Small pieces of Zircaloy-clad 7 and 8 wt. % uranium-zirconium alloys irradiated from 0.4 to 1.5 at. % burnup of the total core atoms in the MTR were heat-treated at temperatures between 510 and 980 C for periods of time from 1 sec to 500 hr. The resulting changes in density, electrical resistiviiy, and microstructure were observed. Tbe core volume changes were found to be a function both of temperature and time at temperature for a given burnup. A decrease in the swelling rate with time was observed. Thus, at 620 C about 40% of the core volume change after 300 hr (37%) occurred in the first 6 hr. At lower temperatures, more time was required for an equivalent fractional volume change. Metallographic changes and gas bubble formation at phase boundaries were observed optically at temperatures as low as 510 C in high burnup material. Bubbles were also observed in the electron microscope. The changes observed appear to be due to the growth of inert gas bubbles formed in the epsilon phase of uranium-zirconium alloy and to a transformation occurring in the delta phase. The effect of irradiation temperature and flux on the nucleation and growth of the gas bubbles is discussed. (auth)

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