Alteration of uraninite from the Nopal I deposit, Pen˜a Blanca District, Chihuahua, Mexico, compared to degradation of spent nuclear fuel in the proposed U.S. high-level nuclear waste repository at Yucca Mountain, Nevada

Abstract At the Nopal I uranium deposit, primary uraninite (nominally UO 2+ x ) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically unsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5–10 μm) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and H 2 O) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated UO 2 under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO 2 ) 2 Si 2 O 7 ·6H 2 O] with lesser amounts of soddyite [nominally (UO 2 ) 2 SiO 4 ·2H 2 O] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical—chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.

[1]  J. Muller,et al.  Paramagnetic Defect Centers in Hydrothermal Kaolinite from an Altered Tuff in the Nopal Uranium Deposit, Chihuahua, Mexico , 1990 .

[2]  P. Taylor,et al.  An X-ray diffraction study of the formation of β-UO2.33 on UO2 pellet surfaces in air at 229 to 275°C , 1980 .

[3]  J. Leroy,et al.  Volcanogenic uranium mineralizations in the Sierra Pena Blanca District, Chihuahua, Mexico; three genetic models , 1991 .

[4]  C. Frondel Systematic mineralogy of uranium and thorium , 1958 .

[5]  David W. Shoesmith,et al.  The corrosion of nuclear fuel (UO2) in oxygenated solutions , 1991 .

[6]  C. Wilson Results from long-term dissolution tests using oxidized spent fuel , 1990 .

[7]  B. Hofmann,et al.  The Krunkelbach uranium deposit, Schwarzwald, Germany; correlation of radiometric ages (U-Pb, U-Xe-Kr, K-Ar, 230 Th- 234 U) , 1991 .

[8]  J. Cramer,et al.  Sandstone-hosted uranium deposits in northern Saskatchewan as natural analogs to nuclear fuel waste disposal vaults , 1986 .

[9]  O. D. Slagle,et al.  Nonuniform oxidation of LWR spent fuel in air , 1991 .

[10]  T. Kotzer,et al.  O, U, and Pb isotopic and chemical variations in uraninite; implications for determining the temporal and fluid history of ancient terrains , 1993 .

[11]  H. Kleykamp,et al.  The chemical state of the fission products in oxide fuels , 1985 .

[12]  C. Frondel Mineral composition of gummite , 1955 .

[13]  J. O. Barner,et al.  Characterization of LWR spent fuel MCC-approved testing material-ATM-101 , 1984 .

[14]  P. Goodell Geology of the Pena Blanca Uranium Deposits, Chihuahua, Mexico , 1981 .

[15]  David W. Shoesmith,et al.  The prediction of nuclear fuel (UO2) dissolution rates under waste disposal conditions , 1992 .

[16]  J. Muller,et al.  Study of two alteration systems as natural analogues for radionuclide release and migration , 1990 .

[17]  R. Ewing,et al.  Alteration of Natural UO2 under Oxidizing Conditions from Shinkolobwe, Katanga, Zaire: A Natural Analogue for the Corrosion of Spent Fuel , 1991 .

[18]  F. Gauthier-Lafaye,et al.  The Francevillian (Lower Proterozoic) uranium ore deposits of Gabon , 1989 .

[19]  David J. Wronkiewicz,et al.  Uranium release and secondary phase formation during unsaturated testing of UO2 at 90°C , 1992 .

[20]  H. Kleykamp The chemical state of LWR high-power rods under irradiation , 1979 .

[21]  L. E. Thomas,et al.  Oxidation of spent fuel in air at 175 to 195°C , 1992 .

[22]  T. Ku,et al.  U-series isochron dating: A generalized method employing total-sample dissolution , 1991 .

[23]  T. Labhart,et al.  U-Pb, U-Xe and U-Kr systematics of a greenschist facies metamorphic uranium mineralization of the Siviez-Mischabel nappe (Valais, Switzerland) , 1989 .

[24]  R. Fleischer Alpha-recoil damage: Relation to isotopic disequilibrium and leaching of radionuclides , 1988 .