The damage structure formed in molybdenum by irradiation in a fast reactor at 650°c

Abstract

Transmission electron microscopy has been used to study the damage structure of commercially pure and zone-refined molybdenum irradiated in a fast reactor to a total dose of ∼3 × 10 22 n/cm 2 at ∼650°C. In all cases the structures consist of coarsely distributed dislocation segments and loops, and a considerably finer distribution of small voids. The voids tend to be ordered on a bcc superlattice parallel to the underlying host lattice. Although the loops are predominantly interstitial in nature, a significant number of small vacacy loops are present in both the commercially pure and zone-refined materials. The formation of vacancy loops during irradiation can only be accounted for by in-site collapse of displacement spikes. The occurrence of such a process implies that the formation and growth of voids is dependent on emission of vacancies from the loops. An important practical consequence of having a high density of voids compared to other sinks for point defects is that the voids themselves act as dominant neutral sinks for vacancies and interstitials, leading to an early saturation in void growth.