Atomic-Scale Study of Dislocation-Point Defect Interaction in UO2 Fuel
Uranium dioxide (UO2) is the primary fuel used in pressurized water reactors (PWRs). Under normal operating conditions and irradiation, the mechanical and microstructural behavior of UO2 evolves due to the accumulation of point defects (vacancies, interstitials, defect clusters) generated by nuclear fission events. These defects alter the thermo-mechanical behavior of the material, particularly through their interaction with dislocations, thereby influencing plasticity, stress relaxation, and ultimately, fuel integrity.
A detailed understanding of the elementary mechanisms governing these interactions is essential for improving the modeling of irradiated fuel mechanical behavior. In particular, the impact of point defects on dislocation mobility remains a key challenge in refining the constitutive laws used in the multi-scale simulation tools of the PLEIADES platform, which is dedicated to predicting fuel behavior under various operating conditions (nominal, transient, and accidental scenarios).
The objective of this study is therefore to analyze, at the atomic scale, the interactions between dislocations and point defects in UO2 in order to quantify their influence on the fundamental plasticity mechanisms. To this end, molecular dynamics calculations will be performed to investigate the effect of different types of point defects (e.g., Frenkel pairs) on dislocation mobility, considering key parameters such as temperature and applied stress. This work will enable the extraction of dislocation mobility laws in the presence of defects, which will serve as input data for micromechanical models used in larger-scale simulations, particularly those implemented in the PLEIADES platform.
Modeling of the MADISON fuel irradiation device for the future JHR reactor
The Jules Horowitz Reactor (RJH), currently under construction at CEA's Cadarache site, will irradiate materials and fuels in support of the French and international nuclear industry, as well as producing radioelements for medical use. To carry out its missions, the reactor will be equipped with numerous experimental devices. In particular, the MADISON device, currently under design, will irradiate 2 or 4 fuel samples under nominal stationary or operational transient conditions. The loop is representative of light-water reactor operating conditions, with single-phase and two-phase forced convection.
The main objective of the Post-Doc is to model the MADISON device and all associated heat exchanges precisely, in order to help determine the overall heat balance during the test and thus improve the accuracy of the linear power imposed on the samples. To this end, a coupled thermal model (describing the fuel rods and device structures) / CFD thermal-hydraulic model (describing the coolant) will be established using the NEPTUNE_CFD/SYRTHES code. The modeling will be validated based on results obtained from similar modeling carried out on the ISABELLE-1 and ADELINE single-rod devices in the OSIRIS and RJH reactors. The proposed approach fits in with the logic of developing digital twins of the RJH experimental devices.
Calculation of the thermal conductivity of UO2 fuel and the influence of irradiation defects
Atomistic simulations of the behaviour of nuclear fuel under irradiation can give access to its thermal properties and their evolution with temperature and irradiation. Knowledge of the thermal conductivity of 100% dense oxide can now be obtained by molecular dynamics and the interatomic force constants[1] at the single crystal scale, but the effect of defects induced by irradiation (irradiation loop, cluster of gaps) or even grain boundaries (ceramic before irradiation) remain difficult to evaluate in a coupled way.
The ambition is now to include defects in the supercells and to calculate their effect on the force constants. Depending on the size of the defects considered, we will use either the DFT or an empirical or numerical potential to perform the molecular dynamics. AlmaBTE allows the calculation of phonon scattering by point defects [2] and the calculation of phonon scattering by dislocations and their transmission at an interface have also recently been implemented. Thus, the chaining atomistic calculations/AlmaBTE will make it possible to determine the effect of the polycrystalline microstructure and irradiation defects on the thermal conductivity. At the end of this post-doc, the properties obtained will be used in the existing simulation tools in order to estimate the conductivity of a volume element (additional effect of the microstructure, in particular of the porous network, FFT method), data which will finally be integrated into the simulation of the behavior of the fuel element under irradiation.
The work will be carried out at the Nuclear Fuel Department of the CEA, in a scientific environment characterised by a high level of expertise in materials modelling, in close collaboration with other CEA teams in Grenoble and in the Paris region who are experts in atomistic calculations. The results will be promoted through scientific publications and participation in international congresses.
References:
[1] Bottin, F., Bieder, J., Bouchet, J. A-TDE
Application of the Hybrid-High-Order (HHO) method for the treatment of non-local effects in crystal plasticity via a micromorph approach
Describing the behavior of materials at the crystalline scale is the subject of much academic research, and is of growing interest in industrial R&D studies. Classically, this description is based on behavior laws describing the local evolution of the material's microstructural state: (visco-)plastic deformation, dislocation density, etc.
The main driving force behind this evolution is resolved shear stress, the projection of the stress tensor on the slip systems.
The formalism of these local constitutive equations (as opposed to non-local constitutive
equations discussed hereafter) is now well established, whether we are considering
infinitesimal or finite transformations, and benefits from special support within the MFront code generator. Thanks to MFront, those constitutive equations can be used in various mechanical solvers at CEA (Manta, Cast3M , Europlexus , AMITEX_FFTP ) and EDF
(code_aster, Manta, Europlexus ).
However, the use of local constitutive equations does not allow to account for many effects.
The aim of the post-doc is to develop a robust numerical strategy for reliably solving
structural problems using non-local crystal plasticity laws, and guaranteeing the
transferability of the constitutive equations between the CEA and EDF codes.