Elasto-viscoplastic behavior of zircaloy-4 tubes: experimental approach and micro-mechanical modelling

Zircaloy-4 tubes containing the fuel of PWR nuclear power plants constitute the first barrier against the dissemination of radioactive elements in the atmosphere. As the physical phenomena in reactor are complex, EDF uses numerical simulations to evaluate and guarantee the mechanical integrity of these tubes. The objective of this thesis is to develop a modelization of the mechanical behavior of zirconium alloys by a predictive approach. To develop the model, an experimental approach has been implemented coupling mechanical tests and metallurgical observations of the plastic deformation mechanisms on fully annealed Zircaloy-4 tubes. For the mechanical tests, we paid particular attention to realize tests whose analysis of results could be directly connected to deformation mechanism at the grains scale (determination of internal and effective stress, of activation volume). To determine the deformation mechanisms, qualitative and quantitative observations in Transmission Electron Microscopy (TEM) have been undertaken. To understand the origin of the mechanical anisotropy and of the viscosity, samples (thin foils) have been taken on tubes submitted to different loading directions and strain rates. The starting point of the modelization is a micro-mechanical approach (self consistent model) used successfully for cubic materials, by taking into account the specificity of the hexagonal structure of zirconium alloys. The choice of constitutive equations to describe the slip mechanisms is directly connected with our experimental approach. Results obtained by the modelization are compared with mechanical tests and TEM observations. Advantages of the model as well as its gaps are discussed. Results obtained show all the interest to extend the methodology used to the case of irradiated materials. (author)