alpha-Al2O3, AlN, beta-Si3N4, and beta-SiC ceramic specimens were irradiated by a 30 MeV electron beam up to 0.01 dpa at 80 degrees C, and they started to recover thermal diffusivity with annealing from 300-500 degrees C. In previous studies, similar ceramics were heavily neutron-irradiated in a fast reactor, JOYO, and the neutron-irradiated beta-Si3N4 and beta-SiC also showed recovery above 400 degrees C, though they were irradiated at 502 degrees C or 738 degrees C. This result is explained by the hypothesis that a 'primary' irradiation occurred at the set temperature, and after that, a 'secondary' irradiation occurred at a lower temperature during the shutdown process of the reactor. In contrast, the neutron-irradiated alpha-Al2O3 ce and AN did not show such unexpected recovery, but showed recovery above 800-900 degrees C. This difference is explained on the basis of structural models of dislocation loops namely, the 'pileup' model and the 'nano-partition' model. (C) 2015 The Authors, Published by Elsevier Ltd.