Electron microscopy of nuclear graphite: A modelling approach

Abstract

Graphite acts as both a major structural component and moderator in nuclear reactors. Upon neutron irradiation, various structural and property changes occur. Property changes include; coefficient of thermal expansion (CTE), Young's modulus and thermal resistivity. This work focuses on the characterisation of irradiated graphite models using both electron energy-loss spectroscopy (EELS) and imaging techniques. A number of models of irradiated graphite have been found, whereby one or more interstitial atoms form between the hexagonal layers. In this work, density functional theory (DFT) modelling is used to predict EEL spectra (carbon K edges) at the spiro-interstitial position, and contrast those to bulk predictions. We observe that for a 'bulk-like' position in the spiro-interstitial model, the carbon K edge shape is similar to that of true bulk, thus confirming the validity of the model used. For the carbon K edge prediction at the spiro-interstitial position, although peaks in the π* region remain in approximately the same energy positions, there is considerable broadening suggesting the presence of strained sp2 bonding character. The σ* peak is significantly altered, both in energy position and intensity relative to the π* region. These observations are arguably consistent with a spiro-interstitial strained from the ideal bonding angles observed in spiro-pentane. Simulations of TEM and HAADF images of the spiro-interstitial model were also performed. These suggested that in a typical TEM, for the (100) orientation, even at a thickness of ∼15Å the interstitial would be difficult to observe. In the case of (S)TEM, a similar situation exists.