Changes in the macroscopic length, lattice parameter and thermal diffusivity of samples, neutron-irradiated to a fluence of up to 4.2 x 1025/m2, were measured. The thermal diffusivity was markedly reduced by irradiation at 390 to 540C. A slight increase in the a-axis parameter and a slight decrease in the c-axis lattice parameter was detected. The extent of the lattice parameter change was very small when compared with the macroscopic length change. Changes in these properties, due to post-irradiation thermal annealing at up to 1500C, were measured. A large fraction of the thermal diffusivity was recovered by annealing, but the macroscopic length did not change significantly during annealing. The change in the lattice parameter exhibited a complicated behaviour. It was proposed that the formation of interstitial loops on planes parallel to the c-axis, the formation of voids during annealing or the difficulty of recovery of point-defects/loops or solid-solution formation, due to a glassy grain-boundary phase, could affect the recovery behavior.

Physical Property Change of Heavily Neutron-Irradiated Si3N4 and SiC by Thermal Annealing. T.Yano, M.Akiyoshi, K.Ichikawa, Y.Tachi, T.Iseki: Journal of Nuclear Materials, 2001, 289[1-2], 102-9