Rim structure formation of isothermally irradiated UO2 fuel discs

UO2 fuel discs, which had been irradiated at an isothermal condition of 550 -630 degreesC to 51, 86 and 90 GWd/t without any restraint pressure, have b een subjected to detailed microstructure observations. elemental analyses a nd density measurements. Their data were compared with previously reported results of high-burnup Zircaloy-clad type fuel pellets, so as to clarify th e effect of pellet-cladding interaction (PCI) restraint on the rim structur e formation. The porous rim structure, accompanying huge bubbles and high p orosity, was recognized for the high-burnup discs of 86 and 90 GWd/t, but n ot for the 51 GWd/t disc. From the good coincidence between porosity increa se and density decrease for the high-burnup discs, it was concluded that th e precipitation and growth of coarsened rim bubbles substantially caused fu el 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 Zirca loy-clad type pellets reported in other literature. possibly resulted from the external PCI restraint effect. The pressure difference between bubble i nternal and external pressures, and vacancy diffusivity would rate-control the growth of rim bubbles. (C) 2001 Elsevier Science B.V. All rights reserv ed.