Graphite (C) is a very important material for nuclear energy applications. Graphite is a moderator used to thermalize neutrons in thermal gas and water-cooled reactors in the United Kingdom and the Soviet Union, respectively.45 Pyrolitic graphite is one of the barrier coating materials used in TRISO coated fuel particles.33 Graphite and carbon composites are also used as plasma-facing materials in fusion reactors.46 Numerous radiation effects studies have been performed on graphite. Nevertheless, the behav­ior of graphite in a radiation damage environment remains poorly understood. This is due primarily to the fact that graphite comes in so many forms and is produced in so many different ways, that in fact, the structure and chemistry of graphite used in nuclear applications is not a well-defined constant. Neverthe­less, there are some aspects of the crystal structure of graphite and the changes in this structure induced by irradiation that are somewhat analogous to the discussion of Al2O3 versus MgAl2O4, presented earlier in this chapter.

Graphite is a hexagonal crystalline material, with an ABAB. .. layer stacking arrangement of carbon sheets. These carbon layers have obvious hexagonal atom patterns in them. However, they are not fully dense triangular atom nets, as would be the case in a close-packed structure. They are so-called graphene sheets, in which the atom pattern is a honeycomb pattern, identical to the cation layer patterns in Al2O3 (see Section 1.05.2.3.1). Each C atom is sur­rounded by three nearest-neighbor C atoms, and the bonding linking each C atom with its neighbors is characterized by sp2 hybridization. The bonding that links adjacent graphene layers is weak, Van der Waals-type bonding.

The interstitial dislocation loops that form in irra­diated graphite, by the condensation of freely migrat­ing interstitial point defects, form (not surprisingly) on (0001) basal planes, between adjacent graphene layers. In some of the earliest work on radiation effects in graphite, this was described as follows47:

When subjected to bombardment with fission neu­trons, primary collisions displace carbon atoms from their normal sites in the layers, driving them to sites between planes (interstitial or interlamellar positions).

This loop nucleation is analogous to the (0001) basal interstitial loops that form in Al2O3 during the initial stages of irradiation (Section 1.05.2.3.1). However, the basal loops in graphite do not grow to any signifi­cant size. Instead, the graphene layers adjacent to interlamellar loop nuclei buckle, which causes a net increase in the c-dimension of the hexagonal material and a concomitant decrease in the a-dimension.48 This buckling is believed to be due to sp3 bond formation between C interstitials and C atoms in the graphene planes.49,50 The overall macroscopic effect of c-axis expansion and a-axis shrinkage is dimensional changes of crystallites within the graphite. Macroscopic radia­tion damage effects in graphite are discussed in detail in Chapter 4.10, Radiation Effects in Graphite.