THERMAL CONDUCTIVITY OF FAST NEUTRON IRRADIATED GRAPHITE

Recent studies of changes in crystal dimensions and of crystal lattice parameters following fast neutron irradiation over a wide range of doses and temperatures have been interpreted in terms of two types of interstitial defect and three types of vacancy defect. It is now possible to estimate the concentration of interstitial atoms and vacancies in each type of defect as a function of neutron dose and irradiation temperature. The relative concentration of each type depends on the irradiation conditions, and in addition the annealing characteristics of each defect type are different. Measurements have been made of the temperature dependence of the principal thermal conductivities of highly oriented pyrolytic graphite irradiated to selected doses at certain temperatures, and of changes in thermal conductivity during post-irradiation annealing. These changes are analysed in detail using a simplified theory of phonon scattering in the graphite lattice and the defect concentrations obtained from the analysis of dimensional changes. More detailed information has been published on the changes of thermal resistivity of reactor grade graphite at ambient temperature under the same conditions and this information is also successfully analysed by the theory. Analysis of these data yields meaningful scattering parameters and gives estimates of defect sizes in reasonable agreement with those obtained by other workers. Considering the temperature dependence of the data on pyrolytic graphite, in the direction parallel to the basal planes at temperatures > 300°K the results can successfully be interpreted in terms of the parameters obtained from the reactor grade graphite analysis. At lower temperatures the resistance is found to increase with decreasing temperature. It is concluded that this is probably due to electron phonon scattering by electronic states associated with point defects. Parallel to the hexagonal axis approximate estimates are made of the thermal resistance changes due to each type of defect. © 1969.