Correlation between microstructure and retention in candidate materials for the first wall

Bakaeva Anastasiia


Noterdaeme Jean-Marie, (UGent),

SCK•CEN Mentor

Terentyev Dmitry
+32 14 33 31 97

SCK•CEN Co-mentor

Malerba Lorenzo
+32 14 33 30 90

Expert group

Structural Materials Modelling and Microstructure

PhD started


Short project description

Retention of Tritium will be one of the key issues in the operation of future fusion devices (i.e. ITER and DEMO). The retention as a physical process is associated to the effects of Hydrogen isotopes being trapped in the first wall material. Generally, the retention of insoluble H/D/Ti and He is associated with the presence of imperfections in a crystalline structure which can be various types of lattice defects, such as vacancies, dislocations and grain boundaries, depending on thermo-mechanical treatment of material's grades. Retention is a crucial factor in such effects as: mechanical wall degradation, fuel management and contamination assessment. In the course of exploitation the microstructure of the first wall material will undergo stress-induced modification due to extensive plastic deformation. Hence, the trapping of Hydrogen in the operating conditions is to be addressed by a combination of expertise in plasma wall interaction and condensed matter science. Which is why experiments involving plasma exposure, deposition assessment and subsequent annealing need to be rationalized in terms of physical processes governing the uptake, bulk migration, bubble nucleation and growth to blisters. Naturally, the latter processes are related to initial microstructure, and its evolution within exposure cycle and subsequent thermal fatigue. By understanding the key physical processes determining retention and degradation of mechanical properties of materials as a function of plasma beam, exposure temperature and material's microstructure, it is possible to make step further towards development of 'controlled/predictable' retention grades with desirable characteristics.

  Even though Tungsten is the baseline armour and divertor material for ITER, its intrinsic brittleness, even without irradiation, is a principal showstopper. Consideration of the reduced activation Ferritic Martensitic steels (RAFMS) is one of the options that will be explored in the European Fusion programme H2020. However, there is a lack of data about Hydrogen retention in the RAFMS grades, which by now are well characterized in terms of their performance under neutron irradiation.

This thesis will therefore contribute to the experimental studies of RAFMS, and it will focus on the mutual effects of severe plastic deformation and Hydrogen retention under various exposure conditions. Limited study on Tungsten grades will also be performed to provide the reference data.

To ensure the proper embedding of the researcher in the fusion technology environment, the work will be mainly carried out in the two laboratories from Belgium and Kingdom of Netherlands, namely: SCK•CEN (Nuclear Research Centre, Mol) and DIFFER  (Netherlands).





The main objective of the thesis is the investigation of Hydrogen-induced retention and subsurface modification in RAFM steels (and Tungsten as reference material). The preceding PhD thesis (done within Trilateral European Collaboration (Belgium, Netherlands and Germany), by E. Zayachuk) has shown that H retention (after exposures at Pilot PSI, DIFFER) is essentially confined within the first few µm of subsurface material. In practice, it means that sub-grains (made of low angle grain boundaries, which are nothing else but dislocation networks) govern trapping of H, nucleation and growth of H-bubbles turning into blisters at a later stage. In parallel, the experimental work on high temperature deformation (also done within TEC collaboration, by S. Hua) shows that after re-crystallization the low angle grain boundaries are completely removed, and in addition the high angle GBs are restructured (most  likely forming high symmetry interfaces to minimize the misfit elastic strain energy).

The objective is therefore to perform a comparative study of Hydrogen retention in 'as received', plastically deformed and recrystallized materials. By performing controlled annealing and using different RAFM and W-based grades it is possible to reach different types of microstructure in terms of volume ratio of low and high angle grain boundaries and their size distribution. The expected outcome of this study is the answer to the following questions:

- what is the effect of sub-grain interfaces composed of dislocation networks on the retention and blistering ?

- how the retention occurs in BCC crystals containing exclusively high angle grain boundary ?

- what is the performance of RAFM steels as compared to Tungsten ?