MOBILIZATION OF URANIUM FROM THE NOPAL I ANALOG AND IMPLICATIONS FOR NUCLEAR WASTE REPOSITORY PERFORMANCE

An important analog feature of the Nopal I uranium (U) deposit is the fractured silicic tuff host rock. In a zone a few tens of meters horizontally around Nopal I, elevated U concentrations in fracture-filling minerals indicate past mobilization and redeposition. While it is not possible to quantify precisely the mass that was transported and not captured, this secondary U enrichment indicates release and transport. Three classes of fracture fills have been studied: iron-rich, carbonate-rich, and opaliferous. Uranium deposition in iron-rich fractures took place during and after oxidative alteration of pyrite to goethite, hematite, jarosite, and amorphous iron oxyhydroxides. Uranium-series isotope systematics suggest this enrichment took place within the past few hundred thousand years, with more recent partial U removal. The carbonate-rich fracture assemblages reflect two environments. Crystalline calcite (up to 50 ppm U) was deposited earlier than 200 ka, while caliches (up to hundreds of ppm U) are less than 140 ka with an episode of higher U contents at 45-55 ka. Leaching studies show that, in most cases, the silica-mineral residues are coeval with the calcite component. Opals (up to thousands of ppm U) are mostly in U-series secular equilibrium; one exception has an age of 54±2 ka. Studies are underway to better understand the physical and chemical environments of deposition of these various fracture materials. Implications of these observations for better understanding the performance of a potential nuclear waste repository at Yucca Mountain include (i) estimating the efficacy of fractures in providing transport pathways, (ii) demonstrating episodes of elevated radionuclide mobility, likely related to increased water flow, (iii) comparing lower-bound release rates, based on estimated U deposition rates, with rates in repository performance assessment analyses, and (iv) quantifying the sorptive properties of iron minerals in a natural setting.

This is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the NRC. The NRC staff views expressed here are preliminary and do not represent a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain.