A Modelling Study on the Fractionation of Uranium Among Minerals During Rock Weathering
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A MODELLING STUDY ON THE FRACTIONATION OF URANIUM AMONG MINERALS DURING ROCK WEATHERING
TOSHIHIKO OHNUKI, TAKASHI MURAKAMI and NOBUYUKI YANASE Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan ABSTRACT
A modelling study has been carried out to understand the effect of rock alteration on the fractionation of uranium among coexisting minerals (chlorite, vermiculite, kaolinite, amorphous and crystalline iron minerals) at the Koongarra ore deposit, Australia. The model considers the chlorite weathering process, its mechanism and rate, and assumes the presence of reversible and irreversible sorption sites in the secondary minerals. The calculated uranium concentrations at the two different sites in the minerals were compared with the results of sequential extraction experiments. Good agreement between the calculated and observed uranium concentrations was obtained only when an appropriate fraction of uranium is fixed to the irreversible sorption sites of the altered clay minerals. However, a conventional Kd model gave inconsistent uranium concentrations. The calculated results show that the crystalline iron minerals sorb uranium during all stages of weathering, and that the uranium fractionation among the minerals varies with time until the end of the weathering. INTRODUCTION
The migration of radionuclides through a rock system is one of the important paths to estimate the impact of radioactive waste disposal on the environment. Because radionuclides are sorbed by minerals in a rock system during the migration, the rock system functions as a barrier against the migration. Minerals, however, are altered to secondary minerals over a geological timescale. This alteration process is not static, but is dynamic. Thus, it is a question how
radionuclides change their sorption characteristics during the alteration process. The weathering at the Koongarra uranium ore deposit, Australia, has resulted in the alteration of chlorite, one of the major minerals of the ore host rock, to vermiculite plus Fe-minerals and then to kaolinite plus Fe-minerals, and in the mobilization of uranium to form 300 m dispersion fan down gradient [1]. An a autoradiography study has shown that uranium concentrations are qualitatively proportional to the extent of the alteration; altered chlorite grains having higher uranium concentration [2]. The uranium fractionations between different minerals in the weathered zone have been measured in a series of sequential extraction experiments [3]. The results suggest that uranium is sorbed by Fe-minerals and clay minerals, and that the uranium fractionations between coexisting minerals are probably related to the alteration of chlorite to secondary minerals. Thus, a modelling study has been undertaken to examine how uranium is fractionated between minerals during the alteration process of chlorite. MODELLING CONCEPT
A conceptual model of chlorite alteration for the present study is schematically shown in Fig. 1. The solid and liquid phases, and
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