Characterisation of Radiation Damage in Nuclear Graphite at the Nanoscale

Graphite is a key component in many of the UK's civil nuclear reactors whose lifetime is heavily dependent on the physical and chemical performance of the graphite. Exposure to neutron radiation at high temperatures (350C) induces complex structural changes over many length scales. This thesis focuses on the nanoscale, an area where a lack of understanding leaves a variety of contentious issues.
Transmission electron microscopy (TEM), electron diffraction, and electron energy loss spectroscopy (EELS) were the three main experimental techniques used to study a range of virgin, electron irradiated, and neutron irradiated graphites. Information gained from energy
filtered TEM, X-ray diffraction, and Raman was also used to compliment these techniques. In situ electron irradiation experiments were conducted at a range of temperatures to better understand the collective effect of thermal annealing and radiation damage.
TEM lattice images were quantified using software provided by the PyroMaN research group to extract information about fringe length and tortuosity as a function of radiation damage. A 3D atomistic modelling technique was also applied to micrographs to produce 3D models of electron irradiated graphite. Electron irradiation resulted in the breaking and bending of basal planes and the fragmentation of crystallites. Analysis of electron diffraction patterns showed a 10% increase in d-spacing and polycrystallisation following electron irradiation.
Low and core loss EEL spectra were collected during in situ electron irradiation which were fitted to extract information about specimen density, planar and non-planar sp2 content, and bond length. Irradiated specimens exhibited a reduced planar sp2 content which was thought to be attributed to the introduction of non-hexagonal rings and inter-planar defects. The reduction in planar sp2 bonded carbon was replaced by non-planar sp2 bonded carbon. Bond lengths were also seen to increase due to an increase in peripheral dangling bonds at crystallite boundaries.
This quantitative analysis methodology was then applied to neutron irradiated specimens to analyse the bulk material and also material found within microcracks, the latter which could have a significant effect on irradiation-induced dimensional change.