Changes to C33 and C44 in HOPG and natural graph­ite crystals have been reported at 50, 650, and 1000 °C53 and at 150 °C.54 For HOPG, the 150 °C data indicated that C33 slightly reduced with increas­ing irradiation (Figure 24) and this was attributed to the increase in ‘C axis lattice spacing. However, there is no clear trend at the other temperatures (Figure 24).

In the case of shear, at a very low temperature of 50 °C there was a significant increase in C44, but at higher temperatures the increase was less (Figure 25). The data for natural crystal showed similar trends but there was significantly more scatter. The trend in the increase in C44 at the lower temperature would go towards explaining the increase in modulus in polycrystalline data at low fluence. However, it is surprising that the increase is only modest at the higher temperatures, although the maximum fast neutron fluence is very low and data is required at the intermediate temperatures.

Seldin and Nezbeda53 also measured the shear strength but unfortunately there is considerable scat­ter and no definite trend.

4.11.11.1 Thermal Conductivity

Taylor et al55 measured the change in thermal conductivity in HOPG with fast neutron
irradiation. The thermal conductivity along the basal planes (the ‘a’ direction) is much greater than the value perpendicular to the basal planes (the V direction). Taylor et al. also measured the change in thermal resistivity in irradiated graphite, and when this data is normalized, the data indicated that thermal resistivity temperature dependence changed with irradiation as given in Figure 26. This is the so-called ‘d’ curve that is used in the United Kingdom to predict thermal resistivity in irradiated graphite.