Laboratory measurements of radon diffusion through multilayered cover systems for uranium tailings

Abstract

Laboratory r.1easurements of radon fluxes and radon concentration profiles were conducted to characterize the effectiveness of multilayer cover systems for uranium tailings. The cover systems utilized soil and clay materials from proposed disposal sites for the Vitro, Durango, Shiprock, Grand Junction and Riverton tailings piles. Measured radon fluxes were in reasonable agreement with values predicted by multilayer diffusion theory. Results obtained by using air-filled porosities in the diffusion calculations were similar to those obtained by using total porosities. r~easured diffusion coefficients were a better basis for predicting radon fluxes than were correlations of diffusion coefficient with moisture or with air porosity. Radon concentration profiles were also fitted by equations for multilayer diffusion in the air-filled space. Layer-order effects in the multilayer cover systems were examined and estimated to amount to 10 to 20 percent for the systems tested. Quality control measurements in support of the multilayer diffusion tests indicated that moisture absorption was not a significant problem in radon flux sampling with charcoal canisters, but that the geometry of the sampler was critical. The geometric design of flux-can samplers was also shown to be important. Enhanced radon diffusion along the walls of the test columns was examined and was found to be insignificant except when the columns had been physically disturbed. Additional moisture injected into two test columns decreased the radon flux, as expected, but appeared to migrate into surrounding materials or to be lost by evaporation. Control of moisture content and compaction in the test columns appeared to be the critical item affecting the accuracies of the experiments.

Predicting long-term moisture contents of earthen covers at uranium mill tailings sites

“…This moisture content was determined by a laboratory measurement using a pressure plate apparatus (Richards 1965). The radon diffusion measurements were made using the transient, short-column method described by Nielson et al (1981). Table 1 lists moisture content and calculated and measured diffusion coefficient results from five pit-run test materials at several compactions.…”

“…The first expression relates radon diffusion to the air filled porosity and was developed by Nelson, Gee and Oster (1980) from existing laboratorymeasured radon diffusion coefficients. The relationship is (A.l5) Nielson et al (1981) have accurately pointed out that this expression fails to describe radon diffusion in the very wet range for soil moisture, (m > 0.95).…”

Multidimensional simulation of radon diffusion through earthen covers

“…The task can be accomplished by placing a canister on the surface of the tailings pile. Various configurations of a canister device for radon flux have been tested (Nielson et al 1981). For short term measurements 8 • (minutes), the canister is often empty and is used simply to accumulate radon gas which is sampled with time.…”