Modelling Study on Uranium Migration in Rocks Under Weathering Condition
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MODELLING STUDY ON URANIUM UNDER WEATHERING CONDITION
MIGRATION IN ROCKS
TOSHIHIKO OHNUKI, TAKASHI MURAKAMI*, HIROSHI ISOBE, TSUTOMU SATO and NOBUYUKI YANASE, Dept. of Environmental Safety Research, JAERI, Tokai, Ibaraki, Japan, *Dept. of Earth Sciences, Ehime University, Matsuyama, Ehime, Japan
ABSTRACT A modelling study has been completed to understand the effect of rock alteration on uranium migration at the Koongarra ore deposit, Australia. The model considers the weathering process, the mechanism and rate of chlorite alteration, a major mineral of the host rock, and assumes the presence of reversible sorption sites of chlorite and the presence of reversible and irreversible sorption sites of the weathering products. One- and two-dimensional, calculated uranium concentrations were compared with those observed. Good agreement between the calculated and observed uranium concentration profiles was obtained only when an appropriate fraction of uranium is fixed to the irreversible sorption sites of Fe-minerals produced during weathering of chlorite. On the other hand, the conventional Kd model failed to estimate an adequate uranium concentration profile. The results suggest that the fixation of uranium to Fe-minerals has dominated the migration of uranium in the vicinity of the Koongarra ore deposit.
INTRODUCTION Migration of radionuclides through rocks is one of the important pathways in the estimate of the impact of radioactive waste disposal on the environment.
Because
some amount of the radionuclides will be sorbed by the minerals of rocks during migration, rocks retard radionuclide migration. Minerals are altered to secondary minerals by water-rock interactions of the Earth's surface over geologic time. This alteration process is not static, but is dynamic. This leads to a question of the mechanism of radionuclides migration under such alteration processes. The uranium migration in the vicinity of the uranium ore deposit at Koongarra, Australia, has been intensively studied [e.g., 1,2]. The host rock of the Koongarra ore deposit is quartz-chlorite schist [2]. Although quartz is resistant to weathering and persists even at the surface, the chlorite has been weathered. X-ray diffraction analysis and scanning electron microscopy have revealed that the sequence of chlorite weathering is simply expressed as a conversion of chlorite -- vermiculite and Fe-minerals -- kaolinite and Fe-minerals. These minerals occur as a function of depth; near the end of the secondary ore deposit, chlorite alone is present at 25 m and deeper, but rapidly decreases in amount between 25 and 24 m, and disappears at 24 m. Vermiculite and Fe-minerals appear at 25 m depth and kaolinite persists at 24 m depth. Vermiculite disappears at 20 m where kaolinite and Fe-minerals are predominant. And kaolinite and Fe-minerals are predominant in the shallower zone above 20 m. The deeper zone under 25 m, the vermiculite presence zone between 20 and 25 m and the shallower zone above 20 m, are, respectively, called the unweathered zone, the transit
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