Preferential vaporization of uranium oxide in the polycrystalline thoria-urania solid solution

The evaporation mechanism and the rate-controlling step were clarified for polycrystalline Th 0.90 U 0.10 O 2.05 and Th 0.75 U 0.25 O 2.13 solid solutions. The evaporation annealing was performed under a flowing air atmosphere at 1650°C for 32 h. The solid solutions showed incongruent evaporation in which uranium oxide preferentially vaporized. The rate of the vaporization was found to be controlled by the cation diffusion in the solid phase and enhanced by the presence of grain boundaries. In the case of the UO 2 -rich solid solution many cracks and pores were produced beneath the surface due to the enhancement.