquid film on the wall and a vapour core).
During a loss of primary coolant accident, the reactor core, containing the fuel rods, increases in temperature until the boiling crisis when the heat flux is high enough. The different regimes of two-phase flows that occur in this type of accident are illustrated in figure 1. A vapour film appears rapidly and thermally insulates the rods, while some liquid remains in the centre of the flow. The rods are dried up, thus their surface are cooled down by the single vapour, and the heat exchange at the wall is reduced [1], which corresponds to the « inverted annular film boiling » flow. When the liquid gradually vaporises, the vapour film thickens and the induced turbulence tends to form waves at the vapour-liquid interface, and to destabilise the interface until the formation of liquid slugs (inverted slug film boiling). Then, the evaporation and fragmentation of these slugs lead to the formation of a dispersed flow with droplets (dispersed film boiling).
The transitions of flow regimes in this configuration are not well-identified [1], [2] although their understanding is significant to study the cooling of a nuclear reactor core. One of the main obstacles in experimental studies is that the walls need to be strongly heated up in order to form and maintain a vapour film, which leads to opaque test sections. Thus, a direct visualisation is particularly complex to obtain, as much as measuring local parameters such as temperature and velocity fields. The experimental results available in the literature on this topic are insufficient to develop a physical model [1], [3], [4], [5].
As a first step towards an accurate identification of the regime transitions, this thesis focuses on the single effect of the hydrodynamics, by coupling experimental and analytical approaches. In order to clarify the physics of the different phenomena, the configuration of a liquid flow inside a gas flow is proposed. Indeed, the interface deformation and the gas and liquid velocities may influence the transition from one regime to another [6], [7]: the smooth interface is therefore perturbed by waves (Kelvin-Helmholtz instabilities) and droplets could be entrained from the interface. A parametric analysis is considered by varying the gas and liquid flow rates and the thickness of the gas film, in order to observe these different phenomena and to understand the influence of each parameter on the regime transitions. An experimental facility has recently been conceived at DM2S/STMF/LE2H to study these transitions by a visualisation of the interface deformations, and may be adapted with new measurements or new methodology if necessary.
Dimensionless numbers will be identified or defined from the experimental results to describe the phenomena. Then, the regime transitions will be characterized, based on these dimensionless numbers, in order to establish a diagram of the transitions of flow regimes.
The combination of the results obtained in this thesis will enable to reinforce the physical models used in the system code CATHARE, developed at CEA for thermal-hydraulic studies about nuclear safety. This thesis presents a strong academic interest thanks to an innovative experimental facility and production of original results. Besides, it also presents an interest on the industrial level since it contributes to enhance the expertise of significant phenomena in the demonstration of nuclear reactor safety.
References:
[1] M. Ishii et G. De Jarlais, « Flow visualization study of inverted annular flow of post-dryout heat transfer region », Nuclear Engineering and Design, 1987.
[2] G. De jarlais, M. Ishii, et J. Linehan, « Hydrodynamic stability of inverted annular flow in an adiabatic simulation », Argonne National Laboratory, CONF-830702-9, 1983.
[3] T. G. Theofanous, « The boiling crisis in nuclear reactor safety and performance », International Journal of Multiphase Flow, vol. 6, no 1, p. 69-95, févr. 1980, doi: 10.1016/0301-9322(80)90040-3.
[4] N. Takenaka, T. Fujii, et others, « Flow pattern transition and heat transfer of inverted annular flow », Int. J. Multiphase Flow, 1989.
[5] M. A. El Nakla, D. C. Groeneveld, et S. C. Cheng, « Experimental study of inverted annular film boiling in a vertical tube cooled by R-134a », International Journal of Multiphase Flow, vol. 37, p. 37-75, 2011.
[6] Q. Liu, J. Kelly, et X. Sun, « Study on interfacial friction in the inverted annular film boiling regime », Nuclear Engineering and Design, vol. 375, 2021.
[7] K. K. Fung, « Subcooled and low quality film boiling of water in vertical flow at atmospheric pressure », PhD Thesis, Argonne National Laboratory, 1981.
Integral measurement of fission products capture cross-section using a combination of oscillation and activation techniques
This thesis is proposed as part of the POSEIDON (Fission Product Oscillation Experiments for Improving Depletion Calculations) project that deals with the integral measurement of the neutron capture and scattering cross-sections of the main fission products contributing to the reactivity loss in irradiated fuel. It consists of measuring the reactivity effect of separated isotope samples using a pile oscillation device, coupled with neutron activation measurements, in three different core spectral configurations : thermal, PWR and epithermal.
Part of the work will be done at CEA IRESNE in Cadarache and part at the Research Center of the Czech Republic, CV Rez. The PhD student will be involved in testing and optimizating the oscillation device that is currently being designed, as well as performing the measurements in the LR-0 Czech experimental reactor. The work at Cadarache will be on the analysis of the measurements with MC simulation tools. Functionalities needed for data analysis will require additional developments of the codes by the student.
The expected impact is a better prediction of the reactivity loss in reactor cores as a function of burn-up. Indeed, even with the most recent international nuclear data libraries, there is an important bias in the estimation of this reactivity loss.
The PhD student will develop competences in experimental and theoretical neutronics. Following job opportunities include R&D laboratories and nuclear industry.
Towards an understanding of the expansive behavior of certain cement-based evaporator concentrates: experimental approach and simplified chemistry-transport-mechanics coupled modeling
In the nuclear industry, evaporation is a commonly used process to reduce the volume of low- or intermediate-level radioactive waste before its conditioning. This results in evaporator concentrates, high-salinity solutions that can contain a wide range of ionic species. These concentrates are then stabilized and solidified in a cement-based matrix, a material with many intrinsic qualities (low cost, availability, ease of implementation, good mechanical resistance, stability under irradiation, etc.). However, the acceptance of cemented waste packages in a repository depends on meeting a number of specifications. For instance, it is necessary to demonstrate the absence of expansion that could damage the matrix when stored in a humid environment.
The thesis will aim to understand the mechanisms governing the volumetric changes of cement matrices when stored underwater. The study will be conducted on synthetic waste, simulated by dissolving salts in water at the desired concentrations. It will begin with an experimental phase that will provide the input data for the building of a simplified physico-chemical model of the cement wasteforms to estimate their macroscopic mechanical behaviour as well as the main leached fluxes.
This research project is aimed at a PhD candidate wishing to develop skills in materials science and open new perspectives for the conditioning of radioactive waste. It will be carried out in collaboration with ONDRAF, the Belgian National Agency for Radioactive Waste Management, and will rely on the expertise of two CEA laboratories: the Laboratory of Formulation and Characterization of Mineral Materials (CEA Marcoule) and the Laboratory for the Study of the Behaviour of Concrete and Clays (CEA Saclay).
Freeze-Casting: ice texturing
The thesis topic focuses on MOX fuels with controlled porosity. The student will have to develop a concentrated aqueous suspension in solid phase, dispersed and stable over time with respect to sedimentation. This suspension will be optimized using an experimental design. The tests to be carried out will typically be zeta potential and rheology measurements. The parameters to be taken into account will be the dry matter content as well as the nature and concentration of certain additives (dispersants, surfactants, organic binders) that can be incorporated into the formulation.
In a second step, the texturing conditions by the controlled growth of ice crystals will be explored, again using an experimental design.
After freeze-drying and sintering, the objective is to obtain a residual porosity controlled in size, morphology and interconnection. The sintered microstructures will be characterized by ceramography, scanning electron microscopy, image analysis and X-ray tomography on a line capable of accommodating radioactive materials.
Atomic-scale study of dislocation mobility in MOX fuel
The transition to carbon neutrality requires a rapid increase in low-carbon energy sources, including nuclear power, which necessitates a deep understanding of irradiated materials. Mixed oxide (MOX) fuel is particularly important as it optimizes the use of nuclear resources and reduces radioactive waste. The mechanical behavior of MOX under irradiation is crucial for ensuring the integrity of the fuel under various operating conditions.
The objective of this thesis is to perform atomistic simulations to understand dislocation mobility, essential for supporting multiscale modeling of the mechanical behavior of MOX. Molecular dynamics calculations will analyze dislocation mobility under different conditions of temperature, stress, plutonium content, and stoichiometric deviations, with the aim of establishing velocity laws. The results of these simulations will enhance micromechanical modeling within the CEA’s PLEIADES simulation platform, which is dedicated to simulating the complete lifecycle of nuclear fuel, from its fabrication to its storage.
The doctoral student will be based at the Fuel Behavior Modeling Laboratory in Cadarache, a dynamic environment with 11 permanent researchers and an equal number of doctoral students. Located in Provence, this center offers a pleasant working environment between the Verdon and Lubéron natural parks. The thesis will be carried out in collaboration with IM2NP, a leading laboratory in materials physics research.
The candidate should have a strong background in materials physics, ideally with experience in small-scale mechanics. These skills can be further developed during an M2 internship at the laboratory. The doctoral student will have the opportunity to present their work through scientific publications and at international conferences, opening up career opportunities in both research and industry.
Modeling of nuclear charge polarization as part of fission yield evaluation: applications to actinides of interest to the nuclear fuel cycle
Nuclear data is crucial for civil nuclear energy applications, being the bridge between the micoscopic properties of nuclei and the “macroscopic good values” needed for cycle and reactor physics studies. The laboratory of physics studies at CEA/IRESNE Cadarache is involved in the evaluation of these nuclear physics observables, in the framework of the JEFF Group and the Coordinated Research Project (CRP) of IAEA. The recent development of a new methodology for thermal neutrons induced fission product yield evaluation (fission product yields after prompt neutron emission) has improved the accuracy of the evaluations proposed for the JEFF-4.0 Library, together with their covariance matrix. To extend the assessments of fission yields induced by thermal neutrons to the fast neutron spectrum, it is necessary to develop a coupling of current evaluation tools with fission fragment yield models (before prompt neutron emission). This coupling is essential to extrapolate the actual studies on thermal fission of 235U and 239Pu to less experimentally known nuclei (241Pu, 241Am, 245Cm) or to study the incident neutron energy dependence of fission yields. One of the essential missing components is the description of the nuclear charge distribution (Z) as a function of the mass of the fission fragments and the incident neutron energy. These distributions are characterized by a key parameter: the charge polarization. This polarization reflects an excess (respectively deficiency) of proton in light (respectively heavy) fission fragments compared to the average charge density of the fissioning nucleus. If this quantity has been measured for the 235U(nth,f) reaction, it is incomplete for other neutron energies or other fissioning systems. The perspectives of this subject concern as much the impact of these new evaluations on the key quantities for electronuclear applications as well as the validation of the fission mechanisms described by microscopic fission models.
Multi-scale modeling of hydrogen diffusion in Ni polycristals
In many applications metallic structural materials face hydrogen-containing environment and at some point the hydrogen enters the metal leading to mechanical properties deterioration and eventually to rupture. The mechanisms of hydrogen embrittlement have been widely studied. Yet, a general, predictive and quantitative model of these phenomena is still missing. This thesis focuses on hydrogen segregation at grain boundaries which is one of the mechanisms identified in hydrogen embrittlement. We aim at modeling the kinetics of the segregation process starting down from the atomic scale. In order to do this, we need to find the equilibrium structures of grain boundaries, identify the segregation sites for each grain boundary and then quantify how each grain boundary affects the diffusion coefficient of hydrogen. All this data will then be fed to a finite element model whose purpose is to compute hydrogen distribution in a polycristalline sample as a function of time, accounting for the specific properties of each grain boundary. These results will be compared with hydrogen permeation experiments which give access to an effective diffusion coefficient, as well as measures localized around a single grain boundary (PANI and SKPFM methods).