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The Peña Blanca Natural Analog Project

Peña Blanca Data in Yucca Mountain Performance Assessment

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Peña Blanca Data in Yucca Mountain Performance Assessment

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	Uranium contents in bulk rock samples collected across the the western margin of the Nopal I deposit (modified from Pearcy et al., 1995). The red line is a fit of the diffusion equation to the U concentrations. The dashed vertical line shows the position of the boundary of visible uranium mineralization.

Natural analog studies at Peña Blanca have provided information for recent performance assessments for the proposed geologic repository at Yucca Mountain.

Application of Peña Blanca data to Yucca Mountain performance assessment has focused primarily on issues of mineral paragenesis resulting from alteration of natural uraninite, source term modeling, and radionuclide transport. Applications have been primarily qualitative. For example, the distribution of uranium in and around fractures at the Nopal I deposit, as described in Pearcy et al. (1995), was used to evaluate matrix diffusion and its significance as a retardartion mechanism in performance modeling of radionuclide transport (CNWRA^®, 1997).

The use of Peña Blanca information in Yucca Mountain performance assessment is summarized in articles appearing in the Proceedings of the Eighth Natural Analouge Working Group (NAWG) meeting (see Murphy et al., in press, in the reference section) and Scientific Basis for Nuclear Waste Management XXIII, Materials Research Society Symposium Proceedings. The abstract for the latter article is shown below.

Scientific Basis for Nuclear Waste Management XXIII

Materials Research Society Symposium Proceedings 608,
pp. 533-544, 2000

Natural Analogs and Performance Assessment for Geologic Disposal of Nuclear Waste

William M. Murphy
Center for Nuclear Waste Regulatory Analyses
Southwest Research Institute, 6220 Culebra Rd.
San Antonio, TX 78238 USA

Abstract

The use of natural analog studies in performance assessments has been widely discussed and debated, but its accomplishment has been limited. Given recognized uncertainties and challenges, scientific contributions to performance assessments and support for the validity of performance assessment models are valuable from all possible sources, including natural analog studies. The conceptual basis for geologic disposal of nuclear waste and for performance assessments relies on scientific expertise based largely on studies of natural systems analogous to possible repository systems, i.e., natural analogs. Natural analog studies offer contributions to model validation based both on inductive and deductive reasoning. The utility of analog studies as a deductive tool in performance assessment is enhanced by specificity of the analog system to the repository system. As geologic sites are selected and repository designs detailed, the use of analog data in supporting deductive performance assessments should increase. Consideration of Yucca Mountain for the proposed US high level nuclear waste repository affords site specificity conducive to applications of natural analog data in performance assessment. The primary use of Peña Blanca natural analog data in recent Yucca Mountain performance assessments stems from observations of mineral products formed by alteration of natural uraninite, an analog of spent fuel. Alternate performance assessment source term models based on the Peña Blanca oxidation rate model and the schoepite solubility model yield lower, yet comparable estimated doses than the base case model in the NRC performance assessment for Yucca Mountain.

The strongest contributions of Peña Blanca analog data to Yucca Mountain performance assessments to date have been provided by the observed correspondence in the mineral products formed by oxidation of spent nuclear fuel in laboratory studies and of uraninite in the Peña Blanca system. These observations have been used as a basis to include the secondary uranyl minerals schoepite, soddyite, uranophane, and Na-boltwoodite in reactive transport simulations for the Department of Energy (DOE) Total System Performance Assessment - Viability Assessment (DOE, 1998; TRW, 1998) to predict the evolution of water chemistry resulting from geochemical interactions in emplacement drifts.

image of chart showing paragenesis of uranium minerals at the Nopal I uranium deposit at Peña Blanca and in laboratory dripping experiments conducted at Argonne National Laboratory

Paragenesis of uranium minerals at the Nopal I uranium deposit at Peña Blanca and in laboratory dripping experiments conducted at Argonne National Laboratory (modified from Murphy, 2000). The dashed, thin, and thick lines for Nopal I represent minor, abundant, and very abundant present occurrences, respectively.

In the recent NRC Total-system Performance Assessment for the Yucca Mountain repository (CNWRA, 1998; NRC, 1999a, 1999b), the sensitivity of performance results was tested using an alternate source term model based on the maximum average oxidation rate of uraninite estimated for the Nopal I uranium deposit and scaled to the proposed repository inventory. Results of this alternate source term model are published in Scientific Basis for Nuclear Waste Management XXII, Materials Research Society Symposium Proceedings. The abstract for the article follows.

Scientific Basis for Nuclear Waste Management XXII

Materials Research Society Symposium Proceedings 556, pp. 551-558, 1999

Alternate source term models for Yucca Mountain performance assessment based on natural analog data and secondary mineral solubility

William M. Murphy* and Richard B. Codell**
*Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238 USA
**U.S. Nuclear Regulatory Commission, Washington, DC 20555

Abstract

Performance assessment calculations for the proposed high level radioactive waste repository at Yucca Mountain, Nevada, were conducted using the Nuclear Regulatory Commission Total-System Performance Assessment (TPA 3.2) code to test conceptual models and parameter values for the source term based on data from the Peña Blanca, Mexico, natural analog site and based on a model for coprecipitation and solubility of secondary schoepite. In previous studies the value for the maximum constant oxidative alteration rate of uraninite at the Nopal I uranium body at Peña Blanca was estimated. Scaling this rate to the mass of uranium for the proposed Yucca Mountain repository yields an oxidative alteration rate of 22 kg yr^-1, which was assumed to be an upper limit on the release rate from the proposed repository. A second model was developed assuming releases of radionuclides are based on the solubility of secondary schoepite as a function of temperature and solution chemistry. Releases of uranium are given by the product of uranium concentrations at equilibrium with schoepite and the flow of water through the waste packages. For both models, radionuclides other than uranium and those in the cladding and gap fraction were modeled to be released at a rate proportional to the uranium release rate, with additional elemental solubility limits applied. Performance assessment results using the Peña Blanca oxidation rate and schoepite solubility models for Yucca Mountain were compared to the TPA 3.2 base case model, in which release was based on laboratory studies of spent fuel dissolution, cladding and gap release, and solubility limits. Doses calculated using the release rate based on natural analog data and the schoepite solubility models were smaller than doses generated using the base case model. These results provide a degree of confidence in safety predictions using the base case model and an indication of how conservatism in the base case model may be reduced in future analyses.

Observations relating to radionuclide transport at Peña Blanca have been recognized as analogous to potential radionuclide transport in the Yucca Mountain system. For example, the importance of flow and radionuclide transport in fractures is strongly supported by the distribution of uranium surrounding the Nopal I deposit (Pearcy et al., 1995). The association of uranium with fracture filling iron oxides and iron oxyhydroxides at Peña Blanca highlights the potential importance of sorption/coprecipitation on oxide mineralization in fractures as a retardation mechanism for radionuclide transport (Prikryl et al., 1997). Although direct use of radionuclide transport data from Peña Blanca are absent in present performance assessments, these studies have contributed to conceptual modeling for performance assessments for Yucca Mountain. In addition, uranium series isotopic studies at Peña Blanca (Pickett and Murphy, 1997) have an untapped potential for applications in performance assessment.

References

CNWRA, 1997, Detailed Review of Selected Aspects of Total System Performance Assessment - 1995, R.G. Baca and M.S. Jarzemba (editors), CNWRA Letter Report to NRC, Center for Nuclear Waste Regulatory Analyses, San Antonio, TX.

CNWRA, 1998, Total-system Performance Assessment (TPA) Version 3.2 Code: Module Descriptions and User's Guide, Center for Nuclear Waste Regulatory Analyses, San Antonio, TX.

Department of Energy, 1998, Viability Assessment of a Repository at Yucca Mountain Total System Performance Assessment, DOE/RW-0508, v. 3, Department of Energy, Las Vegas, NV.

Murphy, W.M., D.A. Pickett, E.C. Pearcy, and D.R. Turner, 2000, Peña Blanca natural analog data in recent performance assessment models for the proposed geologic repository at Yucca Mountain, Nevada, in Eighth EC Natural Analouge Working Group Meeting Proceedings, European Commission, Luxembourg, in press.

Nuclear Regulatory Commission, 1999a, NRC Sensitivity and Uncertainty Analyses for a Proposed HLW Repository at Yucca Mountain, Nevada, Using TPA 3.1 Volume 1: Conceptual Models and Data, (NUREG-1668, 1, 1999).

Nuclear Regulatory Commission, 1999b, NRC Sensitivity and Uncertainty Analyses for a Proposed HLW Repository at Yucca Mountain, Nevada, Using TPA 3.1 Results and Conclusions (NUREG-1668, 2, 1999).

Pearcy, E.C., J.D. Prikryl, and B.W. Leslie, 1995, Uranium transport through fractured silicic tuff and relative retention in areas with distinct fracture characteristics, Applied Geochemistry, Vol. 10, p. 685-704.

Pickett, D.A., and W.M. Murphy, 1997, Isotopic constraints on radionuclide transport at Peña Blanca, in Seventh EC Natural Analogue Working Group Meeting, H. Von Maravic and J. Smellie, eds., EUR 17851 EN, European Commission, Luxembourg, p. 113-122.

Prikryl, J.D., D.A. Pickett, W.M. Murphy, and E.C. Pearcy, 1997, Migration behavior of naturally-occurring radionuclides at the Nopal I uranium deposit, Chihuahua, Mexico, Journal of Contaminant Hydrology, Vol. 26, p. 61-69.

TRW, 1998, Total System Performance Assessment - Viability Assessment (TSPA-VA) Analyses Technical Basis Document, TRW Environmental Safety Systems Inc., Las Vegas, NV, B00000000-01717-4301-00004 Rev. 01.

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.

Contact Information

James D. Prikryl

Peña Blanca Natural Analog Project

(210) 522-5667

jprikryl@swri.org

Related Terminology

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