Magnesium aluminate spinel ceramics with different stoichiometry were irradiated with fast neutrons up to 2.3 × 1024 n/m2 (E > 0.1 MeV) at around 500°C. Near stoichiometric compositions, average length change and corresponding lattice parameter change by the irradiation were very small, but some specimens contracted relatively large amount. These specimens were isochronally annealed up to 1000°C. During the annealing, macroscopic length of the specimens whose length reduced by the irradiation showed stepwise increase at around 650°C, and returned to the pre-irradiation value. On the other hand, macroscopic length of the specimens whose length increased very slightly due to the irradiation almost unchanged. The changes in macroscopic length matched well with the changes in lattice parameter of all specimens due to the irradiation and post-irradiation annealing. The reason of the stepwise increase in length and lattice parameter during annealing may be related to the order-disorder transition of cation distribution in spinel.
