[SY-B5] Irradiation damage in nuclear graphite at the atomic scale
Radiation response and microstructure evolution of nuclear graphite are investigated by combination of both Molecular Dynamics (MD) simulations and experimental observations with mainly High Resolution Transmission Electron Microscopy images (HRTEM) and X-ray diffraction patterns (XRD). Radiation response of single crystal graphite is examined using MD with point defects accumulation method. Results reveal that graphite structure undergoes three stages before amorphisation: (i) an increase of the number of point defects; (ii) a wrinkling of graphene layers pinned by small amorphous pockets; and (iii) a full amorphisation of the structure via percolation of the small amorphous pockets [1]. Each stage can be related to the swelling along the c-axis and the shrinking in the basal plane. In particular rippling contributes significantly to the strain. One filler particle - composed of almost aligned crystallites separated by Mrozowski cracks - is also generated to provide insights on a more realistic microstructure of an-irradiated nuclear graphite.
Subsequently, simulated XRD pattern and HRTEM images have been generated from the MD simulations and are compared with experimental observations [2]. Simulated HRTEM images show many features observed in experimental images in both virgin and irradiated nuclear graphite. Some of these features can be linked unequivocally to defined atomistic configurations. Basal grain boundaries (GBs), Mrozowski cracks, graphene sheets and their folding belong to this category. Conversely, some patterns in simulated HRTEM cannot be related to a unique atomistic configuration and might eventually give rise to misleading interpretation. This is evidenced for edge dislocations in virgin nuclear graphite as well as for residues of graphene layers in highly damaged graphite.
These findings confirm that univocal identification of atomic scales structures in graphite from HRTEM images only is uneasy.
[1] A. Chartier, L. Van Brutzel, B. Pannier, and Ph. Baranek, Carbon 91 (2015) 395.
[2] A. Chartier, L. Van Brutzel, and J. Pageot, Carbon 133 (2018) 224.
Subsequently, simulated XRD pattern and HRTEM images have been generated from the MD simulations and are compared with experimental observations [2]. Simulated HRTEM images show many features observed in experimental images in both virgin and irradiated nuclear graphite. Some of these features can be linked unequivocally to defined atomistic configurations. Basal grain boundaries (GBs), Mrozowski cracks, graphene sheets and their folding belong to this category. Conversely, some patterns in simulated HRTEM cannot be related to a unique atomistic configuration and might eventually give rise to misleading interpretation. This is evidenced for edge dislocations in virgin nuclear graphite as well as for residues of graphene layers in highly damaged graphite.
These findings confirm that univocal identification of atomic scales structures in graphite from HRTEM images only is uneasy.
[1] A. Chartier, L. Van Brutzel, B. Pannier, and Ph. Baranek, Carbon 91 (2015) 395.
[2] A. Chartier, L. Van Brutzel, and J. Pageot, Carbon 133 (2018) 224.