Equiaxed grain growth in swaged UO₂ compact

Temperature gradient was applied on swaged UO₂ compacts prepared from two different starting powders-arc-fused and highly fired, with which changes in microstructure were observed by optical and electron micrography for the purpose of obtaining information on equiaxed grain growth in swaged UO₂ compacts. In addition, microstructural changes under uniform temperature distribution were studied for comparison with the results obtained under temperature gradient. The principal conclusions drawn from the study are the following : (1) The presence of a variety of particle sizes in the starting powder affects equiaxed grain growth which is a characteristic feature of swaged compacts. With the onset of sintering, the pores concentrate locally in the regions where the smaller particles are assembled. The resulting microstructural appearance of the equiaxed grains is similar to that of die-pressed and sintered pellets except for the presence of larger sized pores. (2) There exists an upper limit to equiaxed grain growth in the case of specimens subjected to temperature gradient: In such specimens, while the portions heated to 1,700°∼1,800℃ reveal grain growth with directional tendency, other parts exposed to temperatures above 1,900℃, columnar grain growth. Such difference in grain growth behavior does not appear uniformly. The difference in grain size between specimens uniformly and non-uniformly heated becomes appreciable above 1,700℃. (author)