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.