Grain subdivision and structural modifications by high-energy heavy ions in UO2 with different initial grain size

Irradiation with 84 MeV Xe ions at peak fluence above 7 x 10(17) ions/cm(2) has been carried out on two UO2 samples with different initial grain sizes at 300 degrees C and 500 degrees C. Characterization of both grain structure and bubble morphology has been performed using transmission electron microscopy, image analysis and precession electron diffraction. The bubble size in the large-grained sample was smaller than that in the standard-grain sample, confirming fission gas precipitation resistance in the former. The ion irradiation in the high electronic energy deposition range has caused formation of a large density of dislocations which resulted in grain polygonization for the sample with standard grain size, which was observed in multiple lamellas extracted next to each other at peak dose. The sample with large grains showed inconsistent behavior. In one case, dislocation pileup occurred, but no sub-grain formation was observed. An adjacent location showed sub-grain formation, even if to less extent compared to the standard grain-size sample. These findings suggest a limited polygonization resistance in large-grained UO2 compared to standard-grain UO2. Recrystallization of both sample surfaces was present, leaving unclear if this is a surface effect or a consequence of the highest energy deposition rate in this area.