The swelling of uranium and of a few selected uranium alloys on post-irradiation annealing was investigated by utilizing density measurements in conjunction with the observation of pores in the microstructures of annealed specimens. Specimens were irradiated to about 0.3 at.% burnup in a constrained condition at approximately 275 deg C and were subsequently pulse annealed. The amount of swelling was found to be less than 1% for U specimens that were pulse annealed up to 75 hr at temperatures below 550 deg C; the amount of swelling, however, increased considerably on annealing at temperatures between 550 and 650 deg C. Specimens pulse annealed up to 75 hr at 618 deg C decreased in density by approximately 18%. The swelling was accompanied by the formation of bubbles on grain boundaries in recrystallized regions. The observations suggest that recrystallization is a necessary prerequisite for pronounced swelling in the alpha phase.

Available data on two alloys from the EBR-II driver fuel development program have been utilized in the construction and validation of mechanistic models aimed at elucidating swelling mechanisms in high density uranium alloys. Swelling predictions are made under ATR conditions for U-10Mo fuels, currently under irradiation in the ATR, and for U-10Zr.

Bare and zirconium-clad uranium-10 wt% molybdenum specimens were irradiated in NaK-filled capsules in the MTR. Irradiation conditions varied to include central-core temperatures ranging from 300 to over 1200 deg F, fuel burnups ranging from 0.36 to over 3.0 total at.% and fission rates in the range of 0.35 to 1.9 x 10/sup 14/ fissions/(sec)(cm/sup 3/) of alloy. Other parameters studied included the effects of heat treatment, changes in composition, different fabrication techniques, and changes in cladding thickness on the behavior of the fuel alloy. The objective of the irradiations was to determine the behavior of the fuel alloy under conditions approaching as closely as possible those to be encountered in the Enrico Fermi Atomic Power Plant as they were known at the time. The results indicated that the volume of the fuel alloy would increase conservatively at a rate of about 3.0% per at.% burnup as long as the critical temperature of 1000 to 1100 deg F was not exceeded and the gamma phase of the alloy did not transform during irradiation. If the critical temperature was exceeded, the alloy swelled until rupture or complete disintegration occurred. The occurrence of transformation during irradiation was noted at burnups in the range of 2.5 total at.% at fission rates of 1.5 x 10/sup 14/ to 1.9 x 10/sup 14/ fissions/(sec)(cm/sup 3/) and temperatures of 800 to 1000 deg F. The alloy was normally maintained in the gamma phase during irradiation, even at temperatures below 1100 deg F and at fission rates in the range of 0.7 x 10/sup 14/ fissions per sec. Transformation during irradiation was accompanied by excessive swelling of the alloy. (auth).