Specific Aspects of Internal Corrosion of Nuclear Clad Made of Zircaloy

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^{\circ}C on the internal surface of the clad, a layer of oxidised Zircaloy is formed with a thickness ranging from 5 to 15 $\mu$m. 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

Simulation of Metal/Oxide Interface Mobility: Effects of Mechanical Stresses on Geometrical Singularities

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

Process Optimization and Downscaling of a Single Electron Single Dot Memory

This paper presents the process optimization of a single-electron nanoflash electron memory. Self-aligned single dot memory structures have been fabricated using a wet anisotropic oxidation of a silicon nanowire. One of the main issue was to clarify the process conditions for the dot formation. Based on the process modeling, the influence of various parameters (oxidation temperature, nanowire shape) has been investigated. The necessity of a sharp compromise between these different parameters to ensure the presence of the memory dot has been established. In order to propose an aggressive memory cell, the downscaling of the device has been carefully studied. Scaling rules show that the size of the original device could be reduced by a factor of 2. This point has been previously confirmed by the realization of single-electron memory devices

Mechanisms of High Temperature Degradation of Thermal Barrier Coatings.

Thermal barrier coatings (TBCs) are crucial for increasing the turbine inlet temperature (and hence efficiency) of gas turbine engines. The thesis describes PhD research aimed at improving understanding of the thermal cycling failure mechanisms of electron beam physical vapour deposited (EB-PVD) yttria stabilised zirconia (YSZ) TBCs on single crystal superalloys. The research consisted of three different stages. The first stage involved designing a coupled one-dimensional thermodynamic-kinetic oxidation and diffusion model capable of predicting the concentration profiles of alloying elements in a single-phase γ nickel-rich Ni-Al-Cr ternary alloy by the finite difference method. The aim of this investigation was to improve the understanding of interactions between alloying species and developing oxide. The model demonstrated that in the early stages of oxidation, Al consumption by oxide scale growth is faster than Al replenishment by diffusion towards the scale, resulting in an initial Al depletion in the alloy near the scale. The second stage involved a systematic study of the life-time of TBC systems on different single crystal superalloys. The study aimed at demonstrating that the compatibility of modern nickel-based single crystal superalloys with TBC systems is influenced strongly by the content of alloying element additions in the superalloy substrate. The results can be explained by postulating that the fracture toughness parameters controlling decohesion are influenced strongly by small changes in composition arising from interdiffusion with the bond coat, which itself inherits elemental changes from the substrate. The final stage of study involved a detailed study of different bond coats (two β-structured Pt-Al types and a γ/γ’ Pt-diffusion type) in TBC systems based on an EB-PVD YSZ top coat and a substrate material of CMSX-4 superalloy. Generation of stress in the thermally grown oxide (TGO) on thermal cycling, and its relief by plastic deformation and fracture, were investigated experimentally in detail

Static and dynamic aspects of coupling between creep behavior and oxidation on MC2 single crystal superalloy at 1150 °C

Creep tests were performed on thin wall specimens made of MC2 single crystal superalloy at 1150 °C and under controlled atmosphere. The results highlight the deleterious oxidation effect on creep properties. The assumption that oxidation leads to a non-load-bearing affected zone is insufficient to explain the difference in creep rate that was noticed between tests performed under synthetic air and under hydrogenated argon, and cannot explain the decrease of the strain rate during the tests that were carried out with a change of atmosphere from synthetic air to hydrogenated argon. On the other hand, these experimental results are consistent with vacancy injection due to partial cationic oxidation, which accelerates the creep rate by promoting creep mechanisms controlled by diffusion. The anionic protective alumina scale formed under hydrogenated argon prevents this vacancy flux

A model for the degradation of polyimides due to oxidation

Polyimides, due to their superior mechanical behavior at high temperatures, are used in a variety of applications that include aerospace, automobile and electronic packaging industries, as matrices for composites, as adhesives etc. In this paper, we extend our previous model in [S. Karra, K. R. Rajagopal, Modeling the non-linear viscoelastic response of high temperature polyimides, Mechanics of Materials, In press, doi:10.1016/j.mechmat.2010.09.006], to include oxidative degradation of these high temperature polyimides. Appropriate forms for the Helmholtz potential and the rate of dissipation are chosen to describe the degradation. The results for a specific boundary value problem, using our model compares well with the experimental creep data for PMR-15 resin that is aged in air.Comment: 13 pages, 2 figures, submitted to Mechanics of Time-dependent Material