X-RAY-POWDER DIFFRACTION STUDY OF ANNEALED URANINITE

Natural UO2+x (uraninite) samples were annealed in a reducing atmosphere up to 1200-degrees-C. The initial unit cell parameters of 0.5385, 0.5439 and 0.5458 nm increased after annealing to 0.5469, 0.5447, and 0.5462 nm, respectively. In one sample the unit cell parameter decreased from 0.5463 nm to 0.5456 nm. Migration of point defects in the oxygen sub-lattice occurs at 300-degrees-C. In the interval of 500 to 700-degrees-C, uranium vacancies migrate and cluster. The formation of uranium vacancies is enhanced by the presence of radiogenic Pb. Above 750-degrees-C, the unit cell parameter increases as a result of reduction U6+ to U4+. Uraninite incongruently decomposes at 1100-degrees-C. Density increased after annealing except for one sample in which a large number of bubbles (1-2-mu-m in size) formed. The complex annealing behavior of uraninite is due to the presence of impurities (Pb, Ca, Th, Zr, REE), mixed oxidation states of uranium, and alpha-decay event radiation damage.