Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)

Microcracks with varied length and width are observed in nuclear grade graphite and highly oriented pyrolytic graphite (HOPG) by transmission electron microscopy. In situ observations show that these cracks tend to close up on heating the sample. The crystal dimensional change from in situ electron-irradiation also causes the closure of the cracks. Although some of the cracks may be identifiable as accommodation porosity (i.e. Mrozowski cracks), others appear to have already formed prior to carbonization and graphitization.