Mg–Al spinel ceramics with different stoichiometry (Al2O3/MgO=n; n=1.00, 1.01, 1.10 and 1.48) were irradiated with fast neutrons up to 3.8 × 1023 n/m2 (E > 0.1 MeV) at 170°C, and 5.7 × 1023 n/m2 at 190°C in the Japan Materials Testing Reactor. There was no obvious difference in property change between the two irradiation conditions. The amount of macroscopic length change due to these irradiations increased almost linearly with increasing Al2O3 content. These specimens were isochronally annealed up to 1000°C. During the annealing, macroscopic length of the specimens started to decrease at around 200°C and reduced monotonically up to 550°C. After that, the length increased slightly up to 650°C and then decreased again up to 800°C, showing a small peak. These final values corresponded to the length of the specimens before the irradiation. The shrinkage in length between 200–550°C and 650–800°C may be attributed to the recombination of Frenkel pairs of anions, and the expansion in length between 550–650°C to the order–disorder transition of cation distribution.
