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.