Atmospheric aerosol measurements of 238U and 232Th at Fernald, Ohio, and implications on inhalation dose calculations.

Abstract

Measurements of the aerosol size distribution of 238U and 232Th at the U.S. Department of Energy's Fernald Environmental Management Project site were made to improve the inhalation dose calculations to off-site individuals. For approximately 1 y an 8-stage cascade impactor was co-located with a high volume sampler and operated at the Fernald Environmental Management Project site to collect aerosol samples. During most of the year, the site was dominated by giant particles with more than 70% of the mass of 238U above 15 microm. The seasonal average of the activity median aerodynamic diameter, based on the impactor samples, which excluded particles >15 microm, for both uranium and thorium was approximately 6.5 microm. These numbers reflect the activities at the site and the domination of the resuspension processes. During most sampling periods the size distribution was bimodal. Thorium concentrations were comparable to the uranium concentrations during the late spring and summer period and decreased to approximately 25% of the 238U concentration in the late summer. Fernald Environmental Management Project is required to calculate the maximum allowable dose from airborne emissions, excluding radon, to meet NESHAP compliance. These calculations assume an AMAD of 1 microm. We found that if you combine size distribution information with the latest ICRP 66 lung model you reduce the estimated dose by a factor of 7. Inclusion of the size distribution of radionuclides at any sampling site would substantially improve the dose estimates to the population.