The properties and irradiation-induced changes in graphite crystals have been studied using both ‘natu­rally occurring’ graphite crystals and an artificial product referred to as highly orientated pyrolytic graphite (HOPG), formed by depositing a carbon substrate using hydrocarbon gas6 followed by com­pression annealing at around 3000 °C. HOPG is con­sidered to be the most appropriate ‘model’ material that can be used to study the behavior of artificially produced polycrystalline nuclear graphite. It has a density value near to that of a perfect graphite crystal structure, but perhaps more appropriately, it has imperfections similar to those found in the struc­tures that make up artificial polycrystalline graphite. A detailed description of the properties of graphite can be found in Chapter 2.10, Graphite: Properties and Characteristics.

4.11.2.1 Graphite Crystal Atomic Structure and Properties

In this section, the atomic structure of graphite crys­tal structures is discussed briefly, along with some of the properties relevant to the understanding of the
irradiation behavior of graphite. Graphite can be arranged in an ABAB stacking arrangement termed hexagonal graphite (see Figure 1). This is the most thermodynamically stable form of graphite and has a density of 2.266 gcm—3. The a-spacing is 1.415 A and the c-spacing is 3.35 A.

However, in both natural and artificial graphite stacking faults and dislocations abound.1