Fig. 10-3 Cross-section of Koongarra uranium ore deposit, Australia Uranium migrated to the fractured zone around 1,600 million years ago (a). After a long period of stability, groundwater containing oxygen infiltrated about 2 million years ago (b). Consequently, some portions of uranium were dissolved in the groundwater, and migrated downstream to form a secondary ore deposit (c).

Fig. 10-4 SEM micrograph of a rock sample collected between the primary and secondary ore deposits. A: apatite, SL: uranium concentrated area. This photo indicates that the uranium which migrated from the primary ore deposit was sorbed on apatite, then mineralized to form a uranium mineral.

Fig. 10-5 OM (right) and SEM (left) micrographs of the rock sample collected at the secondary ore deposit. Uranium is concentrated in a yellowish iron mineral at a maximum of 8 wt%.

Underground materials migrate in the groundwater. These materials interact with various rocks (minerals) during this migration. The migration behavior depends on the mineral and the properties of the material. How does a material like uranium behave during migration? In JAERI, scientific research on uranium ore deposits has been conducted in order to examine the future behaviour of materials, after radioactive wastes have been disposed of into underground. Research has been conducted at the Koongarra uranium ore deposit in the northern part of Australia. This ore deposit is about 1,600 million years old, and has been largely stable since. Oxygen containing groundwater infiltrated about 2 million years ago, and as a result, some portions of uranium were dissolved in the groundwater, and it migrated downstream to form a secondary ore deposit; uranium moved only about 60 m (Fig. 10-3). The radioactivities of rock samples were measured, and these samples were also examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (Fig. 10-4, 10-5). As the result, uranium was found to be concentrated in the iron minerals and apatite mineral (a mineral with similar characteristics to the human tooth), which exist in the host rock and weathered rock. We have also carried out accelerated experiments to clarify the mechanism. Finally, we have found that uranium is associated with phosphate, which exists in the groundwater and in minerals. The interaction of the groundwater with these minerals results in the formation of a uranium mineral.

Reference T. Ohnuki et al., Change in Sorption Characteristics of Uranium during Crystallization of Amorphous Iron Minerals, J. Nucl. Sci. Technol., 34, 1153 (1997).

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