As previously discussed, graphite crystal structures, in the form ofHOPG, have been observed to swell in
the ‘C axis direction and contract in the ‘a’ axis direction for all measured fluence and irradiation temperatures. Figure 19 shows early data obtained by Kelly eta/.35 It is clear from Figure 19 that the rate of swelling and shrinkage significantly changes between 200 and 250 0C, indicating that the defect population is becoming more stable above this tem­perature range.

HOPG data is an important input into multiscale models of irradiation damage in graphite.48 For the purpose of understanding irradiation damage in operating reactors, data would be required ideally from 140 to 1400 0C, the maximum fluence being dependent on the irradiation temperature. Unfortu­nately, the dataset is far from complete. The data due to Brockelhurst and Kelly49 is the most complete set of HOPG irradiation data covering the fluence and part of the temperature range appropriate to AGRs (Figure 20). In the same paper, the authors show the effect of final heat treatment, between 2000 and 3000 0C, on the crystal dimensional change rate of HOPG. The data showed that the lower the heat treatment, the faster is the dimensional change rate, indicating that the dimensional change rate ofa poorly graphitized component would be expected to be greater than that of majority of the components.50 Pre­viously, Kelly and Brocklehurst51 had shown that boron doping also significantly increased the dimensional

change rate in HOPG, and this was again reflected in the behavior of doped polycrystalline graphite.50

HOPG high-temperature data was mainly obtained by investigators interested in the behavior of HTR fuel coatings.52 Some of this data is for low-density pyrolytic carbons and it is not always made clear which material the data refers to. Figure 21 shows all the data known to the author, and it is clear that there is some inconsistency.