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View of the cleared surface on
the +10 m level of Nopal I. |
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View of the cleared surface on
the +00 m level of Nopal I. |
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View of the 1 m by 1 m grid on
the +10 m level of Nopal I. |
Uranium transport at the Nopal I
deposit has been studied to evaluate the mechanisms by which
high-level waste components could be transported through
silicic tuff over long time periods. Transport studies were
initiated by clearing portions of Levels +10 and +00 of
loose rock, soil, and debris. The clearing provided a nearly
continuous bedrock exposure across the outcrop of the
deposit and the surrounding tuff.
A reference location framework over the cleared areas was
established by constructing a 1-m x 1-m grid with axes
oriented NS and EW. The 1-m grid was used as a reference for
mapping the occurrence of U minerals, host rock type, and
host rock alteration, as well as fracture locations and
forms.
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Map showing results of a
gamma survey conducted on the cleared areas of
the Nopal I deposit. The green line indicates
the limit of visible U mineralization and marks
the nominal edge of the deposit. Contours have
units of mR/h. Red contours have gamma values of
1.00 mR/h or greater. The 0.45 mR/h contour is
shown in orange and on the Level +10 mimics the
boundary of visible U mineralization. The dotted
blue contour is the 0.15 mR/h boundary of the
continuous area of anomalous U concentration. A
zone of less than 0.45 mR/h gamma values and
sulfate mineralization in the interior of the
orebody on Level +10 is shaded yellow. Figure
modified from Pearcy et al., 1995. |
Transport of
uranium from areas of primary mineralization were determined
by radiometric surveying (see above figure) and analysis of
uranium concentrations and distributions in bulk rock
samples and minerals both within and outside the ore
deposit. Uranium transport was studied along a major
fracture set crosscutting the deposit, along transects
through microfractured tuff adjacent to the major fracture
set, along a transect through generally fractured tuff, and
along micron-scale transects in tuff matrix adjacent to
microfractures. Methods used to analyze uranium included
gamma spectrometry, alpha spectrometry, autoradiography, and
electron microprobe analyses. Results of uranium transport
studies are published in Applied Geochemistry and the
Journal of Contaminant Hydrology. The Applied Geochemistry
article also contains results of geologic, contact gamma,
and comprehensive fracture mapping on the cleared portions
of Levels +10 and +00. The abstracts for these articles are
shown below.
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Applied Geochemistry, Vol. 10, pp.
685-704, 1995
Uranium transport through fractured silicic
tuff and relative retention in areas with
distinct fracture characteristics
English C. Pearcy, James D. Prikryl, and Bret W.
Leslie
Center for Nuclear Waste Regulatory Analyses,
San Antonio, Texas, U.S.A. |
Abstract
The Nopal I uranium (U) deposit, in the Peña
Blanca District, Chihuahua, Mexico, has been
identified as analogous in some regards to the
candidate U.S. high-level waste (HLW) repository
at Yucca Mountain, Nevada. Uranium transport at
the Nopal I deposit has been studied to
investigate mechanisms by which HLW components
could be transported through silicic tuff over
long time periods. This investigation focused on
approximately 1400 m2 of essentially
continuous bedrock outcrop spanning the Nopal I
deposit and surrounding host tuff. Data
collected document: (i) the distribution of U
within and around the Nopal I deposit, (ii) the
distribution and characteristics of the fracture
network within and surrounding the deposit, and
(iii) the transport of U away from the deposit
mainly along fracture paths. Uranium-series
isotopic measurements indicate mobilization of U
along the margin of the deposit within the last
1 Ma and significant U transport at about 54 Ka.
Transport of U away from the Nopal I deposit
along a few relatively continuous mesofractures
achieved maximum distances at least 20 times
greater than transport through the general
fracture network composed of thousands of less
continuous microfractures within and surrounding
the deposit. Uranium transport away from the
deposit appears to be largely independent of
variations in the general fracture network
pattern. Transport of U away from individual
micro- and meso-fractures into homogeneous,
unfractured tuff matrix appears limited to
distances less than 1 mm. At the Nopal I
deposit, matrix diffusion does not appear to
have been an important factor for retardation of
U. This analysis suggests a ranking for U
retention: (i) microfracture network retention >
> mesofracture retention, and (ii) individual
microfracture retention > > matrix retention.
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The Journal of
Contaminant Hydrology article presents data and analyses
of a major fracture set that crosscuts the U deposit at
Nopal I. Materials collected from this fracture set are
absent of U minerals. However, Fe-oxides and Fe-hydroxides
from the fracture were found to exhibit anomalous U
concentrations. Detailed analyses of material collected from
the fracture included: (i) petrographic and mineralogic
characterization; (ii) U distribution; and (iii) U-series
isotopic measurements. Results indicate a complex history of
U mobilization and transport. The abstract for this article
is shown below.
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The trace of the major
fracture set at Nopal I studied in relation to U
transport is visible (see arrows on photo) in
this photo of the Nopal I deposit looking to the
southeast over the southern 2/3 of the cleared
area on Level +10. |
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The major fracture that
divides this photo was the focus of U transport
studies. |
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Journal of Contaminant Hydrology 26
(1997) 61-69
Migration behavior of naturally occurring
radionuclides at the Nopal I
uranium deposit, Chihuahua, Mexico
James D. Prikryl, David A. Pickett, Willliam M.
Murphy, and English C. Pearcy
Center for Nuclear Waste Regulatory Analyses
Southwest Research Institute, 6220 Culebra Road
San Antonio, Texas 78238-5166, U.S.A. |
Abstract
Oxidation of pyrite at the Nopal I uranium
deposit, Peña Blanca district, Chihuahua, Mexico
has resulted in the formation of
Fe-oxides/hydroxides. Anomalous U concentrations
(i.e., several hundred to several thousand ppm)
measured in goethite, hematite, and amorphous
Fe-oxyhydroxides in a major fracture that
crosscuts the deposit and the absence of U
minerals in the fracture suggest that U was
retained during secondary mineral growth or
sorbed on mineral surfaces. Mobilization and
transport of U away from the deposit is
suggested by decreasing U concentrations in
fracture-infilling materials and in goethite and
hematite with distance from the deposit. Greater
than unity 234U/238U
activity ratios measured in fracture-infilling
materials indicate relatively recent (<1 Ma) U
uptake from fluids that carried excess 234U.
Systematic decreases in
234U/238U activity ratios
of fracture materials with distance from the
deposit suggest a multistage mobilization
process, such as remobilization of U from
234U-enriched infill minerals or
differential or diminished transport of
U-bearing solutions containing excess 234U. |
For more
information about the Peña
Blanca Natural Analog Project at SwRI or how you can
contract with SwRI, please contact
James D. Prikryl at
jprikryl@swri.org, or call (210) 522-5667.
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Contact Information |
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James D. Prikryl
Peña Blanca Natural Analog Project
(210) 522-5667
jprikryl@swri.org |
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Related Terminology |
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Sierra Peña Blanca
Nuclear Regulatory Commission
Nopal I uranium deposit
Sierra Peña Blanca
uraninite alteration
Yucca Mountain
performance assessment modeling of
nuclear waste geologic repositories |
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