The core elements of currently operating French nuclear power plants are exposed to multiple extreme conditions such as primary water (pressurized water at 150 bar and around 300 °C) and neutron flux, among others, both affecting the materials’ durability, thus involving security assessment of the plants. In the core reactor, stainless steels are damaged by neutron bombardment, by corrosion, and by a combination of the two effects: the media reactivity is indeed modified by the radiolysis reactions, which impact the corrosion mechanisms and kinetics and, in turn, may affect the hydrogen uptake by the steels. This last point, not investigated yet under radiolysis conditions, is of great interest since once inside the material, hydrogen rapidly diffuses (especially at 300 °C) and can lead to a modification of the mechanical properties of the steel, eventually induce cracking of the element.
This thesis will focus first on the impact of radiolysis phenomena on the hydrogen uptake and corrosion mechanisms (in terms of surface concentration of hydrogen flux) of a stainless steel (316L) exposed to primary water under irradiation. Hydrogen species will be traced by the use of deuterium (non-radioactive, low abundant isotope of hydrogen), and neutron irradiation will by simulated by electron irradiation. A first experimental issue will be to adapt and develop an already existing radiolysis autoclave in order to enable in situ measurement of the deuterium permeation flux (via a mass spectrometer) across a stainless steel membrane exposed to radiolysis conditions. The second step will focus on hydrogen permeation data analysis and investigation of the hydrogen (deuterium) distribution in the material, in correlation with the nature of corrosion layers. Such analyses, mainly based on ion beam analysis and/or mass spectrometry detection, will be carried out on high technology techniques available at CEA or partner laboratories. Finally, the candidate will have to identify the mechanism in play (corrosion and hydrogen uptake), estimate the hydrogen transport kinetics, and propose a model accounting for the simulation of the evolution of hydrogen flux in the steel as function of radiolysis activity.
This thesis touching several topics, the candidate will be involved in many academic collaborations with high level scientists, from CEA and other research institutes, both for experiments/analyses and modelling. Developed skills during these 3 years will surely increase the candidate’s expertise for his/her following carrier in the industrial (including non-nuclear field) or academic domains.