Microstructure Effects on Fracture Strength of U02Fuel Pellets

Microstructure effects on the strength of U02 pellets have been investigated. The strength was measured at room temperature using a biaxial flexure technique in which thin disk specimens, sliced from U02 pellets, were fractured by ring loading in the specimen center. The strength of U02 was reduced, in all cases, by increase in porosity, grain size or pore size. Porosity and pore size had a significant influence, while grain size showed only a weak effect. The diameter of the maximun pore observed in the specimens was close to the critical flaw size calculated from the Griffith criterion. This indicates that the largest pore is the most important preexisting flaw which controls the pellet strength. The grain boundaries associated with the largest pore could play a role as a path for pore extension in the fracture process. The strength of U02 can be expressed by where of is the fracture strength (MPa), PS the largest pore size (μ/m), GS the mean grain size (μm) and P the porosity (%). The equation demonstrated good predictive capabilities regarding strength of U02 pellets having wide variation in microstructure.