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

The dispersion of Rn-222 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 Rn-222 dose normalised to Ra-226 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 Ra-226 activity concentration of 800 Bq kg(-1). The estimated above-background annual dose from Rn-222 to hypothetical receptors at the Aboriginal community at Mudginberri ( similar to 10 km NNW) was 0.005 mSv and at the township of Jabiru ( similar to 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 Rn-222 dose predictions in any single year could be approximately double those of an average year, which suggests that estimates of average Rn-222 dose should potentially be doubled if the assessment aim is to demonstrate compliance with the public dose limit.