Characterisation of the anisotropy of pyrolytic carbon coatings and the graphite matrix in fuel compacts by two modulator generalised ellipsometry and selected area electron diffraction

The safety and performance of the fuel of the high temperature reactor is largely controlled by the anisotropy of the pyrolytic carbon (PyC) coatings in the tristructural isotropic (TRISO) coated fuel particles and the graphite matrix of its fuel compacts. The optical anisotropy factor (OPTAF) and diattenuation of several PyC coatings were characterised by two modulator generalised ellipsometry microscopy. These results were compared to the texture values obtained from selected area electron diffraction patterns by transmission electron microscopy. The differences between “nano-, micro- and macro”-anisotropy are discussed. We established that previously observed sudden increase of PyC coating failure under neutron irradiation at OPTAF values above 1.04 originates from the development of a microstructure with polyhedral growth features and graphene planes with a single preferred orientation within these microstructural features. Three graphite fuel compacts (two cylindrical and one pebble) were also characterised. We have identified that the use of an overcoating on the TRISO particles can result in the formation of a macro-anisotropic area.