Lead-210 profiles in radon barriers, Indicators of long-term Radon-222 transport

摘要

Conventional measurements of fluxes of Rn-222 from engineered earthen covers over uranium milling wastes use accumulation chambers over a 24-h period, making them sensitive to short-term environmental fluctuations. It is unlikely that such measurements represent a long-term average required for assessing cover performance. An alternative flux measurement approach is explored here based on the hypothesis that measurements of Pb-210 (a daughter of Rn-222) concentrations in the barrier material indicate where Rn-222 decayed in the barrier profile and thus indicate Rn transport within the barrier over long times. Samples of radon barrier materials from four mill tailings sites have been analyzed for Pb-210. In many cases, Pb-210 was detectable, was substantially above background values, and had higher concentrations at the bottom of the barrier adjacent to the underlying tailings. In some profiles, Pb-210 was found to decrease to background concentrations part way through the barrier, indicating that all Rn-222 had decayed before reaching the top. In other cases, Pb-210 concentrations were elevated near the top of the barrier, indicating radon migration through the entire radon barrier and possible long-term release of radon. These observations were generally confirmed by radon flux measurements at the top of the radon barrier. Some measured profiles of Pb-210 with depth in the radon barrier can be modeled by simple diffusion, suggesting that the diffusive transport mechanism, which was assumed for the original design of the barriers, is appropriate. In other cases, the Pb-210 profiles are not effectively modeled by diffusion, suggesting that at some locations the transport mechanism is complex and does not conform to the assumed process. Findings from this work indicate that Pb-210 concentrations measured near the top of radon barriers are promising as a monitoring approach for long-term Rn-222 radon fluxes. More detailed profiles of Pb-210 coupled with moisture and de