Nuclear graphite: structural characterisation and effects of irradiation

This thesis investigates graphites used in nuclear materials engineering applications – so called nuclear graphites. As well as outlining a microstructural model for nuclear graphite, it aims to develop a new understanding for nuclear graphite degradation under neutron irradiation and at high temperatures by the use of different characterization techniques and different irradiating species.
Part of the thesis aims to develop a new methodology for understanding nuclear graphite and its damage processes via the correlative use of different characterisation methods and by novel data processing methods applied to characterisation data. This includes use of electron energy loss spectroscopy (EELS) acquired under damage free conditions and analysed using an improved automated fitting method in order to extract information on carbon bonding within the samples. Some of this methodology was developed using in situ electron irradiation of nuclear graphite.
A range of different virgin (unirradiated) nuclear graphite grades were initially investigated and these were compared in terms of the crystallite coherence lengths obtained from X-ray diffraction (XRD) and by the relative proportions of non-graphitic carbon derived from Raman spectra. From both these results and from scanning and transmission electron microscopy (SEM and TEM) an initial model was developed for micro- and nano-structure of the complex composite graphitic material.
Neutron irradiated nuclear graphites were then sourced and analysed using XRD, Raman and TEM/EELS and the effect of degradation was studied as variation of irradiation dose and temperature in terms of the change in crystallite size and carbon bonding. These results indicated a model involving the fragmentation of graphite crystallites and an increase in porosity during irradiation with the presence of some effects due to annealing of defects at higher temperatures. This is similar to a previous model developed for electron irradiation at doses lower than 1 dpa and temperatures lower than 370 °C.