RESUMEN
Experimental investigations and atomistic simulations are combined to study the cesium diffusion processes at high temperature in UO2. After 133Cs implantation in UO2 samples, diffusion coefficients are determined using the depth profile evolution after annealing as measured by secondary ion mass spectrometry. An activation energy of 1.8 ± 0.2 eV is subsequently deduced in the 1300-1600 °C temperature range. Experimental results are compared to nudged elastic band simulations performed for different atomic paths including several types of uranium vacancy defects. Activation energies ranging from 0.49 up to 2.34 eV are derived, showing the influence of the defect (both in terms of type and concentration) on the Cs diffusion process. Finally, molecular dynamics simulations are performed, allowing the identification of preferential Cs trajectories that corroborate experimental observations.
RESUMEN
The second-moment tight-binding variable-charge (SMTB-Q) interatomic potentials have been implemented in the molecular dynamics (MD) code LAMMPS in order to study the static and dynamical properties of uranium dioxide UO2. With respect to a previous work on UO2 the SMTB-Q model has been slightly modified in introducing a splitting energy of the U 5fâ orbitals. This improvement results in a better description of the electronic structure of UO2 namely the gap estimation which is now close to the experimental value (~2 eV). The structural and mechanical properties along with the cohesive energy of bulk UO2 are in good agreement with the experimental data. The ionic charges on uranium and oxygen are respectively equal to 2.86 and -1.43, very close to the Bader charges derived from ab initio calculations. The migration energies and the diffusion coefficient calculated respectively for oxygen vacancy (VO) and oxygen interstitials (IO) in under and over stoichiometry compare well with ab initio calculations and experimental data. The oxygen diffusivity is consistent at high temperature when additional Frenkel thermally formed swamps the effect of single IO and VO defects with recent prediction from EAM semi-empirical potentials. Additionally, a study on phase transitions between high pressure polymorphs of UO2 has been performed and has shown the good transferability of the SMBT-Q potential over different coordination. It is found that the UO2 phases stability order under tensile and compressive stresses, compared with stable fluorite phase at 0 GPa, are respected.
RESUMEN
Molybdenum is an abundant element produced by fission in the nuclear fuel UO2 in a pressurized water reactor. Although its radiotoxicity is low, this element has a key role on the fuel oxidation and other fission products migration, in particular in the case of an accidental scenario. This study aims to characterize the behavior of molybdenum in uranium dioxide as a function of environmental conditions (oxygen partial pressure, high temperature, UO2 oxidation) typical of an accidental scenario. To do so, molybdenum was introduced in UO2 or UO2+ x pellets by ion implantation, a technique that allows us to mimic the production of Mo in the nuclear fuel by fission. Then, thermal treatments at high temperature and different oxygen partial pressures were carried out. The mobility of Mo in UOX samples was followed by secondary ion mass spectrometry (SIMS), while the Mo chemical speciation was investigated by spectroscopic techniques (XANES, Raman). In parallel, ab initio calculations were performed showing the effect of interstitial oxygen atoms on the Mo incorporation sites in UO2. We show that the Mo mobility is directly connected to its chemical state, which in turn, is linked to the redox conditions. Indeed, under reducing atmosphere, Mo is present in UO2 or UO2+ x samples under a metallic state Mo(0). Its mobility, being quite low, is driven by a diffusion mechanism. An increase of pO2 entails the UO2 and Mo oxidation and, as a consequence, a strong release of this element. We show an increase of the Mo release rate with the increase of the UO2+ x hyper-stoichiometry x. After thermal treatment, Mo remaining in the samples is located in the grains under the MoO2 form. Our experimental results are assessed by ab initio calculations showing that in the presence of oxygen Mo atoms adopt in UO2 a local structure close to the octahedral local geometry of Mo oxides.