COMPARISON OF NATURAL BACKGROUND DOSE RATES FOR RESIDENTS OF THE AMARGOSA VALLEY, NV, TO THOSE IN LEADVILLE, CO, AND THE STATES OF COLORADO AND NEVADA

In the latter half of 2005, the U.S. Environmental Protection Agency (U.S. EPA) published a Proposed Rule (40 CFR Part 197) for establishing a dose rate standard for limiting radionuclide releases from the proposed Yucca Mountain high-level radioactive waste repository during the time period from 104 to 106 years after closure. The proposed standard was based on the difference in the estimated dose rate from natural background in the Amargosa Valley and the “average annual background radiation” for the State of Colorado. As defined by the U.S. EPA, “natural background radiation consists of external exposures from cosmic and terrestrial sources, and internal exposures from indoor exposures to naturally-occurring radon. ” On the basis of its assessments, the U.S. EPA estimated that the difference in the dose rate in the two identified areas was 3.5 mSv y−1. The purpose of this paper is to provide an independent evaluation and review of this estimate. One of the first observations was that, because site-specific dose rate measurements for the Amargosa Valley “were not available,” the dose rates for various sources of natural background in that area, used by the U.S. EPA in its assessment, were based on modifications of the average values for the State of Nevada. A second observation was that the conversion coefficient applied in estimating the dose rates due to exposures to indoor radon and its decay products was a factor of >2 higher than the currently internationally accepted value. Further review revealed that site-specific data for many natural background sources in the Amargosa Valley were available. One particularly important observation was that about 91% of the residents of that area live in mobile homes which, due to their construction and design, have indoor radon concentrations comparable to, or less than, those outdoors. For that reason, alone, the U.S. EPA estimate of the average dose rate for residents of the Amargosa Valley, due to exposures to indoor radon, was not valid. For purposes of the comparisons in this paper, site-specific dose rates were estimated for all major natural background sources of exposure to residents of the Amargosa Valley, and those in Leadville, CO. The latter community was selected for comparison because of its altitude (3,200 m) and accompanying relatively high cosmic radiation dose rate, and the fact the size of its population is comparable to that of the Amargosa Valley. Another reason for this selection was that a comparison of the average natural background dose rate in the Amargosa Valley to that for the State of Colorado is not suitable because it fails to consider those locations within the State that have dose rates that are higher than the average. Nonetheless, for completeness, and to provide a number that could be compared to the U.S. EPA estimated difference, similar comparisons of the estimated dose rate in the Amargosa Valley to those for average residents of the States of Colorado and Nevada were included in the assessments that follow. The outcome showed that the estimated dose rates in Leadville, the State of Colorado, and the State of Nevada, were higher than those in the Amargosa Valley by 3.94 ± 1.09, 2.54 ± 2.18, and 0.95 ± 0.82 mSv y−1, respectively. Associated uncertainties were highest for the estimated dose rates due to exposures to radon and its decay products. Had the systematic errors in the radon dose conversion coefficient and the random distribution in radon concentrations been included, the overall uncertainty in the total dose rate estimates could have been as high as 150%.