ATMOSPHERIC DISPERSION AND DEPOSITION OF I-131 RELEASED FROM THE HANFORD SITE

Approximately 2.6 x 10(4) TBg (700,000 Ci) of I-131 were released to t he air from reactor fuel processing plants on the Hanford Site in sout hcentral Washington State from December 1944 through December 1949. Th e Hanford Environmental Dose Reconstruction Project developed a suite of codes to estimate the doses that might have resulted from these rel eases. The Regional Atmospheric Transport Code for Hanford Emission Tr acking (RATCHET) computer code is part of this suite. The RATCHET code implements a Lagrangian-trajectory, Gaussian-puff dispersion model th at uses hourly meteorological and release rate data to estimate daily time-integrated air concentrations and surface contamination for use i n dose estimates. In this model, iodine is treated as a mixture of thr ee species (inorganic gases, organic gases, and particles). Model depo sition parameters are functions of the mixture and meteorological cond itions. A resistance model is used to calculate dry deposition velocit ies. Equilibrium between concentrations in the precipitation and the a ir near the ground is assumed in calculating wet deposition of gases, and irreversible washout of the particles is assumed. RATCHET explicit ly treats the uncertainties in model parameters and meteorological con ditions. Uncertainties in I-131 release rates and partitioning among t he nominal species are treated by varying model input. The results of 100 model runs for December 1944 through December 1949 indicate that m onthly average air concentrations and deposition have uncertainties ra nging from a factor of two near the center of the time-integrated plum e to more than an order of magnitude near the edge. These results indi cate that similar to 10% of the I-131 released to the atmosphere decay ed during transit in the study area, similar to 56% was deposited with in the study area, and the remaining 34% was transported out of the st udy area while still in the air.