THE CORROSION OF URANINITE UNDER OXIDIZING CONDITIONS

Uraninite is the most common uranium mineral, which, because of chemical and structural similarities to synthetic UO2, is a useful natural analogue for UO2 in spent fuel. Uraninite, however, contains "impurities" such as Pb, Ca, Si, U6+, Th, Zr, and lanthanides. These affect the thermodynamic properties of uraninite, the rate of uraninite alteration, and the composition of the corrosion products. Uraninite can contain a significant amount of radiogenic Pb, and the Pb-uranyl oxide hydrates (Pb-UOH) are the most common corrosion products formed by the oxidative alteration of Pb-bearing uraninites. Incongruent alteration of the Pb-UOHs in natural waters produces increasingly Pb-enriched uranyl phases, effectively reducing the amount of U lost from the corrosion rind. This is not true of other uranyl oxide hydrates, such as schoepite, UO3-2H2O, or becquerelite, CaU6O19.11H2O, which can dissolve completely under similar geochemical conditions. The most common end product of Pb-UOH alteration is curite. Curite may provide surface nucleation sites for certain uranyl phosphates, thereby enhancing their formation. Uranyl phosphates are generally less soluble than other uranyl phases. In the absence of Pb, schoepite and becquerelite are the common initial corrosion products. The reaction path for the alteration of Pb-free uraninite results in the formation of uranyl silicates, which are generally more soluble than the uranyl phosphates. Thus, the long-term oxidation behavior for ancient, Pb-bearing uraninite is different from young, Pb-free uraninite. Because the presence of Pb effectively reduces the mobility of uranium in oxidizing waters, the concentration of uranium in ground waters associated with oxidized uranium ore deposits will depend in part on the age of the primary uraninite.