Porosity plays a very important role in the overall dimensional changes induced by irradiation in many graphitic materials. In the present study, a nuclear grade isotropic graphite, H-451, was irradiated in situ in the electron microscope in regions bordering existing pores. Selected area diffraction patterns of different regions of this material showed that graphite grade H-451 is a well-graphitized, single-phase material whose structure is essentially identical to that of highly oriented pyrolytic graphite (HOPG). Therefore, structural changes in the individual crystallites must be the same as found in HOPG in our previous study. Crystallites having either of the two principal orientations parallel to the direction of the electron beam, viz., the c-crystallographic axis direction parallel or normal to the beam, could easily be found in specimens thinned to electron transparency. Both pore opening and closure were detected during irradiation depending upon the crystalline orientation around the pore. With increasing electron dose, elongated pores bound by crystallites with their c axis normal to the pore surface closed, while pores bound by crystallites with their basal plane in the plane of the pore at the observation surface opened up. These effects are consistent with the dimensional changes observed in HOPG. The present results are compared with the macroscopic changes observed under reactor irradiation.
