UO2 fuel discs, which had been irradiated at an isothermal condition of 550-630 °C to 51, 86 and 90 GWd/t without any restraint pressure, have been subjected to detailed microstructure observations, elemental analyses and density measurements. Their data were compared with previously reported results of high-burnup Zircaloy-clad type fuel pellets, so as to clarify the effect of pellet-cladding interaction (PCI) restraint on the rim structure formation. The porous rim structure, accompanying huge bubbles and high porosity, was recognized for the high-burnup discs of 86 and 90 GWd/t, but not for the 51 GWd/t disc. From the good coincidence between porosity increase and density decrease for the high-burnup discs, it was concluded that the precipitation and growth of coarsened rim bubbles substantially caused fuel swelling. The wide variety of rim bubble sizes and porosities at a given local burnup, which was recognized in the present PCI-free discs and Zircaloy-clad type pellets reported in other literature, possibly resulted from the external PCI restraint effect. The pressure difference between bubble internal and external pressures, and vacancy diffusivity would rate-control the growth of rim bubbles.
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