Radon dispersion modeling and dose assessment for uranium mine ventilation shaft exhausts under neutral atmospheric stability.

In the present study, the roles of atmospheric wind profiles in the neutral atmosphere and surface roughness parameters in a complex terrain were examined to determine their impacts on radon ((222)Rn) dispersion from an actual uranium mine ventilation shaft. Simulations were completed on (222)Rn dispersion extending from the shaft to a vulnerable distance, near the location of an occupied farmhouse. The eight dispersion scenarios for the ventilation shaft source included four downwind velocities (0.5, 1.0, 2.0 and 4.0 m s(-1)) and two underlying surface roughness characteristics (0.1 m and 1.0 m). (222)Rn distributions and elevated pollution regions were identified. Effective dose estimation methods involving a historical weighting of wind speeds in the direction of interest coupled to the complex dispersion model were proposed. Using this approach, the radiation effects on the residents assumed to be outside at the location of the farm house 250 m downwind from the ventilation shaft outlet were computed. The maximum effective dose rate calculated for the residents at the outside of the farm house was 2.2 mSv y(-1), which is less than the low limit action level of 3-10 mSv y(-1) recommended by the International Commission on Radiological Protection (ICRP) occupational exposure action level for radon.