THE ROLE OF CLAY MINERALS IN YUCCA MOUNTAIN VOLCANICS AND ALLUVIUM ON TRANSPORT OF RADIONUCLIDES

Our previous studies on transport of radionuclides in saturated Yucca Mountain alluvium have suggested that fine particles, rich in smectite and clinoptilolite, possess a larger radionuclide retention capacity than the bulk alluvium material. To enhance our knowledge of the role of clay minerals on transport of radionuclides in the saturated zone near Yucca Mountain, we have carried out studies of sorption of Np (V), Pu (V), and U (VI) onto clay-rich particles obtained from Yucca Mountain volcanics and alluvium by a settling-velocity separation method (Chipera S.J. et al., 1998). Our results indicate that particle size smaller than 0.25 mm (diameter) of these materials are composed of more than 96 weight percent clay minerals. Preliminary results indicate that Np (V) sorption capacity of these clay minerals is about 100 times larger than that of the primary quartz and feldspar minerals in the Yucca Mountain rocks. These results suggest that fine clay particulates in the saturated zone may play a prime role in radionuclide transport near Yucca Mountain, both from the standpoint of attenuating radionuclide transport, and from the standpoint of facilitating the transport of radionuclides that are strongly sorbed to clay minerals which become mobilized as colloids.