One of the strategic research programs at the NMS Institute within SCK•CEN is related to radiation damage of existing reactor pressure vessel materials. The reactors of Gen-II/III are being operated worldwide in the perspective of long term operation (LTO), typically 60 years or even 80 years for the US reactors. Consequently, understanding better the post-irradiation behavior of structural materials is mandatory to ensure their adequate operation in safe conditions to supply of electricity over the coming decades.
An extensive irradiation program including post-irradiation testing and non-destructive radiation damage evaluation is being presently performed within the EU H2020 project NOMAD (2017-2021). Beside very important and unique experimental data on the mechanical behavior of reactor pressure vessel (RPV) steels after neutron irradiation, a better understanding of the relationship between non-destructive signals (electric, electromagnetic and ultrasonic) and mechanical properties (tensile and Charpy impact) will be made. The NOMAD project aims to demonstrate the feasibility of such a correlation between the NDE signals and material damage (embrittlement) through a number of calibrations on materials exhibiting different radiation damage levels. The tools that will be developed will play a key role in evaluating residual lifetime of nuclear components, in particular within a long term operation perspective. The combination of microstructural examination, non-destructive damage evaluation signals and mechanical properties is the key -issue to understand the non-destructive signals and mechanical properties.
Non-destructive evaluation (NDE) to assess the microstructure of structural materials is not new and promising results were already obtained. However, it is the first time that an extensive experimental database is available where multiple techniques are used, either for NDE or for the mechanical properties and microstructure addressing irradiation effects.
While the NOMAD project aims to explore the possibility to assess irradiation damage non-destructively, the main objectives of the proposed PhD is to scientifically investigate the relationship between microstructure, non-destructive signals and the resulting mechanical properties changes.
A large database on the mechanical properties including tensile and Charpy impact properties is already available. Various non-destructive damage evaluation techniques including electric, magnetic and ultrasonic techniques are also available. This database will be completed by a few additional tests where needed together with microstructural examination of a selected number of specimens addressing the effect of irradiation temperature on the underlying damage mechanisms. A variety of microstructural techniques will be performed, including transmission electron microscopy (TEM) for the highly embrittled materials, atom probe tomography (ATP) in collaboration with the University of Rouen, small angle neutron scattering (SANS) in collaboration with Coventry University and PAS (positron annihilation spectroscopy) available at SCK•CEN. The combination of the mechanical properties, the non-destructive damage evaluation and the microstructure will be critically analyzed with the aim to provide a rational to all available data. Thermal annealing treatments will also be used to better characterize the radiation damage evolution.