THE INFLUENCE OF SPECIMEN ROUGHNESS ON THE RATE OF FORMATION OF U3O8 ON UO2 IN AIR AT 250-DEGREES-C

Disks of sintered, unirradiated UO2 fuel with different surface roughn ess were oxidized in air at 250 degrees C. X-ray powder diffractometry was used to quantify the rate of U3O8 formation. The variation of the rate of U3O8 formation was most significant when the particle size of the polishing media was in the range 1-18 mu m. The time to reach a g iven percentage conversion of UO2 to U3O8 decreased by about a factor of 4 between the smoothest and roughest surfaces. This corresponds to an increase of about two orders of magnitude in the composite nucleati on-and-growth rate constant, kappa. This effect is attributed to a com bination of increased surface area and increased surface density of nu cleation sites, as well as a crystallographic orientation phenomenon. (C) 1998 Published by Elsevier Science B.V. All rights reserved.