Rim structure formation and high burnup fuel behavior of large-grained UO2fuels

Irradiation-induced fuel microstructural evolution of the sub-divided grain structure, or rim structure, of large grained UO2 pellets has been examine d through detailed PIEs, Besides standard grain size pellets with a grain s ize range of 9-12 mu m. two types of undoped and alumino-silicate doped lar ge-grained pellets with a range of 37-63 mu m were irradiated in the Halden heavy water reactor up to a cross-sectional pellet average burnup of 86 GW J/t. The effect of grain size on the rim structure formation was quantitati vely evaluated in terms of the average Se depression in the pellet outside region measured by EPMA, based on its lower sensitivity for Se enclosed in the coarsened rim bubbles. The Se depression in the high burnup pellets abo ve 60 GWd/t was proportional to d(-0.5)-d(-1.0) (d: grain size), and the tw o types of large-grained pellets showed remarkable resistance to the rim st ructure 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 forma tion mechanism of rim structure, in which tangled dislocation networks are organized into the nuclei for recrystallized or sub-divided grains. In addi tion to higher resistance to the microstructure change, the large-grained p ellets showed a smaller swelling rate at higher burnups and a lower fission gas release during base irradiation. (C) 2000 Elsevier Science B.V. All ri ghts reserved.