Modelling the dispersion of radon-222 from a landform covered by low uranium grade waste rock.

The dispersion of 222 Rn from the planned remediation of the Ranger U mine in the wet-dry tropics of Northern Australia was modelled. Dry and wet season contour maps of 222 Rn dose normalised to 226 Ra activity concentration in the proposed waste rock substrate on the remediated landform were developed. Three example exposure scenarios were assessed based on an anticipated waste rock 226 Ra activity concentration of 800 Bq kg-1 . The estimated above-background annual dose from 222 Rn to hypothetical receptors at the Aboriginal community at Mudginberri (∼10 km NNW) was 0.005 mSv and at the township of Jabiru (∼7 km W) was 0.033 mSv. The estimated above-background annual dose for the hypothetical worst case scenario, representing a receptor 1 km WNW of the landform centroid during the dry season and at the centroid during the wet season, was 0.13 mSv. Variability analysis on the 20 y meteorological dataset used in the dispersion modelling showed that the dry and wet season 222 Rn dose predictions in any single year could be approximately double those of an average year, which suggests that estimates of average 222 Rn dose should potentially be doubled if the assessment aim is to demonstrate compliance with the public dose limit.