TEM analysis of pellet-cladding bonding layer in high burnup BWR fuel

Detailed analysis of the pellet-cladding bonding layer in high burnup nuclear fuel has been done by transmission electron microscopy (TEM). A specimen was prepared from the fuel, which had been irradiated to the pellet average bumup of 49 GWd/tU (1.2 × 10²¹ fissions/cm³) in a boiling water reactor (BWR). A 20 μm thick bonding layer which consisted of two regions was observed. In one region from the inner surface of the Zr liner cladding to 12-13 μm away, the main species identified was ZrO₂ with a small amount of dissolved UO₂ also present. This ZrO₂ consisted of a mixture of cubic polycrystals of a few nanometers in size and an amorphous phase, but no monoclinic crystal, which is the stable ZrO₂ phase at temperatures below 1170°C, was found. In a second region from the pellet surface to about 7 μm away, both a cubic solid solution of (U, Zr)O₂ and an amorphous phase existed, in which the concentrations of UO₂ and ZrO₂ changed continuously. The formation of substitutional solid solution progressed during irradiation due to a strong contact between cubic UO₂ and ZrO₂, which induced subsequent mutual diffusion of U and Zr. Phase transformation from monoclinic to cubic ZrO₂ and amorphization were discussed in connection with fission damage.