English Abstract Within phase I of the INTRAVAL project, the Koongarra
uranium ore deposit which is located in the Alligator Rivers Region of the
Northern Territory (Australia), was selected as a test case. This test case
attributes to the understanding of the processes relevant for the modelling
of long term radionuclide migration through the geosphere from a nuclear
waste repository. The specific objective of this INTRAVAL test case is to
develop a consistent picture of the processes that have controlled the
transport of radionuclides in the weathered zone and the time scale over
which they have operated. At the Koongarra site, the primary uranium ore
consists of uraninite (UO2). The reaction path calculations modelling the
alteration of uraninite and biotite, resulted in direct transformation of
biotite to kaolinite. Precipitation of chlorite did not occur during the
calculations, which is in agreement with the present hypothesis of
hydrothermal alteration of biotite to chlorite at Koongarra. The modelling
resulted in the precipitation of the uranyl silicate, soddyite. However,
soddyite is not identified at Koongarra and hence, the formation of this
uranyl silicate mineral during the modelling, can not explain the occurrence
of the uranyl silicate zone at Koongarra. The formation of soddyite during
the calculations is supposed to be caused by the use of inadequate
thermodynamic data.
