High magnification SEM observations for two types of granularity in a high burnup PWR fuel rim

Rim microstructure of an UO2 pellet irradiated up to 61 GWd/tU, observed with high magnification scanning electron microscopy, presented two types of subgrains: polyhedral and round subgrains. Round subgrains were also observed on the surface of fabrication pores along the pellet radius. These round-shaped subgrains seem to be associated to free surface rearrangement. This new type of round subgrains was proved to be independent of the rim effect. Observations of the fuel structure in the vicinity of rim area evidenced two types of defects specific to rim effect: planar defects and small intergranular pores. Hypothesis is made that the accumulation of planar defects could generate polyhedral subgrains, a new description of the rim effect is proposed including this planar defect. These observations prove that a careful examination of subgrains morphology is necessary to identify rim effect. (C) 1998 Elsevier Science B.V. All rights reserved.