Long-Term and Episodic Modeling or Atmospheric Dispersion of 131I for the Oak Ridge Dose Reconstruction Study. Shyam K. Nair, A. Iulian Apostoaei, and F. Owen Hoffman, SENES Oak Ridge, Inc., Center for Risk Analysis, 102 Donner Drive Oak Ridge, TN 37830; and Frank A. Gifford, 109 Gorgas Lane, Oak Ridge, TN 37830
Between 1944 and 1956, 131I was released to the atmosphere as a result of the dissolution of spent fuel slugs from the graphite reactor located at the Oak Ridge National Laboratory for the separation of radioactive lanthanum. One of the primary objectives of the Oak Ridge Dose Reconstruction Study was to estimate the excess risks of developing thyroid cancers from the intake of 131I. The region surrounding the graphite reactor represents a unique ridge-valley topography in which the wind speeds and directions are strongly dominated by the local terrain features. When modeling the dispersion of 131I, modifications of the winds by local terrain must be taken into consideration. Local climate in the region is also subject to higher frequencies of foggy conditions, calmer winds, and inversion conditions. Ingestion of contaminated milk from locally grazing cows and goats by the local population constituted the most important pathway of human exposure. Therefore, increased wet deposition in the humid climate of Oak Ridge is another consideration that must be taken into account to accurately estimate the concentrations of 131I on pasture grass. After its release, radioiodine is distributed in the atmosphere in reactive-elemental, nonreactive organic, and particulate forms. Speciation of radioiodine into the three forms during the dispersion is another process-level detail that must be accounted for. Separate conceptual models were developed to describe the short- and long-term dispersion, chemistry, and deposition of 131I. Uncertainties in the knowledge of input parameters and processes were addressed by simulating the codes (ISCLT and ISCST) in a Monte Carlo-based uncertainty analysis framework. The results of the analysis are presented as time- and space-dependent probability distributions of the levels of contamination on the pasture grass.
Work supported by Tennessee Department of Health through subcontract with McLaren/Hart Environmental Services, Inc.