Mechanisms for the Formation of a Perched Water Zone in Fractured Tuff: a Natural Analogue Study

1996 ◽  
Vol 465 ◽  
Author(s):  
E. G. Woodhouse ◽  
R. L. Bassett
Keyword(s):  
Structural Features ◽  
Yucca Mountain ◽  
Nuclear Waste Repository ◽  
Arid Environments ◽  
Perched Water ◽  
Natural Analog ◽  
Welded Tuff ◽  
Geochemical Properties ◽  
Water Zone ◽  
Mountain Site

ABSTRACTPerched water zones have been identified in the fractured, welded tuff in the semi-arid to arid environments of Yucca Mountain, Nevada and near Superior, Arizona. An understanding of the formation of such zones is necessary in order to predict where future perched water might form at Yucca Mountain, the proposed site of a high-level nuclear waste repository. The formation or growth of a perched zone near a repository is one of the factors to be considered in the risk assessment of the Yucca Mountain site.The Apache Leap Research Site near Superior, Arizona is a natural analog to the Yucca Mountain site in terms of geology, hydrology, and climate. Information used to study possible mechanisms for the formation of the perched zone included data regarding isotopie and geochemical properties of the waters in and above the perched water zone; measured hydrologie parameters of the perched zone; geophysical and measured parameters of the tuff; megascopic and microscopic observations of the tuff, including mineralogical, alteration, and structural features; and the lateral and vertical extent of perched water in the region.Aquifer test, geophysical, geochemical, and radioisotopic data show that fractures are the means by which water is recharging the perched zone. The reduced hydraulic conductivity of the formation in the perched zone appears to result from both a severe reduction in matrix porosity and permeability caused by welding, devitrification, and vapor phase crystallization; and by an increase in fracture filling which restricts the pathways for flow.

Unsaturated Zone Waters From the Nopal I Natural Analog, Chihuahua, Mexico - Implications for Radionuclide Mobility at Yucca Mountain

1999 ◽  
Vol 556 ◽  
Author(s):  
David A. Pickett ◽  
William M. Murphy
Keyword(s):  
Unsaturated Zone ◽  
Yucca Mountain ◽  
Nuclear Waste Repository ◽  
Spent Fuel ◽  
Path Modeling ◽  
Rock Interaction ◽  
Colloidal Transport ◽  
Natural Analog ◽  
Initial Silica ◽  
Radionuclide Release

AbstractChemical and U-Th isotopic data on unsaturated zone waters from the Nopal I natural analog reveal effects of water-rock interaction and help constrain models of radionuclide release and transport at the site and, by analogy, at the proposed nuclear waste repository at Yucca Mountain. Geochemical reaction-path modeling indicates that, under oxidizing conditions, dissolution of uraninite (spent fuel analog) by these waters will lead to eventual schoepite precipitation regardless of initial silica concentration provided that groundwater is not continuously replenished. Thus, less soluble uranyl silicates may not dominate the initial alteration assemblage and keep dissolved U concentrations low. Uranium-series activity ratios are consistent with models of U transport at the site and display varying degrees of leaching versus recoil mobilization. Thorium concentrations may reflect the importance of colloidal transport of low-solubility radionuclides in the unsaturated zone.

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

1999 ◽  
Vol 556 ◽  
Author(s):  
Bret W. Leslie ◽  
David A. Pickett ◽  
English C. Pearcy
Keyword(s):  
Source Term ◽  
Activity Ratio ◽  
Decay Chain ◽  
Yucca Mountain ◽  
Nuclear Waste Repository ◽  
Natural Analog ◽  
Ore Body ◽  
Term Model ◽  
High Level ◽  
Source Term Model

AbstractThe Nopal I uranium (U) deposit, Peñia Blanca, Mexico is a source term and contaminant transport natural analog to the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. In an attempt to characterize the mobilization and potential transport of adionuclides in the unsaturated zone at the Nopal I deposit, vegetation growing on ore piles was analyzed for 238U, 235U, and 232Th decay-series isotopes. Specimens of Phacelia robusta growing on highgrade piles of U ore were collected and analyzed by alpha autoradiography, and by alpha and gamma spectrometry. Activities for U, thorium (Th), and radium (Ra) isotopes (Bq/kg dried plant) were 300, 1000, and 7000 for 238U, 230Th, and 226Ra, respectively. The 226Ra activities in these specimens are among the highest ever measured for plants; furthermore, the plant-to-soil 226Ra concentration ratio is higher than expected. These results demonstrate the large mobility and bio-availability of Ra in the Nopal I environment, and support previous indications of recent loss of 226Ra from the ore body. Comparison between the activities of 238U and 232Th decay-chain Th isotopes in the plants and in the ore substrate indicate that relative mobilization into pore solutions of 228Th > 230Th > 232Th, in a ratio of about 50 – 25:4:1, respectively. The similarity of the plant's 234U/238U activity ratio (˜1.2) to that of a caliche deposit that formed adjacent to the Nopal ore body around 54 ka suggests the 234U/238U activity ratio of U released from the ore is approximately 1.2. The U and 236Ra isotope activities of the plants and ore substrate, and solubility considerations, are used to assess a source term model of the potential Yucca Mountain repository. These results suggest the use of a natural analog source term model in performance assessments may be non-conservative.

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

1991 ◽  
Vol 257 ◽  
Author(s):  
William M. Murphy ◽  
English C. Pearcy
Keyword(s):  
Nuclear Waste ◽  
Source Term ◽  
Yucca Mountain ◽  
Performance Assessments ◽  
Nuclear Waste Repository ◽  
Advective Transport ◽  
Silicate Minerals ◽  
Natural Analog ◽  
Rate Limiting ◽  
Nuclear Waste Forms

ABSTRACTThe 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 PeÑa 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 PeÑa 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 PeÑa Blanca is calculated to be 0.032 tons of UO2 per year using geologic constraints on the amount of oxidation and the available time.

scholarly journals Creep in Topopah Spring Member welded tuff. Yucca Mountain Site Characterization Project

10.2172/82462 ◽  
1995 ◽  
Author(s):  
R.J. III Martin ◽  
P.J. Boyd ◽  
J.S. Noel ◽  
R.H. Price
Keyword(s):  
Site Characterization ◽  
Yucca Mountain ◽  
Welded Tuff ◽  
Mountain Site

Modeling Geochemical Stability of Cement Formulations for use as Shaft Liner and Sealing Components at Yucca Mountain(1)

1990 ◽  
Vol 212 ◽  
Author(s):  
Mark A. Gardiner ◽  
Thomas E. Hinkebein ◽  
Jonathan Myers
Keyword(s):  
Ground Water ◽  
Nuclear Waste ◽  
Geochemical Modeling ◽  
Cementitious Materials ◽  
Yucca Mountain ◽  
Nuclear Waste Repository ◽  
Preliminary Estimate ◽  
Mountain Site ◽  
Compositional Changes ◽  
Waste Repository

ABSTRACTThe geochemical modeling codes EQ3NR/EQ6 were used to model the interaction of cementitious materials with ground water from the Yucca Mountain proposed nuclear waste repository site in Nevada. This paper presents a preliminary estimate of the compositional changes caused by these interactions in the ground water and in the cement-based compounds proposed for use as sealing and shaft liner materials at the Yucca Mountain site.

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