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

Published online by Cambridge University Press:  10 February 2011

Bret W. Leslie ,
David A. Pickett  and
English C. Pearcy
Bret W. Leslie
Affiliation:
U.S. Nuclear Regulatory Commission, MS T-7C6, Washington, D.C. 20555–0001
David A. Pickett
Affiliation:
Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road, San Antonio, Texas 78238–5166
English C. Pearcy
Affiliation:
Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road, San Antonio, Texas 78238–5166
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Abstract

The 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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 556: Symposium QQ – Scientific Basis for Nuclear Waste Management XXII , 1999 , 833
Copyright
Copyright © Materials Research Society 1999

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