Grain Growth in Sintered Uranium Dioxide: II, Columnar Grain Growth

Evidence is presented which shows that columnar grains can be induced to grow in high-density sintered uranium dioxide specimens by applying a steep temperature gradient at temperatures above 1700°C but below the melting point of 2800°C. Columnar growth apparently is a result of the migration of large transverse voids, whose individual widths define the cross sections of the grains, up a temperature gradient by a sublimation process. The grains grown by this process have a (111) preferred orientation along their columnar axis. A consequence of such void migration in operating fuel elements containing solid UO₂ pellets is the formation of a central void bounded by a region of oxide exhibiting columnar growth.