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 222Rn 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 222Rn 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 222Rn 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) × 10−6 m2 s−1. For an infinitely thick layer of the dry material, the average value of the 222Rn exhalation flux density relative to the 226Ra activity concentration was estimated as (5.3 ± 0.3) × 10−4 Bq m−2 s−1 per Bq kg−1. From the diffusion length, the waste rock material was characterised as both a source and attenuator of 222Rn for its proposed use as the surface cover on the final landform of the remediated Ranger uranium mine. •We experimentally determined 222Rn diffusion and exhalation characteristics of waste rock.•The 222Rn diffusion length in dry waste rock material was 1.9 m.•The 222Rn diffusion coefficient in dry waste rock material was 7.3 × 10−6 m2 s−1.•The 222Rn exhalation flux density relative to 226Ra activity concentration was 5.3 × 10−4 Bq m−2 s−1 per Bq kg−1.