Effect of Fast Neutron Bombardment on Physical Properties of Graphite: A Review of Early Work at the Metallurgical Laboratory

Abstract

Fast neutron bombardment of artificial graphite increases the longitudinal elastic modulus and electrical resistivity and decreases the thermal conductivity. Annealing at temperatures higher than the bombardment temperature reverses the neutron-induced property changes, but complete recovery does not occur until annealing temperatures approach 2000°C. These effects are considered to be the results of the displacement of carbon atoms from normal positions in graphite crystallites to interstitial positions and the production of lattice vacancies. The displaced atoms are trapped in a variety of metastable interstitial trapping centers and, as a result, increase the internal energy of the graphite. Annealing causes interstitial atoms to move to more stable positions, thus releasing stored energy and causing recovery of other neutron-induced changes. Hall effect studies indicate that the neutron-induced disturbances (interstitial atoms and clusters of atoms, and lattice vacancies) are electron traps. Data are presented and the implications of various observations are discussed.