Rim structure formation of isothermally irradiated UO2 fuel discs

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.