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 burnup of 49 GW d/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. (orig.)