The mechanism of dimensional changes in the crystals of graphites and carbons under fast neutron irradiation

The accumulation of irradiation damage in graphite crystals under neutron irradiation at temperatures between 150°c and 650°c is discussed using recent ideas on nucleation of planar clusters of interstitial atoms and the collapse of lines of vacant lattice sites produced by chance sequences of atomic displacement. It is shown that a simple kinetic model based on these concepts accounts for the build-up of damage to very high levels and also, using estimated values of the dimensional changes due to various defects, accounts for the magnitude of the observed crystal dimensional changes. The model explains qualitatively the small magnitude of the physical property changes at the higher temperatures in this range in comparison with the crystal dimensional changes.