Thermal annealing of nuclear graphite during in-situ electron irradiation

We have investigated the in-situ electron irradiation of nuclear graphite within a 200 kV transmission electron microscope at temperatures between 83 K and 473 K. For each temperature, nuclear grade Pile Grade A graphite specimens were subject to a fluence of ca. 1022 electrons cm−2, and transmission electron micrographs and selected area diffraction patterns were collected during electron beam exposure. By considering a critical fluence, at which the graphite (002) d-spacing increased by 10%, a temperature threshold for damage has been determined. Below ca. 420 K, electron irradiation caused significant net structural damage: fragmenting basal planes and producing a tortuous nanotexture. Above this temperature the effects of thermal annealing became more prevalent, maintaining the structure even at much higher fluences. We have derived activation energies for the annealing processes operative in these two temperature regimes and, via a comparison with theoretical predictions have, for the first time, associated these with specific recovery processes.