Some Preliminary Results on Defects in Irradiated UO2 Single Crystals as Revealed by Transmission Electron Microscopy

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Abstract
Vapor‐grown single crystals of nearly stoichiometric UO2 have been thinned for transmission electron microscopy. The as‐grown crystals show a dense pattern of precipitates and may contain loops due to quenched‐in vacancies but contain hardly any dislocations. Two models for the production of loops are proposed. Dislocation networks (small angle boundaries) are produced on annealing the crystals. Irregular patterns of dislocations arise when sufficient thick parts of the crystal foils are treated with heat shocks in the microscope suddenly increasing the beam current. After exposure to 1015 nvt the crystal foils show fission tracks, short segments of straight dislocation lines and black dots, which are partly distributed at random and partly are aligned on fission tracks. Interaction between fission tracks and dislocations has been observed. The conditions for contrast production on fission tracks have been investigated. The experimental findings can be explained by a model of a displacement field around the fission tracks.

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