Radon-222 diffusion length and exhalation characteristics of uraniferous waste rock and application to mine site remediation in the Australian wet-dry tropics.

The diffusion length of Rn in uraniferous waste rock was determined through a novel experiment. Large PVC columns were filled to different depths in the range from 0.5 m to 3.0 m with waste rock material from the Ranger uranium mine and the build-up of Rn activity concentration in the column headspace above the material was measured after closing the columns with a lid. Measurements were made approximately one month after filling the columns and again after approximately one and two years. The average Rn diffusion length derived from the measurements was 1.9 ± 0.2 m in the dry material. The corresponding diffusion coefficient was (7.3 ± 0.7) × 10 m s. For an infinitely thick layer of the dry material, the average value of the Rn exhalation flux density relative to the Ra activity concentration was estimated as (5.3 ± 0.3) × 10 Bq m s per Bq kg. From the diffusion length, the waste rock material was characterised as both a source and attenuator of Rn for its proposed use as the surface cover on the final landform of the remediated Ranger uranium mine.