Evaluation of In-Service Radon Barriers over Uranium Mill Tailings Disposal Facilities (NUREG/CR-7288, Volume 1)

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Publication Information

Manuscript Completed: November 2021
Date Published: March 2022

Prepared by:
M. Williams2
M. Fuhrmann3
M. Stefani1
A. Michaud1
W. Likos1
C. Benson1
W. Waugh2

1Geological Engineering, University of Wisconsin-Madison
1415 Engineering Drive
Madison, WI 53706 USA

2RSI EnTech, LLC
Grand Junction CO 81503

3U.S. Nuclear Regulatory Commission

Mark Fuhrmann, NRC Project Manager

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Earthen final covers over uranium mill tailings and associated wastes were investigated at four sites that had been in service for approximately 20 years: Falls City in Texas, Bluewater in New Mexico, Shirley Basin South in Wyoming, and Lakeview in Oregon. Test pits were excavated, radon fluxes were measured, soil morphological observations made, and samples were collected to determine saturated hydraulic conductivity, soil water characteristic curves, Pb-210 concentrations, and related properties. Similar procedures were conducted at natural analogue sites – nearby locations of undisturbed natural ground used as an indicator of the very long-term state of the covers. Saturated hydraulic conductivity of the Rn barriers at three of the four sites typically fell within the range recommended to represent long-term in-service conditions (1.0×10−7 to 5.0×10−6 m/s), regardless of depth or thickness of the cover or radon barrier. These saturated hydraulic conductivities are 2 to 3 orders of magnitude higher than the common 1.0×10−9 m/s design criterion established for low-conductivity Rn barriers. One Rn barrier was an exception, with some hydraulic conductivities as low as 10-11m/s. A slight increase over the as-built Rn flux was evident for some of the barriers. However, the intentionally biased sampling procedure and differences between the methods used in this study for measuring Rn flux relative to those used in the as-built condition precluded making inferences regarding sitewide in-service Rn fluxes. Rn fluxes were higher in regions where woody vegetation (mesquite, salt bush, and bitterbrush) or aggressive insects had established on the cover, suggesting that the vegetation and insects affected the performance of the barrier in these locations. These higher fluxes are attributed to soil structure induced by root activity and insect burrowing in the Rn barrier, as well as higher Rn diffusion coefficients associated with lower water saturation in areas influenced by root water uptake. A method to use Pb-210 concentration profiles to quantify long-term (decades) Rn-222 fluxes was developed. Soil morphology within the covers is evolving toward a more structured condition, and in some places appears to be approaching a state comparable to the natural analogues.

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