DIMENSIONAL CHANGES IN GRADE H-451 NUCLEAR GRAPHITE DUE TO ELECTRON-IRRADIATION

Porosity plays a very important role in the overall dimensional change s induced by irradiation in many graphitic materials. In the present s tudy, a nuclear grade isotropic graphite, H-451, was irradiated in sit u in the electron microscope in regions bordering existing pores. Sele cted area diffraction patterns of different regions of this material s howed that graphite grade H-451 is a well-graphitized, single-phase ma terial whose structure is essentially identical to that of highly orie nted pyrolytic graphite (HOPG). Therefore, structural changes in the i ndividual crystallites must be the same as found in HOPG in our previo us study. Crystallites having either of the two principal orientations parallel to the direction of the electron beam, viz., the c-crystallo graphic axis direction parallel or normal to the beam, could easily be found in specimens thinned to electron transparency. Both pore openin g and closure were detected during irradiation depending upon the crys talline orientation around the pore. With increasing electron dose, el ongated pores bound by crystallites with their c axis normal to the po re surface closed, while pores bound by crystallites with their basal plane in the plane of the pore at the observation surface opened up. T hese effects are consistent with the dimensional changes observed in H OPG. The present results are compared with the macroscopic changes obs erved under reactor irradiation.