A Study of the Annealing Behavior of Neutron Irradiated Graphite

Recycle is being considered as a disposal option for irradiated graphite from gas cooled reactors. Thermal annealing was performed on irradiated graphite samples to establish what fraction of in-crystal (displacement damage) and ex-crystal (pore generation) damage could be recovered. The sample dimensions and electrical resistivity were measured after isochronal annealing at 500, 1000, 1500, and 2000 C. Sample dimensions were unaffected by annealing. Some fraction of the resistivity was unannealable at 2000 C, this behavior which was attributed to (a) structural changes resulting from pore generation (also causing dimensions to be unaffected by annealing), or, (b) defect structures in irradiated graphite that persist even at an annealing temperature of 2000 C. The resistivity annealing behavior was different in low irradiation temperature samples, Tirr < ∼400 C and high irradiation temperature samples Tirr > ∼400 C, which indicated defect structures established during irradiation were different in these two temperature regimes. Moreover, the extent of recovery was much greater in the specimens with higher irradiation temperatures, and was small for lower dose low temperature specimens. Evidently, some in-crystal defects that scatter or trap electrons are still present in the crystal lattice after annealing and their tenacity depends upon the irradiation temperature and accumulated neutron dose.