Rim structure formation and high burnup fuel behavior of large-grained UO2 fuels

Irradiation-induced fuel microstructural evolution of the sub-divided grain structure, or rim structure, of large-grained UO2 pellets has been examined through detailed PIEs. Besides standard grain size pellets with a grain size range of 9–12 μm, two types of undoped and alumino-silicate doped large-grained pellets with a range of 37–63 μm were irradiated in the Halden heavy water reactor up to a cross-sectional pellet average burnup of 86 GWd/t. The effect of grain size on the rim structure formation was quantitatively evaluated in terms of the average Xe depression in the pellet outside region measured by EPMA, based on its lower sensitivity for Xe enclosed in the coarsened rim bubbles. The Xe depression in the high burnup pellets above 60 GWd/t was proportional to d−0.5–d−1.0 (d: grain size), and the two types of large-grained pellets showed remarkable resistance to the rim structure formation. A high density of dislocations preferentially decorated the as-fabricated grain boundaries and the sub-divided grain structure was localized there. These observations were consistent with our proposed formation mechanism of rim structure, in which tangled dislocation networks are organized into the nuclei for recrystallized or sub-divided grains. In addition to higher resistance to the microstructure change, the large-grained pellets showed a smaller swelling rate at higher burnups and a lower fission gas release during base irradiation.