Spinel ceramics with various stoichiometries were bombarded with fast neutrons (> 0.1MeV) up to a fluence of 2.3 x 1024/m2 at about 500C. In the case of near-stoichiometric compositions, the average length changes and the corresponding lattice parameter changes which were caused by the irradiation were very small; although some specimens contracted by relatively large amounts. These specimens were isochronally annealed at up to 1000C. During annealing, the macroscopic lengths of specimens whose length had been reduced by irradiation exhibited step-wise increases at about 650C, and returned to the pre-irradiation value. On the other hand, the macroscopic lengths of specimens whose length had increased very slightly due to irradiation remained almost unchanged. The changes in macroscopic length agreed well with the changes in lattice parameter of all of the specimens during irradiation and annealing. It was suggested that the step-wise increase in length and lattice parameter during annealing was related to an order-disorder transition of the cation distribution.

T.Yano, H.Sawada, A.Insani, H.Miyazaki, T.Iseki: Nuclear Instruments and Methods in Physics Research B, 1996, 116[1-4], 131-5