Scanning electron photomicrograph of acicular uranophane filling a void within the orebody at Nopal I. Isotopic analyses were conducted on acicular uranophane collected from fractures within the orebody at Nopal I. Field of view is 0.1 mm across

Seventh EC Natural Analogue Working Group Meeting Proceedings,
EUR 17851 EN, Luxembourg, European Commission, p. 113-122, 1997

Isotopic Constraints on Radionuclide Transport at Peña Blanca

David A. Pickett and William M. Murphy

Center for Nuclear Waste Regulatory Analyses
Southwest Research Institute, 6220 Culebra Rd.

San Antonio, TX 78238 USA

Abstract

²³⁸U-series and U/Pb isotopic systematics have been interpreted at the Nopal I natural analog, Peña Blanca, Mexico, in terms of U mineral history and the nature and timing of U transport. The analog consists of a vertical, roughly cylindrical uranium deposit, hosted by fractured, silicic tuff, with U contained chiefly in uranyl phases. Three samples of uranophane, the most abundant uranyl phase in the deposit, yield ²³⁸U/206Pb and ²³⁵U/207Pb internal and external isochron ages of 3.4 and 3.2 Ma, respectively. These ages represent an episode of oxidation of uraninite. Activity data on uranium-series nuclides ²³⁰Th, ²³⁴U, and ²³⁸U from rocks surrounding the deposit indicate a relatively recent, episodic history of U mobilization. ²³⁴U and ²³⁰Th excesses point to a multistage, and in some cases shifting, history dominated by early mobilization of U out of the primary uranium deposit and deposition outside, followed by episodic partial U depletion from rocks outside the deposit. The data indicate that these processes acted over the past few hundred thousand years to the present. Uranium transport in percolating, oxidizing meteoric waters is the preferred dominant process, consistent with isotopic data on present-day perched and seep waters collected at the site. The isotopic and chemical data reveal a history of (i) uraninite deposition several million years ago, (ii) deposition of uranyl silicates during a single period around 3 Ma, and (iii) complex episodic U mobilization and remobilization in the past few hundred thousand years. Because of the strong analogy between conditions at Peña Blanca and Yucca Mountain, the demonstrated timing, episodicity, and complexity of transport at Peña Blanca suggest that such conditions can be expected at Yucca Mountain and are likely to be important to radionuclide transport at the proposed repository.

Eighth EC Natural Analogue Working Group Meeting Proceedings,
Luxembourg, European Commission, in press

Uranium Chemistry and Isotopy in Waters and Rocks at
Peña Blanca, Mexico
David A. Pickett and William M. Murphy
Center for Nuclear Waste Regulatory Analyses
Southwest Research Institute, 6220 Culebra Rd.
San Antonio, TX 78238 USA

Abstract

A geochemical chronology of uranium (U) behavior is described for the Nopal I natural analogue, which is hosted in 44 Ma tuffs. Available evidence points to primary U mineralization at around 8 Ma, followed by oxidative alteration as recent as 3 Ma. The past few million to several hundred thousand years were marked by open-system U mobilization, with episodes of U release and deposition at around 400 and 54 ka. This timing information has implications for attempts to model release and transport processes in nuclear waste repositories.

Perched water was collected from a 10.7 m-deep uncased borehole on Level +10 (BH-12). The hole contained water for several years and was capped three months prior to sampling.

A seep-water collection system was constructed in the Level +00 adit. Plastic sheets were suspended from the ceiling and walls, leading to funnel filters feeding plastic collection bottles.

Scientific Basis for Nuclear Waste Management XXII
Materials Research Society Symposium Proceedings 556, pp. 809-816, 1999

Unsaturated zone waters from the Nopal I Natural analog,
Chihuahua, Mexico - Implications for radionuclide mobility at
Yucca Mountain
David A. Pickett and William M. Murphy
Center for Nuclear Waste Regulatory Analyses
Southwest Research Institute, 6220 Culebra Rd.
San Antonio, TX 78238, USA

Abstract

Chemical 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 concentration 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.