Papers by Author: Virgil Optasanu

Abstract: The present study evaluates the influence of the residual stress on the oxidation rate of Zr at 650°C. It is shown that the oxidation rate can be substantially decreased by shot-peening. A systematic instigation of the various factors (such as the residual stress, the surface pollution and the chemical composition of the oxidation atmosphere) is realized to separate their influence on the oxidation. The influence of an annealing made after the shot-peening is also considered. One show that the pollution of the surface during the shot-peening process is not responsible for the oxidation rate decrease.

Abstract: Hydrogen diffusion in metals is still an ongoing topic of research due to its technical relevance (hydrogen embrittlement, hydrogen storage...). In the last decades, significant progress in understanding the time evolution of the hydrogen concentration in solids was completed. This paper presents a modeling of hydrogen diffusion with a general and thermodynamically based diffusion concept coupled with mechanical and chemical aspects. This model was previously used to simulate the oxidation of a metal [1][2]. This concept has been upgraded to offer a thoroughly macroscopic behavior law used to simulate hydrogen diffusion in metal parts under mechanical loadings. The thermodynamic approach of the stress-diffusion coupling was implemented in a finite element code in order to study the hydrogen diffusion mode into a strained metal. Simulations were performed on a cylindrical austenitic steel tank under important internal pressure. The results of this study allow us to understand how hydrogen diffusion and mechanical stresses are mutually induced and modified.

Abstract: In PWR, the Zircaloy based clad is the first safety barrier of the fuel rod, it must prevent the dispersion of the radioactive elements, which are formed by fission inside the UO₂ pellets filling the clad. We focus here on internal corrosion that occurs when the clad is in tight contact with the UO₂ pellet. In this situation, with temperature of 400 °C on the internal surface of the clad, a layer of oxidised Zircaloy is formed with a thickness ranging from 5 to 15 µm. In this paper, we will underline the specific behaviour of this internal corrosion layer compared to wet corrosion of Zircaloy. Simulations will underline the differences of stress field and their influences on corresponding dissolved oxygen profiles. The reasons for these differences will be discussed as function of the mechanical state at inner surface of the clad which is highly compressed. Differences between mechanical conditions generated by an inner or outer corrosion of the clad are studied and their influences on the diffusion phenomena are highlighted.

Abstract: During the last decade, an increasing importance has been given to the feedback of mechanical stresses on the chemical diffusion and, further, on corrosion. Many works point the active role of stresses on the material ageing especially on their negative consequences leading to the damaging of structures. Based on a theoretical study and using numerical tools and experimental results our previous works [1, on stress/diffusion coupling, highlight the strong influence of stress field on the diffusion process. The aim of the present paper is to describe the influence of some particular morphologies of the metal/oxide interface on both diffusion and oxidation process. The oxidation is assumed to be driven by a mass conservation law (Stefan's law) while the diffusion coefficient of oxygen in metal is locally influenced by the stress field. The stability of a waved-shape interface is studied in both cases: simple diffusion and coupled stress/diffusion process. In this purpose we have developed an original numerical model using a virtual metal/oxide interface of a mono-material with oxygen concentration-dependent parameters, which allows to operate easily with any shape of interface and to use simple finite element meshes. Furthermore, in order to underline in a more obvious way the consequences of mechanical stress on the diffusion process, a particular geometry is studied.

Abstract: Our recent modelling works and corresponding numerical simulations realized to describe the UO₂ oxidation processes confirm the theory showing that an applied mechanical strain can strongly affect the local oxygen diffusion in a stressed solid. This result allows us to assume that stress field, previously applied at the surface of a metallic sample on several microns, will delay the degradation during its oxidation. Considering this hypothesis, we implemented a FEM simulation code developed in our laboratory to numerically investigate some different stress fields applied on a sample sub-surface, that might significantly modify the volume diffusion of oxygen during the oxidation process. The results of our simulations are presented and discussed from the perspective to study the consequences of a surface mechanical treatment on the durability of a metallic material.

Abstract: The growth of a U3O7 oxide layer during the anionic oxidation of UO2 pellets induced very important mechanical stresses due to the crystallographic lattice parameters differences between UO2 and its oxide. These stresses, combined with the chemical processes of oxidation, can lead to the cracking of the system, called chemical fragmentation. We study the crystallographic orientation of the oxide lattice growing at the surface of UO2, pointing to the fact that epitaxy relations at interface govern the coexistence of UO2 and U3O7. In this work, several results are given: - Determination of the epitaxy relations between the substrate and its oxide thanks to the Bollmann’s method; epitaxy strains are deduced. - Study of the coexistence of different domains in the U3O7 (crystallographic compatibility conditions at the interface between two phases: Hadamard conditions). - FEM simulations of a multi-domain U3O7 connected to a UO2 substrate explain the existence of a critical thickness of U3O7 layer.

Abstract: The aim of the present work is to introduce a thermodynamic model to describe the growth of an oxide layer on a metallic substrate. More precisely, this paper offers a study of oxygen dissolution into a solid, and its consequences on the apparition of mechanical stresses. They strongly influence the oxidation processes and may be, in some materials, responsible for cracking. To realize this study, mechanical considerations are introduced into the classical diffusion laws. Simulations were made for the particular case of uranium dioxide, which undergoes the chemical fragmentation. According to our simulations, the hypothesis of a compression stress field into the oxidised UO2 compound near the internal interface is consistent with the interpretation of the mechanisms of oxidation observed experimentally.