UO2/Zry-4 chemical interaction layers for intact and leak PWR fuel rods

In this study, the UO2 pellet–Zry-4 cladding interfaces of intact and leak PWR fuel rods were examined with the help of an optical microscope and a scanning electron microscope to investigate typical chemical interaction layers formed at the pellet–cladding interface during the normal reactor operations. The two intact and two leak fuel rods with the burnup of between 35,000 and 53,000 MWD/MTU were selected to evaluate the effects of gap–gas compositions and fuel burnup on the chemical interaction layer formation. Based on the optical and scanning electron micrographs, it is found that the intact fuel rod generates apparently one interaction layer of (U,Zr)O2−x at the interface, whereas the leak fuel rod generates apparently two interaction layers of ZrO2−x and (U,Zr)O2−x. These interaction layers for the intact and leak fuel rods were predicted by several diffusion paths drawn on a U–Zr–O ternary phase diagram. The variations of chemical element compositions around the interface of one intact rod were generated by an electron probe micro-analyzer to confirm the interaction layers at the pellet–cladding interface. The interaction layer growth rates of the ZrO2−x and (U,Zr)O2−x phases were estimated, using the layer thicknesses and the reaction times.