Atomic displacements due to fast neutron irradiation modify the ‘crystallite’ dimensions and most of their material properties. Neutron energies of around 60 eV are required to permanently displace carbon atoms from the lattice. However, most damage in graphite is due to fast neutron energies >0.1 MeV; a typical thermal reactor has neutron energies of up to 10 MeV, with an average of 2 MeV. High-energy neutrons knock an atom out of the lattice, leading to a cascade of secondary knock-ons. This process knocks atoms into interstitial positions between the basal planes, leaving vacant positions within the lat­tice. Many of the interstitial atoms will immediately find and fill these vacancies. However, others may form semistable Frenkel pairs or other small clusters or ‘semistable’ clusters. With increasing fast neutron damage, the stability, size, and number of these clus­ters will change depending on the irradiation tem­perature. The higher the irradiation temperature, the larger are the interstitial clusters or ‘loops.’ This process leads to considerable expansion in the graph­ite crystal ‘c’ axis. Conversely, vacancy loops also form and grow in size with increased irradiation temperature. It has been postulated that this process will cause the lattice to collapse leading to the ‘a’ axis shrinkage observed on irradiating graphite crystal structures. This process is illustrated in Figure 15.

Thrower32 carried out an extensive review of transmission electron microscopic (TEM) studies of defects in graphite, particularly those produced by fast neutron irradiation. He demonstrated that interstitial loops and vacancy loops could be distin­guished by tilting the specimen. He was able to observe vacancy loops in graphite irradiated only at and above 650 °C, whereas interstitial loops and defects were observed at all temperatures of interest to reactor graphite. It is proposed that the dimen­sional change in bulk polycrystalline graphite may be understood by eqn [29]33:

Ac

ffi +

c where Lc is the crystal dimension perpendicular to the basal plane, ‘c’ is the atomic lattice parameter, and r0 and r1 are the mean defect radius and mean half separation of defects in the basal plane, respectively. However, it was noted that this does not completely explain the expansion. In order to explain basal plane contraction it is necessary to postulate that vacancy lines cause the collapse of the basal planes.34-36

More recent atomistic calculations due to Telling and Heggie37 have sought to explain the process by the ‘buckling’ of basal planes until they twist round upon themselves. This latter explanation is more satisfying as it accounts for the atomistic bonding around the edges of the interstitial loops and vacan­cies. However, more HRTEM (high-resolution trans­mission electron microscopy) observations and other techniques are required to validate these theories.

Whichever mechanism is correct, empirical observations made on HOPG, and some natural crystal flakes,35 show that graphite crystal structures expand in V axis and shrink in the ‘a’ axis, the degree of deformation being a function of fast neu­tron fluence and irradiation temperature. Crystal dimensional change is discussed in more detail later in this section.