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Dive into the research topics where English C. Pearcy is active.

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Featured researches published by English C. Pearcy.


Applied Geochemistry | 1994

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

English C. Pearcy; James D. Prikryl; William M. Murphy; Bret W. Leslie

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.


Applied Geochemistry | 1995

Uranium transport through fractured silicic tuff and relative retention in areas with distinct fracture characteristics

English C. Pearcy; James D. Prikryl; Bret W. Leslie

Abstract The Nopal I uranium (U) deposit, in the Pefia 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 distributions 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.


Journal of Contaminant Hydrology | 1997

Migration behavior of naturally occurring radionuclides at the Nopal I uranium deposit, Chihuahua, Mexico

James D. Prikryl; David A. Pickett; William M. Murphy; English C. Pearcy

Oxidation of pyrite at the Nopal I uranium deposit, Pena 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 234U238U activity ratios measured in fracture-infilling materials indicate relatively recent ( < 1 Ma) U uptake from fluids that carried excess 234U. Systematic decreases in 234U238U 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.


MRS Proceedings | 1992

Oxidative Alteration of Uraninite at the Nopal I Deposit, Mexico: Possible Contaminant Transport and Source Term Constraints for the Proposed Repository at Yucca Mountain

Bret W. Leslie; English C. Pearcy; James D. Prikryl

The Nopal I uranium deposit at Pena Blanca, Mexico is being studied as a natural analog of the proposed high-level nuclear waste repository at Yucca Mountain. Identification of secondary uranium phases at Nopal I, and the sequence of their formation after uraninite oxidation, provides insight into the source term for uranium, and suggests that uranophane may control uranium release and transport in a silici, tuffaceous, chemically oxidizing, and hydrologically unsaturated environment. Possible constraints on contaminant transport at Nopal I are derived from the spatial distribution of uranium and from measurements of {sup 238}U decay-series isotopes. The analyses indicate that flow of U-bearing fluids was influenced strongly by fracture density, but that the flow of these fluids was not restricted to fractures. Gamma spectroscopic measurements of {sup 238}U decay-series isotopes indicates secular equilibrium, which suggests undetectable U transport under present conditions.


MRS Proceedings | 1991

Source-Term Constraints for the Proposed Repository at Yucca Mountain, Nevaida, Derived from the Natural Analog at PeÑa Blanca, Mexico

William M. Murphy; English C. Pearcy

The source term for nuclear waste repository performance assessments can be constrained by the solubilities of radioelement-bearing solids and/or the rates of release of radioelements from nuclear waste forms. Both solubility and rate limits for the proposed repository at Yucca Mountain, Nevada, can be assessed using information from the natural analog at PeNa Blanca, Mexico. Petrographic and field relations indicate that uraninite oxidation and transformation to secondary uranyl silicate minerals have been rapid relative to mass transport of uranium out of the PeNa Blanca system. The rate limiting process for uranium removal is likely to be advective transport in groundwaters with uranium contents controlled by interactions with uranyl silicate minerals such as uranophane. A maximum limit on the rate of uraninite oxidation at PeNa Blanca is calculated to be 0.032 tons of UO 2 per year using geologic constraints on the amount of oxidation and the available time.


Journal of Contaminant Hydrology | 1998

A test of long-term, predictive, geochemical transport modeling at the Akrotiri archaeological site

William M. Murphy; English C. Pearcy; Ronald T. Green; James D. Prikryl; Sitakanta Mohanty; Bret W. Leslie; Ashok Nedungadi

Abstract A study of elemental transport at the Akrotiri archeological site on the island of Santorini, Greece, has been conducted to evaluate the use of natural analog data in support of long-term predictive modeling of the performance of a proposed geologic repository for nuclear waste at Yucca Mountain, Nevada. Akrotiri and Yucca Mountain have many analogous features including silicic volcanic rocks, relatively dry climates, and oxidizing, hydrologically unsaturated subsurface conditions. Transport of trace elements from artifacts buried in volcanic ash 3600 years ago at Akrotiri is analogous to transport of radioactive wastes in the proposed repository. Subtle evidence for a plume of Cu, Zn, and Pb has been detected by selective leaching of packed earth and bedrock samples collected immediately beneath the site where bronze and lead artifacts were excavated. The geologic setting of the artifacts and the hydraulic properties of the enclosing media were characterized. A numerical model of the type used in repository performance assessments was developed for elemental transport at the site. Site characterization data were used to build the model but no prior information on the nature of the contaminant plume was provided to the modelers. Some model results are qualitatively consistent with field data, including the small amount of material transported, limited amounts of sorbed material, and relatively elevated sorption on a packed earth layer, However, discrepancies result from incomplete representation of heterogeneity and complexity and poorly constrained model parameters. Identification of such system characteristics and model limitations in relevant systems is a major contribution that analog studies can contribute in support of repository modeling.


Nuclear Science and Engineering | 1999

Design of a subcritical assembly for conducting neutron activation analyses in the field

Mark S. Jarzemba; James Weldy; English C. Pearcy

Two subcritical assemblies (consisting of a subcritical reactor plus a neutron emitter such as 252 Cf) designed for conducting neutron activation analyses in the field are described. The size of the assemblies has been minimized (compared to conventional, graphite-moderated assemblies) to allow for field portability. Although less powerful than using a research reactor as the source of neutrons, these assemblies will provide an adequate source of neutrons for detecting gold concentrations in rock or soil samples down to the limits of economic importance. Using afield-portable source of neutrons eliminates the need for shipping samples back to the reactor for analysis, which may be important for reasons of sample security and measurement turnaround time. The two subcritical assemblies are composed of natural uranium metal as the multiplying material and high-density polyethylene as the moderator, and they have k eff approximately equal to 0.8 for the smaller assembly (∼692-kg assembly mass) and 0.9 for the larger assembly (∼3059-kg assembly mass). The larger assembly was found to be more desirable from a neutronics standpoint; however, it may be too massive to maintain field portability. It was found that the optimal location for the irradiation facility (a 4.0-cm-high, 2.0-cm-diam right cylinder) in the subcritical assemblies is the grid location as close to the neutron emitter location as possible. It was also found that the total, epithermal plus thermal (i.e., neutron energy <0.5 eV), and thermal (i.e., neutron energy <0.05 eV) volume-averaged neutron fluxes were as follows (assuming a neutron emitter source strength of 10 9 n/s): 1.72 x 10 8 , 4.47 x 10 7 , and 2.15 x 10 7 cm -2 .s -1 for the smaller assembly, and 3.43 x 10 8 , 9.09 x 10 7 , and 4.37 x 10 7 cm -2 .s -1 for the larger assembly. Although the purpose for which the assembly was designed was for conducting neutron activation analyses for gold, the assemblies should also work equally well for analyzing sample compositions of other elements at both the bulk and trace levels.


Nuclear Science and Engineering | 1999

Experimental Determination of Detection Limits for Performing Neutron Activation Analysis for Gold in the Field

Mark S. Jarzemba; James Weldy; English C. Pearcy; Jim Prikryl; David A. Pickett

Measurements are presented of gold concentration in rock/soil samples by delayed neutron activation analysis using a device and method that are potentially field portable. The device consists of a polyethylene moderator and {sup 252}Cf as the source of neutrons for activating the samples and a high-purity germanium detector to measure the 412-keV gamma-ray emissions from activated gold. This information is used to extract the gold concentration in the sample. Two types of samples were investigated: (1) pure SiO{sub 2} doped with a known amount of gold chloride and (2) US Geological Survey standards. The former types were used to evaluate optimum device performance and to calibrate the device and method. The latter types were used to show typical system performance for the intended application (field exploration for gold deposits). It was found that the device was capable of determining gold concentrations to {approximately}10 ppb with a turnaround time (the sum of irradiation, decay, and counting times) of {approximately}10 days. For samples where the gold concentration was much higher (i.e., gold ore), turnaround times are {approximately}2 days and could be shortened further by sacrificing accuracy (e.g., lessening irradiation, decay, and counting times) or by augmenting source strength.


Archive | 2002

Field analysis of geological samples using delayed neutron activation analysis

English C. Pearcy; Mark S. Jarzemba; James Weldy


MRS Proceedings | 1999

Vegetation-Derived Insights on the Mobilization and Potential:Transport of Radionuclides from the Nopal I Natural Analog Site, Mexico

Bret W. Leslie; David A. Pickett; English C. Pearcy

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William M. Murphy

Southwest Research Institute

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Bret W. Leslie

Southwest Research Institute

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James D. Prikryl

Southwest Research Institute

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David A. Pickett

Southwest Research Institute

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James Weldy

Southwest Research Institute

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Mark S. Jarzemba

Southwest Research Institute

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Ashok Nedungadi

Southwest Research Institute

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Hersh K. Manktala

Southwest Research Institute

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Ronald T. Green

Southwest Research Institute

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Sitakanta Mohanty

Southwest Research Institute

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