Effect of grain size on recrystallization in high burnup fuel pellets

The effect of grain size on recrystallized structure formation in the outer region of high burnup UO₂ fuel pellets was studied by optical microscopy, SEM, EPMA, XRD and TEM. Specimens were prepared from three kinds of fuels with different grain size (the standard pellet (grain size: 9 μm), the undoped large-grained pellet (51 μm) and the alumino-silicate-doped large-grained pellet (45 μm)), irradiated up to an average pellet burnup of 60 GWd/t in the Halden Reactor. The TEM observations showed that recrystallized structures were formed in a region from the middle to the edge (relative radius: r/r₀ = 0.7-1.0) of all fuel pellets, though they were less likely to form in the undoped large-grained pellet and the alumino-silicate-doped large-grained pellet than in the standard one. This result agreed qualitatively with the results obtained from optical microscope observations of the whole pellet region, SEM fractographs, and measurements of Xe concentration in the fuel matrix by EPMA. The effects of grain size and irradiation temperature on recrystallized structure formation were discussed in connection with fission damage accumulation.