Comparison of Risks Calculated Using U.S. Environmental Protection Agency External Slope Factors and Radiological Field Measurements at the Idaho National Engineering Laboratory and the Potential Aspects on the Ri/Fs Process.* S. T. Dundon, IT Corporation, 5301 Central Avenue NE, Suite 700, Albuquerque, NM 87108; R. Filemyr, IT Corporation, 557 Oppenheimer Road, Suite 200, Los Alamos, NM 87544; and K. J. Holdren, EG&G Idaho, P. 0. Box 1625, Idaho Falls, ID 83415
The risks to occupational receptors from direct exposure to penetrating radiation at the Idaho National Engineering Laboratory's BORAX-1 and SL-1 burial grounds were calculated using (a) the EPA external slope factor approach, and (b) 1992 radiological field measurements. Identical exposure parameters (i.e., 8 hours/day, 250 days/year, for 25 years) were used in each calculation. The calculations were performed in support of the remedial investigation baseline risk assessment for the two sites. The main contributor to the external dose rate was Cs-137 (specifically, the daughter Ba-137m). The calculated risks were 1 x 10-2 and 3 x 10-1 for BORAX-1 and SL-1, respectively, using estimated pCi/g activities for Cs-137 and the EPA external slope factor method. Risks were also calculated using the existing mrem/hr field measurement data. Based on the field measurements, the risks were approximately 5 x 10-4at BORAX-1 and 3 x 10-3 at SL-1, a range from near background risk of 3 x 10-4to an approximate order of magnitude greater than the risk due to background radiation. EPA methods generated higher calculated risks primarily because of conservatively estimated source terms used in the risk equation and from the model uncertainties present in the assumed source geometry (infinite plane and volume, as compared to multiple point sources). The two order-of-magnitude difference in the risk estimates have significant impact on the consideration of remedial action objectives and, ultimately, on the selection of appropriate future actions by federal and state agencies.
*Work supported by EG&G-Idaho and the U.S. Department of Energy under Contract #DE-AC07-761DO1570.