Fabienne Ribeiro

In situ hydrogen loading on zirconium powder

Commercial-grade Zr powder loaded with hydrogen in situ and phase transformations between various Zr and ZrHx phases have been monitored in real time.

Atomistic study of porosity impact on phonon driven thermal conductivity: Application to uranium dioxide

We present here an analytical method, based on the kinetic theory, to determine the impact of defects such as cavities on the thermal conductivity of a solid. This approach, which explicitly takes into account the effects of internal pore surfaces, will be referred to as the Phonon Interface THermal cONductivity (PITHON) model. Once exposed in the general case, this method is then illustrated in the case of uranium dioxide. It appears that taking properly into account these interface effects significantly modifies the temperature and porosity dependence of thermal conductivity with respect to that issued from either micromechanical models or more recent approaches, in particular, for small cavity sizes. More precisely, it is found that if the mean free path appears to have a major effect in this system in the temperature and porosity distribution range of interest, the variation of the specific heat at the surface of the cavity is predicted to be essential at very low temperature and small sizes for sufficiently large porosity.

A novel method for calculating the energy barriers for carbon diffusion in ferrite under heterogeneous stress.

A novel method for accurate and efficient evaluation of the change in energy barriers for carbon diffusion in ferrite under heterogeneous stress is introduced. This method, called Linear Combination of Stress States, is based on the knowledge of the effects of simple stresses (uniaxial or shear) on these diffusion barriers. Then, it is assumed that the change in energy barriers under a complex stress can be expressed as a linear combination of these already known simple stress effects. The modifications of energy barriers by either uniaxial traction/compression and shear stress are determined by means of atomistic simulations with the Climbing Image-Nudge Elastic Band method and are stored as a set of functions. The results of this method are compared to the predictions of anisotropic elasticity theory. It is shown that, linear anisotropic elasticity fails to predict the correct energy barrier variation with stress (especially with shear stress) whereas the proposed method provides correct energy barrier variation for stresses up to ∼3 GPa. This study provides a basis for the development of multiscale models of diffusion under non-uniform stress.

Theoretical study of xenon adsorption in UO2 nanoporous matrices

We present a theoretical study of xenon incorporation in UO2 nanocavities, by means of Grand Canonical Monte Carlo calculations based on semi-empirical potentials. We first characterize the reconstruction of the matrix around an empty cavity which leads to a stoechiometry change from UO2 to UO in this region. Then, we determine xenon adsorption isotherms which exhibit an abrupt transition from a dilute phase to a dense one and an increase in the density of the latter phase as a function of temperature. This last result is attributed to a vibrational entropy effect by means of a mean field analysis. Finally, the pressure calculation inside the bubble proves the limitations of the usual mesoscopic models based on gas state behaviour.

How to derive tight-binding spd potentials? Application to zirconium.

We propose here a general methodology to derive tight-binding potentials accounting for spd hybridization in transition metals, dealing simultaneously with electronic structure and energy properties. This methodology is illustrated for zirconium which is largely used for technological applications, in particular in the nuclear industry, and whose modelling is known to be complex and challenging. Such potentials are very promising. Their fits have a clear physical meaning with a limited amount of parameters and their complexity can be adjusted as a function of the problem under consideration.

Effect of magnetism on surface segregation in FeNi alloys

Modelling the segregation of the various chemical species in the vicinity of crystallographic defects in FeNi alloys is essential because it affects the macroscopic properties of these materials, which are widely used in technological applications. We present here a theoretical study of surface segregation, within a mean-field approach based on the tight-binding Ising model grounded on density functional theory calculations. The most important result is that, although FeNi presents none of the driving forces (i.e. surface energy, size mismatch) which generally favour surface enrichment in the same element in the whole range of concentrations, there exists a wide temperature range in which Ni is found to segregate at the surface irrespective of the concentration. This is due to a complex interplay between magnetic and ordering/phase separation effects.

Atomistic modelling of residual stress at UO2 surfaces.

Modelling oxide surface behaviour is of both technological and fundamental interest. In particular, in the case of the UO2 system, which is of major importance in the nuclear industry, it is essential to account for the link between microstructure and macroscopic mechanical properties. Indeed micromechanical models at the mesoscale need to be supplied by the energetic and stress data calculated at the nanoscale. In this framework, we present a theoretical study, coupling an analytical model and thermostatistical simulation to investigate the modifications induced by the presence of a surface regarding atomic relaxation and energetic and stress profiles. In particular, we show that the surface effective thickness as well as the stress profile, which are required by micromechanical approaches, are strongly anisotropic.