Estimating worker exposure to the decay of radon for the proposed olympic dam open pit

BHP Billiton has recently completed an environmental impact statement (EIS) for the proposed expansion of the Olympic Dam copper and uranium operation in South Australia. The expansion includes the construction of one of the world's largest open pit mines, with a diameter of some 6km and a depth of 1 km. The unprecedented size of the proposed pit presented challenges when making assessments of radiation doses to workers. Doses were assessed for each of the main exposure pathways, with assessment of potential doses from gamma radiation and airborne radioactive dust being relatively straightforward. However, potential doses from inhalation of the decay products of radon required a more detailed assessment. The overall approach was to model radon decay product (RnDP) exposures using such parameters as; estimated radon emanation rates, assumed equilibrium factors and pit ventilation rates. In addition to these factors, the Olympic Dam region is subject to seasonal and nocturnal temperature inversions and it was was expected that these effects would also be present within the pit. A conservative approach was used which considered exposure during 'normal' conditions and exposure during the inversion conditions. Experimental work was conducted in other large open pits to confirm the assumptions made. The presentation outlines the approach and modeling used to determine the RnDP doses and the additional testwork that was conducted to estimate the RnDP dose for miners working at the base of the pit.