Uranium Fixation During Uranium Migration Under an Oxidizing Condition
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URANIUM FIXATION DURING URANIUM MIGRATION UNDER AN OXIDIZING CONDITION Takashi Murakami*, Katsuyuki Tsuzuki*, Tsutomu Sato**, Hiroshi Isobe** and Toshihiko Ohnuki** *Ehime University, Matsuyama, Ehime 790, Japan, **Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
ABSTRACT A rock specimen, collected downstream of the Koongarra uranium ore deposit, Australia, was examined mainly by high resolution transmission electron microscopy in order to understand the uranium fixation mechanism. Uranium was found to exist as sal6eite (Mg(U02)2(PO4)2. 10H20) microcrystals of 1 - 20 nm scattered between iron minerals (mainly goethite and hematite) of 2 - 50 nm. The microtextural relationship between sal6eite and the iron minerals revealed that the iron minerals function as catalyst for the formation of sal6eite. The intermediate metamict microstructures of the sal6eite microcrystals are consistent with the estimated formation age of sal6eite, 1 to 3 x 106 years. Uranium has been, thus, fixed as sal6eite downstream as well as in the secondary ore deposit. Sal6eite in the secondary ore deposit showed completely periodic to fully metamict microstructures, suggesting that sal6eite, a major uranium mineral in the secondary ore deposit, probably began to form a few million years ago and continued to form for the next million years. INTRODUCTION For the long-term safety assessment of high-level waste management, it is necessary to understand the long-term migration behavior of actinides because of their long half-lives. However, it is difficult to complete field observations and measurements for actinides, such as Pu, Np, and Am, though the field observations and
measurements are important in the safety assessment. Uranium migration from a uranium ore deposit can be a good natural analogue of actinide migration from a repository. The Koongarra uranium ore deposit, Australia, from which the release of uranium has been occurring by water-rock interactions over a geologic time, is an appropriate natural analogue site. The Koongarra ore deposit consists of a primary and secondary ore deposit. In the primary ore deposit, uraninite (U02+x), the primary uranium mineral, has been partly altered to uranyl-lead oxides and uranyl silicates [1, 2]. In the secondary ore deposit, below the surface to 20 m depth, uranium is found as uranyl phosphates,
mostly sal6eite,
Mg(U02)2(PO4)2.10H20
[1, 3], resulting from the dissolution of
sklodowskite, MgSi2U2011.7H20 and apatite, Ca5(P04)3F [3]. The uranium redistribution downstream of the two ore deposits has been strongly affected by weathering of the host rock, a quartz-chlorite schist; the weathered area having a higher uranium concentration than the unweathered area [4]. Weathering has resulted in the formation of vermiculite, kaolinite, ferrihydrite, goethite, and hematite [5]. The uranium is most closely associated with the iron minerals among the weathered products of chlorite [6]. Although the presence of uranium and its association with the weathered products downstream have
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