As discussed previously (see Section 4.11.11.2), when irradiated, graphite crystallites, as simulated using HOPG, expand significantly in the V direction and shrink in the ‘a direction. These dimensional changes are reflected in the behavior of polycrystal­line graphite, but the volumetric changes, although relatively large are much smaller than those seen in HOPG. The reason for this is attributed to the many microcracks, which range in size from the nano — to microscale; see Figure 32. While these cracks can accommodate the crystal growth in the ‘c,’ the shrink­age in the V direction will directly be reflected in the polycrystalline behavior. However, as previously

discussed, the crystallite dimensional change rate is much greater below ^300 °C than above that temperature.