Cecilie Duhamel

Stress Corrosion Cracking of Alloy 600 in PWR Primary Water : Influence of Chromium, Hydrogen and oxygen Diffusion

Alloy 600, a nickel base alloy containing 15 % chromium, is used in the primary circuit of Pressurized Water Reactor (PWR). This alloy is well-known to be susceptible to Stress Corrosion Cracking (SCC) in PWR primary water. Despite the fact that many laboratory studies have been performed and that many models are proposed in the literature, the mechanisms involved are still not well-known. In some models, the transport of species (oxygen, hydrogen and chromium) has a key role. Therefore, experiments and calculations have been performed to study the transport of chromium, hydrogen and oxygen in Alloy 600 and in model microstructures. The results lead to the conclusion that the transport of oxygen and hydrogen cannot be considered as the rate-controlling steps. The asymmetric aspect of the crack tip and of the chromium depletion ahead of the crack lead to the conclusion that chromium diffusion could play a significant role in the mechanism.

Oxidation of nickel-base welds 182 and 82 in simulated primary water of pressurised water reactors

Abstract To date, nickel-base welds 182 and 82 used in pressurised water reactors have shown a higher susceptibility to stress corrosion cracking (SCC) during laboratory tests than in power plants. However, the increasing number of cracks reported in American, Swedish and Japanese nuclear power plants on Alloys 182/82 enlightens the need for a predictive initiation model of SCC. Initiation of SCC involves several variables interacting together such as material microstructure and composition, primary water chemistry and loading history. Building such a model requires studying each contribution of individual phenomenon, in order to have a better understanding of the mechanisms involved at a local scale in crack initiation. Before SCC initiation, some grain boundaries exposed to primary water present intergranular oxidation reaching several hundreds of nanometres. The oxide penetration kinetics could be modified by several parameters such as local metallurgy, dissolved hydrogen content of the primary water and mechanical history before oxidation. The grain boundaries affected by oxidation during a tensile test demonstrate a lower resistance to failure compared to ‘healthy’ grain boundaries. The main objective of this study is to determine key local parameters controlling grain boundary oxidation in order to calibrate an oxidation model which will be integrated to SCC initiation modelling.

Micro-mechanical characterization of lead-free solder joints in power electronics

The following study is motivated by the need to capture the elasto-viscoplastic behavior of a “real” industrial power module lead-free solder joint. In this work, we carried out a numerical design of experiments in order to forecast the ability of an experimental bending system to identify the specimen material properties. As a proof of principle, the micro-mechanical elastic behavior of power module copper substrates was then characterized thanks to the development of an innovative in-situ micro-mechanical bending test under an optical profilometer. An inverse Finite-Element Method has been applied in order to identify the material properties of test specimens designed directly out of industrial assemblies and not from bulk solder for good representativity. The results show that identified copper Youngs modulus values are lower than that of a bulk material. It will be defined as such in the next identificatio n step targeting the solder joint.

Initiation of PWSCC in Nickel Base Alloy 182

To date, welded nickel base alloy 182 used in Pressurized Water Reactors (PWRs) components have shown a higher susceptibility to Primary Water Stress Corrosion Cracking (PWSCC) during laboratory tests than in power plants. However, an increasing number of cracks reported in American, Swedish and Japanese nuclear power plants on Alloy 182 enlighten the need for a predictive initiation model of PWSCC. Initiation of PWSCC involves several factors such as material, environment and loading history, interacting with each other. Building such a model first requires to focus on these parameters separately, in order to have a better understanding of the involved mechanisms at a local scale in crack initiation. This study focuses on the correlation between EBSD/strain field results to improve the accuracy of the actual initiation model [1] involving local parameters.Copyright