Fracture properties of sintered UO2 ceramic pellets with duplex microstructure

The fracture behavior of sintered ceramic UO2 material is studied in light of microstructural (grain size and porosity) parameters. Using Vicker's indentation technique the fracture properties have been evaluated and compared for different microstructural conditions present in the same sample. In a sintered pellet sample, duplex microstructure was obtained by addition of additives like SiO2 and Fe2O3. The duplex microstructure consisted of (1) high-density region with fine intragranular porosity present in coarse sized grain and (2) low-density region having intragranular porosity with fine grain size. The fracture toughness in the low-density region was higher as compared to the high-density region. This was attributed to crack tip deflection and branching at the grain boundary due to porosity. Acoustic emission signals were obtained during indentation. These were used to characterize the crack initiations and propagation during indentation. Toughening due to porosity with fine grain size has been explained using a mathematical model. These observations are in line with the observations of a tough rim structure formed at high burn-up. The rim has similar microstructural features to those observed in the low-density region of the sample used in the current study. In the end it is concluded that toughening in these ceramics can be achieved by suitable control of porosity and microstructure