1 PhD Position in Physics on Modelling of Diffusion in the Corrosion Layer of the 690 alloy in France | Atomic Energy and Alternative Energies Commission

The 690 alloy is currently used in the vapor generators in pressurized water reactors. Its oxidation leads to the formation of an oxide layer made of Cr2O3 nodules close to the substrate, then a chromium rich spinel (Ni,Fe)Cr2O4 and hydroxides. The corrosion process of the 690 alloy relies on the ionic/electronic migration throughout the corrosion layer. The only continuous layer is the (Ni,Fe)Cr2O4 spinel. This layer drives the diffusion in particular the nickel diffusion.

Nickel is indeed released in the primary water during the corrosion process. It readily transforms to the highly radiotoxic 60Co under irradiations. Because of that, it is if utmost importance to known in details the diffusion mechanisms of nickel in the Ni-Fe-Cr oxides

Bibliographic survey evidences that the ionic/cationic diffusion throughout spinel structures is described using the Dieckmann mechanisms. Such description is valid for magnetite (Fe3O4) mono-crystals. It considers that cationic diffusion is driven by the oxygen partial pressure. In this model, magnetism and grain boundaries are not considered, although they drastically change the diffusion trends. Experiments evidence their roles: grain-boundaries are short-cuts for diffusion at high oxygen partial pressure while they act as barriers at low oxygen partial pressure. This low oxygen partial pressure is relevant to the vapor generators in pressurized water reactors.

The aim of the thesis is to investigate in the atomic scale modeling framework the cationic diffusion in spinels grain boundaries and in bulk as a function of oxygen partial pressure and magnetism. This thesis will be fed by previous work done on spinel grain boundaries [1].

The calculations will be done in the framework of quantum mechanics (DFT) and molecular dynamics (using ReaxFF potentials). Both simulations may benefit from the CalPhad modeling of the corrosion layer.