X-ray diffraction studies of irradiated beryllium oxide

Abstract

The lattice parameter changes which occur on neutron irradiation of hot pressed beryllium oxide to integrated fission neutron doses of 1 × 10 19 nvt to 3.5 × 10 20 nvt at 75–100° C and to doses of 1–1.5 × 10 20 nvt at 500–650° C are reported. In agreement with other workers, it is shown that an anisotropic lattice expansion occurs, the expansion in the c direction being greater than the expansion in the a direction. The lattice expansion at irradiation temperatures of 500–600° C is almost a factor of three smaller than for equivalent neutron doses at 75–100° C. The volume expansion calculated from X-ray data is shown to agree with the macroscopic volume expansion. At doses of 3.5 × 10 20 nvt and above at temperatures of 75–100° C, crumbling and cracking of the specimens occurs because of the intergranular stresses caused by the anisotropic expansion. The disagreement between various workers as to the degree of this anisotropy is explained in terms of the effect on the lattice parameters of the intergranular stress in solid polycrystalline specimens. The annealing behaviour of the lattice parameter changes is described and it is shown that temperatures of 1400–1500° C are required for complete recovery of the damage to occur. Line broadening effects are discussed in a qualitative manner and it is suggested that defect clusters are formed in the basal planes of the hexagonal beryllium oxide lattice.