Neutron radiation damage is a significant problem for graphite moderated nuclear reactors where graphite serves as both the neutron moderator and a key structural component. Exposure to neutron radiation introduces a variety of chemical and physical property changes, such as a reduction in the thermal conductivity, increase in Young’s modulus and dimensional change, creating cracks [1]. Understanding the damage processes experienced by irradiated nuclear graphite over a range of length scales is essential in predicting the lifetime of the material, which influences the overall lifetime of the reactor.

In this study, electron irradiation is used as a surrogate for neutron irradiation. A multi-faceted approach has been applied to transmission electron microscopy (TEM)/ electron energy loss spectroscopy (EELS) data obtained from nuclear grade graphite exposed to the electron beam for differing time periods.  This involved deriving initial 3D structural models from a series of 2D TEM images at differing stages of damage, these models were then used to derive theoretical TEM and EELS data which were then in-turn compared to back to the experimental data from the same sample; finally these models were used to predict mechanical and transport properties relevant to reactor operation. 

TEM, selected area electron diffraction (SAED) patterns and EEL spectra were collected periodically (along the graphite [100] orientation) during electron beam exposure at 200 kV for 13 minutes (total fluence = 2 × 10⁸ e-nm^-2). Electron micrographs were subject to 2D image analysis to measure the change in (002) fringe length and tortuosity, following the method outlined in [2]. The EELS series were analysed to extract information about bond length and ratio of non-planar to planar sp² bonded carbon, following the method outlined in [3]. In recent years, a reconstruction procedure has been developed, called image guided atomistic reconstruction (IGAR), that allows large and realistic representations of disordered, yet anisotropic, graphite-based carbons to be built [4–6], from data inferred from their TEM images (Figure 1). The IGAR procedure was applied to the experimental TEM image series to produce 13 separate models at differing stages of electron beam induced damage.  These models provide information about the different atomic environments of the carbon atoms within the structure, e.g. whether they are 2, 3 or 4-fold, sp² bonded or defective. They were used to produce simulated TEM lattice images, which were analysed in the same way as experimental TEM images; the analysis results were then compared.  EEL spectra were also calculated from the reconstructed models using the plane wave density functional theory [7,8] (DFT) code CASTEP [9]. These were analysed in the same way as the experimental spectra (Figure 2). Experimental and theoretical data were compared and showed a reasonable correlation (Figures 3 and 4): the proportion of non-planar to planar sp² bonded carbon was observed to increase following a fluence of 2 × 10⁸ e-nm^-2 and the C-C bond length was also observed to increase.

These reconstructed models bridge the gap between the primary damage obtained in former molecular dynamics studies which can only cope with short timescales, and the severe damage observed in TEM images and EEL spectra after prolonged exposure to neutrons or electrons.

[1]        D.E. Baker, Nucl. Eng. Des. 14 (1970) 413–444.

[2]        P.I. Raynal et al., Carbon Conf., 2010, pp. 1–2.

[3]        B.E. Mironov et al., Carbon 83 (2015) 106–117.

[4]        J.-M. Leyssale et al., Appl. Phys. Lett. 95 (2009) 231912.

[5]        J.-M. Leyssale et al., Carbon 50 (2012) 4388–4400.

[6]        B. Farbos et al., Carbon 80 (2014) 472–489.

[7]        P. Hohenberg, W. Kohn, Phys. Rev 136 (1964).

[8]        W. Kohn, L.J. Sham, Phys. Rev 140 (1965).

[9]        S.J. Clark et al., Zeitschrift Für Krist. 220 (2005) 567–570. 

Figures:

IGAR reconstruction after 12 minutes of electron beam exposure showing a range of atomic environments, coloured accordingly (4-fold C4 red atoms, 3-fold sp2 C6 white atoms, defected 3-fold non-planar sp2 C6a,b orange and blue atoms).

Example of fitted EEL core loss spectrum using the 5-Gaussian fitting outlined in [3]

Comparison of data for planar and non-planar sp2 calculations from experimental and simulated spectra and directly from the IGAR models.

Comparison of data for bond length from experimental EELS and directly from the IGAR models.

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