Alteration of uranium minerals in the Koongarra deposit, Australia: Unweathered zone

Abstract

Abstract The alteration behavior of natural uranium minerals may be used as a natural analogue for the long-term behavior of actinide-bearing ceramic waste forms. Specifically, uraninite (UO 2+ x ) has been used as a natural analogue in the study of the processes and alteration products that are important in the corrosion of the UO 2 in spent nuclear fuel. Uranium minerals from the Koongarra deposit, Northern Territory of Australia, were examined in order to understand the alteration processes of the primary ore body in the unweathered zone, which extends at depths greater than 20 m from the surface. The primary mineral is uraninite with less than 2 wt% of CaO and 10 wt% of PbO. Uraninite has been altered to uranyl-lead oxides and uranyl silicates by the ground water which contains Si (10 mg/1), Mg (25 mg/l) and PO 4 (100 μg/l) at the present time. Uraninite reacted to form sklodowskite, a uranyl-Mg silicate (MgSi 2U 2O 11 · 7H 2O). Lead in uraninite is not released to form kasolite, a uranyl-Pb silicate (PbSiUO 6 · 2H 2O), but was enriched in residual phases such as curite (Pb 3U 8O 27 · 5H 2O) and wo¨lsendorfite (PbU 2O 7 · 2H 2O). Primary uraninite contains Ca, but Ca has an apparently high mobility and was not retained in the U-Pb oxides or uranyl silicates. Kasolite is observed with Pb-free sklodowskite in the uranyl silicates region formed by the alteration of uraninite and U-Pb oxides. Kasolite crystals are euhedral and acicular. This suggests that radiogenic Pb, produced in sklodowskite, was released to form kasolite. Redistribution of Pb in uranyl silicates may take place continuously in the uranyl silicates region of the deposit.