Field and Laboratory Measurement of Radon Flux and Diffusion for Uranium Mill Tailings Cover Systems
Benson, Craig H.
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Disposal Facilities for uranium mill tailings have been constructed as required by the Uranium Mine Tailing Radiation Control Act (UMTRCA) of 1978. Nearly all of these facilities rely on a low-permeability, compacted clay surface cover to control the rate at which contaminants migrate in the gas and water phase from the tailings into the environment. The primary engineered component of the surface cover is typically referred to as the “low permeability Radon barrier, (or Rn Barrier). The Rn Barrier is designed to have low hydraulic conductivity and low gaseous diffusivity to effectivity control the emission of Radon and other contaminants into the environment. Several of the UMTRCA sites are over 20 years of service life. It is well understood that near-surface clay covers can experience significant structure development (e.g., cracking) due to process related to environmental exposure, including seasonal wetting and drying and bio-intrusion from vegetation and animal activity. Over many seasons, there is the potential that the Rn Barrier system may not contain contaminants as well as when the barrier was initially constructed. Studies conducted on UMTRCA sites have indicated significant variability in Radon Flux (Rn Flux) emanating through the top of the Rn Barrier (e.g., from as little as 0.3 pCi/m2-s to as much as 200 pCi/m2-s) (CNWRA 2012). When UMTRCA was first passed, the intended service life of cover systems was 1,000 years and Rn Flux was stipulated to not exceed 20 pCi/m2-s. This study first seeks to develop and calibrate alternative measurement techniques for accurately and reliably measuring Rn Flux at UMTRCA field sites. The study is motivated partly to address limitations in current regulatory measurement techniques, where Radon surface flux is measured using Activated Carbon (AC) placed directly on the cover surface and Radon is measured by passive sorption. Advances in Radon measurement technologies have led to the development of continuous, electronic Radon monitors. These monitors are capable of measuring Radon concentration at user-specified intervals and thus can be used to measure the buildup of Radon inside a closed sampling chamber placed on the cover system over time. Continuous data afforded by this approach provides significant advantages for measuring Rn flux over the historical AC measurement approach. The calibration effort of this study employed an electronic RAD7 Radon detection system (Durridge Company Inc.) to measure Rn flux emanating from two Radon sources in the laboratory (granite aggregate and a poured concrete slab). Results were used to develop an experimental protocol for field measurements and to identify and characterize variables that affect the measurements. AC canisters were placed into chambers containing the Radon sources in tandem with the electronic radon monitors so that radon concentrations measured using the two approaches could be directly compared. The AC to RAD7 Rn Concentration ratio averaged 0.58 (+/- 0.07) for five tests under the same conditions. A second suite of experiments was conducted to examine the effects of process variables on the measurements, including relative humidity, chamber size, exposure duration, and Rn flux. Similar differences in the two techniques were observed. The lab analysis properly corrected varying levels of exposure to RH before Rn exposure to the same Rn concentration (198.5 +/- 23.8 Bq/m3), but that concentration consistently averaged 0.60 (+/- 0.07) in the AC to RAD7 Rn Concentration Ratio. Varying chamber size, Rn flux, and exposure time all failed to produce statistically significant trends with R2 = 0.73, 0.42, and 0.28 respectively. Field measurements were conducted at a uranium mill tailings disposal cell located in Falls City, Texas. Continuous (RAD7) and passive (AC canisters) Radon detectors were deployed at six test pits that were excavated to expose the Rn barrier at the site and to measure radon flux at locations representative of different site conditions. Test pit locations were selected to compare varying vegetation conditions, Rn barrier thicknesses, and underlying tailings activities. Rn flux measured at the top of the barrier within the six test pits ranged from 0.06 - 24.74 pCi/m2-s. Radon flux measurements were also conducted directly on top of the tailings layer, which was exposed by excavating through the Rn Barrier. Radon flux from the tailings layer was measured as high as 1148.07 pCi/m2-s. Results from this suite of measurements were used to assess performance of the alternative Radon measurement approaches in a typical field application. The AC to RAD7 Rn concentration ratio was consistent with the laboratory measurements, averaging 0.59 with a variance of 0.07. Corresponding ratio of Rn flux between the two devices was 0.60-0.93 depending on the size of the chamber and length of the test. Finally, a laboratory procedure was developed to simulate desiccation and structure development inside compacted clay samples and the associated effects on gaseous Radon diffusion coefficient. Experiments were conducted in a modified flexible wall permeameters to measure Radon diffusion coefficients through several wetting and drying cycles. Results were used to understand how structure develops in different types of clays that may be considered representative of compacted cover materials and to quantify the associated effects on Radon diffusivity. Measured Rn diffusion coefficients across all clays ranged from a maximum of 3.98e-7 m2/s to a minimum of 1.24e-09 m2/s. These values fall within the expected diffusion coefficients boundaries of air (1e-5 m2/s) and water (1e-9 m2/s).